Main Page: Difference between revisions

From formulasearchengine
Jump to navigation Jump to search
No edit summary
mNo edit summary
Line 1: Line 1:
{{Main|Quantum triviality}}
{{Use dmy dates|date=August 2012}}
{{about|algorithms specific to paging|outline of general cache algorithms (e.g. processor, disk, database, web)|Cache algorithms}}
In a [[computer]] [[operating system]] that uses [[paging]] for [[virtual memory]] [[memory management|management]], '''page replacement algorithms''' decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a [[page fault]] occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.


In [[physics]], the '''Landau pole''' or the '''Moscow zero''' is the [[energy scale|momentum (or energy) scale]] at which the [[coupling constant]] (interaction strength) of a [[quantum field theory]] becomes infinite. Such a possibility was pointed out by the physicist [[Lev Landau]] and his colleagues.<ref>[[Lev Landau]], in {{cite book| title=Niels Bohr and the Development of Physics|editor=[[Wolfgang Pauli]]|publisher=Pergamon Press| year=1955| location=London}}</ref> The fact that coupling constants depend on the momentum (or length) scale is one of the basic ideas behind the [[renormalization group]].
When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the ''quality'' of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.


Landau poles appear in theories that are not [[asymptotic freedom|asymptotically free]], such as [[quantum electrodynamics]] (QED) or {{math|''φ''<sup>4</sup>}} theory—a [[scalar field]] with a [[quartic interaction]]—such as may describe the [[Higgs boson]]. In these theories, the renormalized coupling constant grows with energy. A Landau pole appears when the coupling constant becomes infinite at a finite energy scale. In a theory intended to be complete, this could be considered a mathematical inconsistency. A possible solution is that the renormalized charge could go to zero as the cut-off is removed, meaning that the charge is completely screened by quantum fluctuations ([[vacuum polarization]]). This is a case of [[quantum triviality]],<ref>
The page replacing problem is a typical [[online problem]] from the competitive analysis perspective in the sense that optimal deterministic algorithm is known.   
{{cite journal
| author=[[David J E Callaway|D. J. E. Callaway]]
| year=1988
| title=Triviality Pursuit: Can Elementary Scalar Particles Exist?
| journal=[[Physics Reports]]
  | volume=167
| issue=5 | pages=241–320
| doi=10.1016/0370-1573(88)90008-7
|bibcode = 1988PhR...167..241C }}</ref> which means that quantum corrections completely suppress the interactions in the absence of a cut-off.


Since the Landau pole is normally calculated using [[perturbation theory (quantum mechanics)|perturbative]] one-loop or two-loop calculations, it is possible that the pole is merely a sign that the perturbative approximation breaks down at strong coupling. [[Lattice field theory]] provides a means to address questions in quantum field theory beyond the realm of perturbation theory, and thus has been used to attempt to resolve this question. Numerical computations performed in this framework seems to confirm Landau's conclusion that QED charge is completely screened for an infinite cutoff.<ref>{{cite journal|last=Göckeler|first=M.|coauthors=R. Horsley, V. Linke, P. Rakow, G. Schierholz, and H. St&uuml;ben|year=1998|title=Is There a Landau Pole Problem in QED?|journal=[[Physical Review Letters]]|volume=80|doi=10.1103/PhysRevLett.80.4119|url=http://link.aps.org/doi/10.1103/PhysRevLett.80.4119| pages=4119–4122| bibcode=1998PhRvL..80.4119G|arxiv = hep-th/9712244|issue=19 }}</ref><ref>{{cite journal| last=Kim| first=S.| coauthors=John B. Kogut and Lombardo Maria Paola|date=2002-01-31|journal=[[Physical Review D]]| doi=10.1103/PhysRevD.65.054015| url=http://link.aps.org/doi/10.1103/PhysRevD.65.054015|volume=65|pages=054015|arxiv = hep-lat/0112009 |bibcode = 2002PhRvD..65e4015K| title=Gauged Nambu–Jona-Lasinio studies of the triviality of quantum electrodynamics|issue=5 }}</ref><ref>{{cite journal| last1=Gies| first1=Holger|last2=Jaeckel| first2=Joerg| title=Renormalization Flow of QED| journal=[[Physical Review Letters]]| date=2004-09-09| volume=93| doi=10.1103/PhysRevLett.93.110405 |url=http://link.aps.org/doi/10.1103/PhysRevLett.93.110405| page=110405| bibcode=2004PhRvL..93k0405G|arxiv = hep-ph/0405183|issue=11}}</ref>
== History ==
Page replacement algorithms were a hot topic of research and debate in the 1960s and 1970s.
That mostly ended with the development of sophisticated LRU approximations and working set algorithms. Since then, some basic assumptions made by the traditional page replacement algorithms were invalidated, resulting in a revival of research. In particular, the following trends in the behavior of underlying hardware and user-level software have affected the performance of page replacement algorithms:


==Brief history==
* Size of primary storage has increased by multiple orders of magnitude. With several gigabytes of primary memory, algorithms that require a periodic check of each and every memory frame are becoming less and less practical.
According to Landau, [[Alexei Alexeyevich Abrikosov|Abrikosov]], [[Isaak Markovich Khalatnikov|Khalatnikov]],<ref>L. D. Landau, A. A. Abrikosov, and I. M. Khalatnikov, Dokl. Akad. Nauk SSSR 95, 497, 773, 1177 (1954).</ref> the relation of the observable charge {{math|''g''<sub>obs</sub>}} with the “bare” charge {{math|''g''<sub>0</sub>}} for renormalizable field theories when {{math|Λ ≫ ''m''}} is given by expression
* Memory hierarchies have grown taller. The cost of a CPU cache miss is far more expensive. This exacerbates the previous problem.
* [[Memory locality|Locality of reference]] of user software has weakened. This is mostly attributed to the spread of [[object-oriented programming]] techniques that favor large numbers of small functions, use of sophisticated data structures like [[Tree data structure|tree]]s and [[hash table]]s that tend to result in chaotic memory reference patterns, and the advent of [[garbage collection (computer science)|garbage collection]] that drastically changed memory access behavior of applications.


: <math> g_{obs}=\frac{g_0}{1+\beta_2 g_0 \ln \Lambda/m} \qquad\qquad\qquad (1)</math>
Requirements for page replacement algorithms have changed due to differences in operating system [[Kernel (computer science)|kernel]] architectures. In particular, most modern OS kernels have unified virtual memory and file system caches, requiring the page replacement algorithm to select a page from among the pages of both user program virtual address spaces and cached files. The latter pages have specific properties. For example, they can be locked, or can have write ordering requirements imposed by [[journaling file system|journaling]]. Moreover, as the goal of page replacement is to minimize total time waiting for memory, it has to take into account memory requirements imposed by other kernel sub-systems that allocate memory. As a result, page replacement in modern kernels ([[Linux]], [[FreeBSD]], and [[Solaris (operating system)|Solaris]]) tends to work at the level of a general purpose kernel memory allocator, rather than at the higher level of a virtual memory subsystem.


where {{mvar|m}} is the mass of the particle, and {{math|Λ}} is the momentum cut-off. If {{math|''g''<sub>0</sub> < ∞}} and {{math|Λ → ∞ }} then {{math|''g''<sub>obs</sub> → 0}} and the theory looks trivial. In fact, inverting Eq.1, so that {{math|''g''<sub>0</sub>}} (related to the length scale {{math|Λ<sup>−1</sup>}} reveals an accurate value of {{math|''g''<sub>obs</sub>}}:
== Local vs. global replacement ==
Replacement algorithms can be ''local'' or ''global''.


: <math>  g_0=\frac{g_{obs}}{1-\beta_2 g_{obs} \ln \Lambda/m}. \qquad\qquad\qquad (2)</math>
When a process incurs a page fault, a local page replacement algorithm selects for replacement some page that belongs to that same process (or a group of processes sharing a [[Memory management (operating systems)#Partitioned allocation|memory partition]]).
A global replacement algorithm is free to select any page in memory.


As {{mvar|Λ}} grows, the bare charge {{math|''g''<sub>0</sub> {{=}} ''g''(Λ)}} increases, to diverge at the renormalization point
Local page replacement assumes some form of memory partitioning that determines how many pages are to be assigned to a given process or a group of processes. Most popular forms of partitioning are ''fixed partitioning'' and ''balanced set'' algorithms based on the [[working set]] model. The advantage of local page replacement is its scalability: each process can handle its page faults independently without contending for some shared global data structure.


: <math>  \Lambda_{Landau} = m \exp\left\{ \frac{1}{\beta_2 g_{obs}} \right\}.\qquad\qquad\qquad (3)</math>
== Precleaning ==
Most replacement algorithms simply return the target page as their result. This means that if target page is ''dirty'' (that is, contains data that have to be written to the stable storage before page can be reclaimed), I/O has to be initiated to send that page to the stable storage (to ''clean'' the page). In the early days of virtual memory, time spent on cleaning was not of much concern, because virtual memory was first implemented on systems with [[full duplex]] channels to the stable storage, and cleaning was customarily overlapped with paging. Contemporary commodity hardware, on the other hand, does not support full duplex transfers, and cleaning of target pages becomes an issue.


This singularity is the '''Landau pole''' with a ''negative residue'', {{math|''g''(Λ) ≈ −Λ<sub>Landau</sub> /(''β''<sub>2</sub>(Λ − Λ<sub>Landau</sub>))}}.
To deal with this situation, various ''precleaning'' policies are implemented. Precleaning is the mechanism that starts I/O on dirty pages that are (likely) to be replaced soon. The idea is that by the time the precleaned page is actually selected for the replacement, the I/O will complete and the page will be clean. Precleaning assumes that it is possible to identify pages that will be replaced ''next''. Precleaning that is too eager can waste I/O bandwidth by writing pages that manage to get re-dirtied before being selected for replacement.


In fact, however, the growth of {{math|''g''<sub>0</sub>}} invalidates Eqs.1,2 in the region {{math|''g''<sub>0</sub> ≈ 1}}, since these were obtained for {{math|''g''<sub>0</sub> ≪ 1}}, so that the exact reality of the Landau pole becomes doubtful.
==Anticipatory paging==
{{seealso|Paging}}
Some systems use [[demand paging]] -- waiting until a page is actually requested before loading it into RAM.


The actual behavior of the charge {{math|''g''(''μ'')}} as a function of the momentum scale {{mvar|μ}} is determined by the [[Murray Gell-Mann|Gell-Mann]]–[[Francis E. Low|Low]] equation<ref>{{cite journal|last1=[[Murray Gell-Mann|Gell-Mann]]| first1=M.| last2=Low| first2=F. E.|year=1954|journal=[[Physical Review]]|title=Quantum Electrodynamics at Small Distances| volume=95| doi=10.1103/PhysRev.95.1300| url=http://link.aps.org/doi/10.1103/PhysRev.95.1300|pages=1300–1320|bibcode = 1954PhRv...95.1300G | issue=5}}</ref>
Other systems attempt to reduce latency by guessing which pages not in RAM are likely to be needed soon, and pre-loading such pages into RAM, before that page is requested. (This is often in combination with pre-cleaning, which guesses which pages currently in RAM are not likely to be needed soon, and pre-writing them out to storage).


: <math> \frac{dg}{d \ln \mu} =\beta(g)=\beta_2 g^2+\beta_3 g^3+\ldots \qquad\qquad\qquad (4)  </math>
When a page fault occurs, "anticipatory paging" systems will not only bring in the referenced page, but also the next few consecutive pages (analogous to a [[prefetch input queue]] in a CPU).


which gives Eqs.1,2 if it is integrated under conditions {{math|''g''(''μ'') {{=}} ''g''<sub>obs</sub>}} for {{math|''μ'' {{=}} ''m''}} and {{math|''g''(''μ'') {{=}} ''g''<sub>0</sub>}} for {{math|''μ'' {{=}} Λ}}, when only the term with {{math|''β''<sub>2</sub>}} is retained in the right hand side. The general behavior of {{math|''g''(''μ'')}} depends on the appearance of the function {{math|''β''(''g'')}}.
The [[Paging#Swap_prefetch|swap prefetch]] mechanism goes even further in loading pages (even if they are not consecutive) that are likely to be needed soon.


According to the standard classification,<ref>N. N. Bogoliubov and D. V. Shirkov, Introduction to the Theory of Quantized Fields, 3rd ed. (Nauka, Moscow, 1976; Wiley, New York, 1980).</ref> there are three qualitatively different cases:
==The (h,k)-Paging problem==
The (h,k)-Paging problem is a generalization of the model of paging problem: Let h,k be positive integers that <math>h \leq k</math>. We measure the performance of an  algorithm with cache of size <math>h \leq k</math> relative to the [[theoretically optimal page replacement algorithm]]. If <math>h<k</math> we provide the optimal page replacement algorithm with strictly less resource.


*(a) if {{math|''β''(''g'')}} has a zero at the finite value {{math|''g''<sup>∗</sup>}}, then growth of {{mvar|g}} is saturated, i.e. {{math|''g''(''μ'') → ''g''<sup>∗</sup>}} for {{math|''μ'' → ∞}};
The (h,k)-Paging problem is a way to measure how an online algorithm performs by comparing it with the performance of the optimal algorithm, specifically, separately parameterizing the cache size of the online algorithm and optimal algorithm.


*(b) if {{math|''β''(''g'')}} is non-alternating and behaves as {{math|''β''(''g'') ∝ ''g<sup>α</sup>''}} with {{math|''α'' ≤ 1}} for large {{mvar|g}}, then the growth of {{math|''g''(''μ'')}} continues to infinity;
==Marking algorithms==
Marking algorithms is a general class of paging algorithms. For each page, we associate it with a bit called its mark. Initially, we set all pages as unmarked. During a stage of page requests, we mark a page when it is first requested in this stage. A marking algorithm is such an algorithm that never pages out a marked page.


*(c) if {{math|''β''(''g'') ∝ ''g<sup>α</sup>''}} with {{math|''α'' > 1}} for large {{mvar|g}}, then {{math|''g''(''μ'')}} is divergent at finite value {{math|''μ''<sub>0</sub>}} and the real Landau pole arises: the theory is internally inconsistent due to indeterminacy of {{math|''g''(''μ'')}} for {{math|''μ'' > ''μ''<sub>0</sub>}}.
If ALG is a marking algorithm with a cache of size k, and OPT is the optimal algorithm with a cache of <math>h \leq k</math>. Then ALG is <math> \dfrac{k}{k-h+1}</math>-competitive. So every marking algorithm attains the <math> \dfrac{k}{k-h+1}</math>-competitive ratio.


Landau and [[Isaak Pomeranchuk|Pomeranchuk]] <ref>L.D.Landau, I.Ya.Pomeranchuk, Dokl. Akad. Nauk SSSR 102, 489 (1955); I.Ya.Pomeranchuk, Dokl. Akad. Nauk SSSR 103, 1005 (1955).</ref> tried to justify the possibility (c) in the case of QED and {{math|''φ''<sup>4</sup>}} theory. They have noted that the growth of {{math|''g''<sub>0</sub>}} in Eq.1 drives the observable charge {{math|''g''<sub>obs</sub>}} to the constant limit, which does not depend on {{math|''g''<sub>0</sub>}}. The same behavior can be obtained from the functional integrals, omitting the quadratic terms in the action. If neglecting the quadratic terms is valid already for {{math|''g''<sub>0</sub> ≪ 1}}, it is all the more valid for {{math|''g''<sub>0</sub>}} of the order or greater than unity: it gives a reason to consider Eq.1 to be valid for arbitrary {{math|''g''<sub>0</sub>}}. Validity of these considerations on the quantitative level is excluded by non-quadratic form of the {{mvar|β}}-function. Nevertheless, they can be correct qualitatively. Indeed, the result {{math|''g''<sub>obs</sub> {{=}} const(''g''<sub>0</sub>)}} can be obtained from the functional integrals only for {{math|''g''<sub>0</sub> ≫ 1}}, while its validity for {{math|''g''<sub>0</sub> ≪ 1}}, based on Eq.1, may be related with other reasons; for {{math|''g''<sub>0</sub> ≈ 1}} this result is probably violated but coincidence of two constant values in the order of magnitude can be expected from the matching condition. The [[Monte Carlo method|Monte Carlo]] results <ref>{{cite doi|10.1016/0550-3213(84)90246-3|noedit}}</ref> seems to confirm the qualitative validity of the Landau–Pomeranchuk arguments, though a different interpretation is also possible.
LRU and CLOCK  are marking algorithms while FIFO is not a marking algorithm.


The case (c) in the Bogoliubov and Shirkov classification  corresponds to the [[quantum triviality]] in full theory (beyond its perturbation context), as can be seen by  a [[reductio ad absurdum]].  Indeed, if {{math|''g''<sub>obs</sub> < ∞}}, the theory is internally inconsistent. The only way to avoid it, is for {{math|''μ''<sub>0</sub> → ∞}}, which is  possible only for {{math|''g''<sub>obs</sub> → ∞}}. It is a widespread belief  that both QED and {{math|''φ''<sup>4</sup>}} theory are trivial in the continuum limit. In fact, available information confirms only “Wilson triviality”, which just amounts to positivity of {{math|''β''(''g'')}} for {{math|''g'' ≠ 0}} and can be considered as firmly established. Indications of “true” quantum triviality are not numerous and allow different interpretations.
==Conservative algorithms==
An algorithm is conservative, if on any consecutive request sequence containing k or fewer distinct page references, the algorithm will incur k or fewer page faults.  


== Phenomenological  aspects ==
If ALG is a conservative algorithm with a cache of size k, and OPT is the optimal algorithm with a cache of <math>h \leq k</math>. Then ALG is <math> \dfrac{k}{k-h+1}</math>-competitive. So every conservative algorithm attains the <math> \dfrac{k}{k-h+1}</math>-competitive ratio.
In a theory intended to represent a physical interaction where the coupling constant is known to be non-zero, Landau poles or triviality may be viewed as a sign of incompleteness in the theory. For example, QED is usually not believed to be a complete theory on its own and contains a Landau pole. Conventionally QED forms part of the more fundamental [[electroweak theory]]. The {{math|U(1)<sub>Y</sub>}} group of electroweak theory also has a Landau pole which is usually considered to be a signal as a need for an embedding into a [[Grand Unified Theory]]. The grand unified scale would provide a natural cutoff well below the Landau scale, preventing the pole from having observable physical consequences.


The problem of the Landau pole in QED is of pure academic interest. The role of {{math|''g''<sub>obs</sub>}} in Eqs. 1, 2 is played by the [[fine structure constant]] {{math|''α'' ≈ 1/137}} and the Landau scale for QED is estimated as 10<sup>286</sup> eV, which is far beyond any energy scale relevant to observable physics. For comparison, the maximum energies accessible at the [[Large Hadron Collider]] are of order 10<sup>13</sup> eV, while the [[Planck scale]], at which [[quantum gravity]] becomes important and the relevance of [[quantum field theory]] itself may be questioned, is 10<sup>28</sup> eV.
LRU, FIFO and CLOCK are conservative algorithms.


The [[Higgs boson]] in the [[Standard Model]] of [[particle physics]] is described by {{math|''φ''<sup>4</sup>}} theory. If the latter has a Landau pole, then this fact is used in setting a "triviality bound" on the Higgs mass.  The bound depends on the scale at which new physics is assumed to enter and the maximum value of the quartic coupling permitted (its physical value is unknown). For large couplings, non-perturbative methods are required. Lattice calculations have also been useful in this context.<ref>For example, {{cite journal| last=Heller| first=Urs| author2=Markus Klomfass |author3=Herbert Neuberger |author4=Pavols Vranas |date=1993-09-20|journal=[[Nuclear Physics B]]| volume=405| doi=10.1016/0550-3213(93)90559-8| pages=555–573|arxiv = hep-ph/9303215 |bibcode = 1993NuPhB.405..555H|title=Numerical analysis of the Higgs mass triviality bound|issue=2–3 }}, which suggests {{math|''M<sub>H</sub>'' < 710 GeV}}.</ref>


== Recent  developments ==
==Page replacement algorithms==
Solution of the Landau pole problem requires calculation of the Gell-Mann–Low function {{math|''β''(''g'')}} at arbitrary {{mvar|g}} and, in particular, its asymptotic behavior for {{math|''g'' → ∞}}. This problem is very difficult and was considered as hopeless for many years: by diagrammatic calculations one can obtain only few expansion coefficients {{math|''β''<sub>2</sub>, ''β''<sub>3</sub>, ...}}, which do not allow to investigate the {{mvar|β}} function in the whole. The progress became possible  after development of the [[Lev Lipatov|Lipatov]] method for calculation of large orders of perturbation theory:<ref>L.N.Lipatov, Zh.Eksp.Teor.Fiz. 72, 411 (1977) [Sov.Phys. JETP 45, 216 (1977)].</ref> now one can try to interpolate the known coefficients {{math|''β''<sub>2</sub>, ''β''<sub>3</sub>, ...}} with their large order behavior and to sum the perturbation series. The first attempts of reconstruction of the {{mvar|β}} function witnessed on triviality of {{math|''φ''<sup>4</sup>}} theory. Application of more advanced summation methods gave the exponent {{mvar|α}} in the asymptotic behavior {{math|''β''(''g'') ∝ ''g<sup>α</sup>''}} a value close to unity. The hypothesis  for the asymptotics {{math|''β''(''g'') ∝ ''g''}} was recently confirmed analytically for {{math|''φ''<sup>4</sup>}} theory and QED.<ref>I. M. Suslov,  JETP 107, 413 (2008); JETP 111, 450 (2010); http://arxiv.org/abs/1010.4081, http://arxiv.org/abs/1010.4317.  I. M. Suslov, JETP 108,  980 (2009), http://arxiv.org/abs/0804.2650. </ref> Together with positiveness of {{math|''β''(''g'')}}, obtained by summation of series, it gives the case (b) of the Bogoliubov and Shirkov classification, and hence the Landau pole is absent in these theories. Possibility of omitting the quadratic terms in the action suggested by Landau and Pomeranchuk is not confirmed.
There are a variety of page replacement algorithms
<ref>
[http://www.cs.uiowa.edu/~jones/opsys/fall95/notes/0908.html "Lecture Notes"] by Douglas W. Jones 1995
</ref>:


==References==
===The theoretically optimal page replacement algorithm===
<references/>
The theoretically optimal page replacement algorithm (also known as OPT, [[Clairvoyance|clairvoyant]] replacement algorithm, or [[László Bélády|Bélády's]] optimal page replacement policy)<ref>[http://www.read.cs.ucla.edu/111/2006fall/notes/lec11 2006fall:notes:lec11 [CS111&#93;<!-- Bot generated title -->]</ref><ref>[http://cat.inist.fr/?aModele=afficheN&cpsidt=15530156 Characterization of Web reference behavior revisited: Evidence for Dichotomized Cache management<!-- Bot generated title -->]</ref><ref>[http://www.cs.uiowa.edu/~jones/opsys/fall95/notes/0908.html 22C:116, Notes, 8 September 1995<!-- Bot generated title -->]</ref> is an algorithm that works as follows: when a page needs to be swapped in, the [[operating system]] swaps out the page whose next use will occur farthest in the future. For example, a page that is not going to be used for the next 6 seconds will be swapped out over a page that is going to be used within the next 0.4 seconds.


{{DEFAULTSORT:Landau Pole}}
This algorithm cannot be implemented in the general purpose operating system because it is impossible to compute reliably how long it will be before a page is going to be used, except when all software that will run on a system is either known beforehand and is amenable to the static analysis of its memory reference patterns, or only a class of applications allowing run-time analysis. Despite this limitation, algorithms exist{{Citation needed|date=June 2008}} that can offer near-optimal performance &mdash;<!--  on the first run of a program, (conflicts with following sentense) --> the operating system keeps track of all pages referenced by the program, and it uses those data to decide which pages to swap in and out on subsequent runs. This algorithm can offer near-optimal performance, but not on the first run of a program, and only if the program's memory reference pattern is relatively consistent each time it runs.
[[Category:Quantum field theory]]
 
[[Category:Renormalization group]]
Analysis of the paging problem has also been done in the field of [[online algorithm]]s. Efficiency of randomized online algorithms for the paging problem is measured using [[amortized analysis]].
 
===Not recently used===
The not recently used (NRU), sometimes known as the Least Recently Used (LRU), page replacement algorithm is an algorithm that favours keeping pages in memory that have been recently used. This algorithm works on the following principle: when a page is referenced, a referenced bit is set for that page, marking it as referenced. Similarly, when a page is modified (written to), a modified bit is set. The setting of the bits is usually done by the hardware, although it is possible to do so on the software level as well.
 
At a certain fixed time interval, the clock interrupt triggers and clears the referenced bit of all the pages, so only pages referenced within the current clock interval are marked with a referenced bit. When a page needs to be replaced, the [[operating system]] divides the pages into four classes:
 
<ol start=0>
<li>not referenced, not modified
<li>not referenced, modified
<li>referenced, not modified
<li>referenced, modified
</ol>
 
Although it does not seem possible for a page to be not referenced yet modified, this happens when a class 3 page has its referenced bit cleared by the clock interrupt. The NRU algorithm picks a random page from the lowest category for removal. Note that this algorithm implies that a modified (within clock interval) but not referenced page is less important than a not modified page that is intensely referenced.
 
NRU is a marking algorithm, so it is <math> \dfrac{k}{k-h+1}</math>-competitive.
 
===First-in, first-out===
The simplest page-replacement algorithm is a FIFO algorithm. The first-in, first-out (FIFO) page replacement algorithm is a low-overhead algorithm that requires little book-keeping on the part of the [[operating system]]. The idea is obvious from the name - the operating system keeps track of all the pages in memory in a queue, with the most recent arrival at the back, and the earliest arrival in front. When a page needs to be replaced, the page at the front of the queue (the oldest page) is selected. While FIFO is cheap and intuitive, it performs poorly in practical application. Thus, it is rarely used in its unmodified form. This algorithm experiences [[Bélády's anomaly]].
 
FIFO page replacement algorithm is used by the [[VAX/VMS]] operating system, with some modifications.<ref name="Silber">Abraham Silberschatz, Peter Baer Galvin, Greg Gagne. ''Operating Systems Concepts (Seventh Edition).'': Wiley 2005. p. 339.</ref>
Partial second chance is provided by skipping a limited number of entries with valid translation table references,<ref>[http://mx.isti.cnr.it/cgi-bin/conan?key=Sys_Parameters~TBSKIPWSL&title=VMS%20Help&referer= VMS Help<!-- Bot generated title -->]</ref> and additionally, pages are displaced from process working set to a systemwide pool from which they can be recovered if not already re-used.
 
FIFO is a conservative algorithm, so it is <math> \dfrac{k}{k-h+1}</math>-competitive.
 
===Second-chance===
A modified form of the FIFO page replacement algorithm, known as the Second-chance page replacement algorithm, fares relatively better than FIFO at little cost for the improvement. It works by looking at the front of the queue as FIFO does, but instead of immediately paging out that page, it checks to see if its referenced bit is set. If it is not set, the page is swapped out. Otherwise, the referenced bit is cleared, the page is inserted at the back of the queue (as if it were a new page) and this process is repeated. This can also be thought of as a circular queue. If all the pages have their referenced bit set, on the second encounter of the first page in the list, that page will be swapped out, as it now has its referenced bit cleared. If all the pages have their reference bit set then second chance algorithm degenerates into pure FIFO.
 
As its name suggests, Second-chance gives every page a "second-chance" - an old page that has been referenced is probably in use, and should not be swapped out over a new page that has not been referenced.
 
===Clock===
Clock is a more efficient version of FIFO than Second-chance because pages don't have to be constantly pushed to the back of the list, but it performs the same general function as Second-Chance. The clock algorithm keeps a circular list of pages in memory, with the "hand" (iterator) pointing to the last examined page frame in the list. When a page fault occurs and no empty frames exist, then the R (referenced) bit is inspected at the hand's location. If R is 0, the new page is put in place of the page the "hand" points to, otherwise the R bit is cleared. Then, the clock hand is incremented and the process is repeated until a page is replaced.<ref name="Tanenbaum">Andrew S. Tanenbaum. ''Modern Operating Systems (Second Edition)''. pp. 218 (4.4.5). 2001.</ref>
 
==== Variants on Clock ====
* GCLOCK: Generalized clock page replacement algorithm.<ref>[http://portal.acm.org/citation.cfm?id=320276 Sequentiality and prefetching in database systems]</ref>
* Clock-Pro keeps a circular list of information about recently-referenced pages, including all M pages in memory as well as the most recent M pages that have been paged out. This extra information on paged-out pages, like the similar information maintained by [[adaptive replacement cache|ARC]], helps it work better than LRU on large loops and one-time scans.<ref>[http://www.cse.ohio-state.edu/hpcs/WWW/HTML/publications/papers/TR-05-3.pdf "CLOCK-Pro: An Effective Improvement of the CLOCK Replacement" by Song Jiang, Feng Chen, and Xiaodong Zhang, 2005]</ref>
* WSclock.<ref>
"WSCLOCK—a simple and effective algorithm for virtual memory management"
by Richard W. Carr and John L. Hennessy, 1981
[http://portal.acm.org/citation.cfm?id=806596]
[http://infolab.stanford.edu/~manku/quals/zpapers/81-wsclock.pdf.gz]
</ref> The "aging" algorithm and the "WSClock" algorithm are probably the most important page replacement algorithms in practice.<ref>
[http://www.cs.nyu.edu/courses/spring09/V22.0202-002/wsclock-davis.html "WSClock"]
by Allan Gottlieb
</ref><ref>
[http://www.informit.com/articles/article.aspx?p=25260&seqNum=11 "Page Replacement Algorithms"]
by Andrew S. Tanenbaum 2002
</ref>
* Clock with Adaptive Replacement (CAR) is a page replacement algorithm that has performance comparable to [[Adaptive Replacement Cache|ARC]], and substantially outperforms both LRU and CLOCK<ref>{{cite conference
| author =  Sorav Bansal and Dharmendra S. Modha
| title = CAR: Clock with Adaptive Replacement
| booktitle = In Proceedings of the USENIX Conference on File and Storage Technologies (FAST)
| pages = 187--200
| year = 2004
| url = http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.105.6057
| accessdate =19 February 2012
}}</ref>. The algorithm CAR is self-tuning and requires no user-specified magic parameters.
 
CLOCK is a conservative algorithm, so it is <math> \dfrac{k}{k-h+1}</math>-competitive.
===Least recently used===
The least recently used page (LRU) replacement algorithm, though similar in name to NRU, differs in the fact that LRU keeps track of page usage over a short period of time, while NRU just looks at the usage in the last clock interval. LRU works on the idea that pages that have been most heavily used in the past few instructions are most likely to be used heavily in the next few instructions too. While LRU can provide near-optimal performance in theory (almost as good as [[Adaptive Replacement Cache]]), it is rather expensive to implement in practice. There are a few implementation methods for this algorithm that try to reduce the cost yet keep as much of the performance as possible.
 
The most expensive method is the linked list method, which uses a linked list containing all the pages in memory. At the back of this list is the least recently used page, and at the front is the most recently used page. The cost of this implementation lies in the fact that items in the list will have to be moved about every memory reference, which is a very time-consuming process.
 
Another method that requires hardware support is as follows: suppose the hardware has a 64-bit counter that is incremented at every instruction. Whenever a page is accessed, it gains a value equal to the counter at the time of page access. Whenever a page needs to be replaced, the [[operating system]] selects the page with the lowest counter and swaps it out. With present hardware, this is not feasible because the OS needs to examine the counter for every page in memory.
 
Because of implementation costs, one may consider algorithms (like those that follow) that are similar to LRU, but which offer cheaper implementations.
 
One important advantage of the LRU algorithm is that it is amenable to full statistical analysis. It has been proven, for example, that LRU can never result in more than N-times more page faults than OPT algorithm, where N is proportional to the number of pages in the managed pool.
 
On the other hand, LRU's weakness is that its performance tends to degenerate under many quite common reference patterns. For example, if there are N pages in the LRU pool, an application executing a  loop over array of N + 1 pages will cause a page fault on each and every access. As loops over large arrays are common, much effort has been put into modifying LRU to work better in such situations. Many of the proposed LRU modifications try to detect looping reference patterns and to switch into suitable replacement algorithm, like Most Recently Used (MRU).
 
====Variants on LRU====
# [http://www.ics.forth.gr/dcs/Activities/papers/2000.WCW.caching_search.pdf LRU-K] improves greatly on LRU with regard to locality in time. It's also known as LRU-2, for the case that K=2. LRU-1 (i.e. K=1) is the same as normal LRU.
# The [[Adaptive Replacement Cache|ARC]]<ref>Megiddo & Modha, ''[http://www.almaden.ibm.com/cs/people/dmodha/arcfast.pdf ARC: A Self-tuning, low overhead replacement cache]''</ref> algorithm extends LRU by maintaining a history of recently evicted pages and uses this to change preference to recent or frequent access. It is particularly resistant to sequential scans.
 
A comparison of ARC with other algorithms (LRU,MQ,2Q,LRU-2,LRFU,LIRS) can be found in Megiddo & Modha.<ref>Nimrod Megiddo & Dharmendra S. Modha, ''{{PDFlink|[http://www.almaden.ibm.com/cs/people/dmodha/ARC.pdf Outperforming LRU with an Adaptive Replacement Cache Algorithm]|123&nbsp;KB}}'', IEEE Computer Magazine, pp. 58-65, April 2004.</ref>
 
LRU is a marking algorithm, so it is <math> \dfrac{k}{k-h+1}</math>-competitive.
 
===Random===
Random replacement algorithm replaces a random page in memory. This eliminates the overhead cost of tracking page references. Usually it fares better than FIFO, and for looping memory references it is better than LRU, although generally LRU performs better in practice. [[OS/390]] uses global LRU approximation and falls back to random replacement when LRU performance degenerates, and the [[Intel i860]] processor used a random replacement policy (Rhodehamel 1989).
 
===Not frequently used===
 
The not frequently used (NFU) page replacement algorithm requires a counter, and every page has one counter of its own which is initially set to 0. At each clock interval, all pages that have been referenced within that interval will have their counter incremented by 1. In effect, the counters keep track of how frequently a page has been used. Thus, the page with the lowest counter can be swapped out when necessary.
 
The main problem with NFU is that it keeps track of the frequency of use without regard to the time span of use. Thus, in a multi-pass compiler, pages which were heavily used during the first pass, but are not needed in the second pass will be favoured over pages which are comparably lightly used in the second pass, as they have higher frequency counters. This results in poor performance. Other common scenarios exist where NFU will perform similarly, such as an OS boot-up. Thankfully, a similar and better algorithm exists, and its description follows.
 
The not frequently used page-replacement algorithm generates fewer page faults than the least recently used page replacement algorithm when the page table contains null pointer values.
 
===Aging===
The aging algorithm is a descendant of the NFU algorithm, with modifications to make it aware of the time span of use. Instead of just incrementing the counters of pages referenced, putting equal emphasis on page references regardless of the time, the reference counter on a page is first shifted right (divided by 2), before adding the referenced bit to the left of that binary number. For instance, if a page has referenced bits 1,0,0,1,1,0 in the past 6 clock ticks, its referenced counter will look like this: 10000000, 01000000, 00100000, 10010000, 11001000, 01100100. Page references closer to the present time have more impact than page references long ago. This ensures that pages referenced more recently, though less frequently referenced, will have higher priority over pages more frequently referenced in the past. Thus, when a page needs to be swapped out, the page with the lowest counter will be chosen.
 
Note that aging differs from LRU in the sense that aging can only keep track of the references in the latest 16/32 (depending on the bit size of the processor's integers) time intervals. Consequently, two pages may have referenced counters of 00000000, even though one page was referenced 9 intervals ago and the other 1000 intervals ago. Generally speaking, knowing the usage within the past 16 intervals is sufficient for making a good decision as to which page to swap out. Thus, aging can offer near-optimal performance for a moderate price.
 
===Techniques for hardware with no reference bit===
Many of the techniques discussed above assume the presence of a reference bit associated with each page.  Some hardware has no such bit, so its efficient use requires techniques that operate well without one.
 
One notable example is [[VAX]] hardware running [[OpenVMS]]. This system knows if a page has been modified, but not necessarily if a page has been read. Its approach is known as Secondary Page Caching.  Pages removed from working sets (process-private memory, generally) are placed on special-purpose lists while remaining in physical memory for some time.  Removing a page from a working set is not technically a page-replacement operation, but effectively identifies that page as a candidate.  A page whose backing store is still valid (whose contents are not dirty, or otherwise do not need to be preserved) is placed on the tail of the Free Page List.  A page that requires writing to backing store will be placed on the Modified Page List.  These actions are typically triggered when the size of the Free Page List falls below an adjustable threshold.
 
Pages may be selected for working set removal in an essentially random fashion, with the expectation that if a poor choice is made, a future reference may retrieve that page from the Free or Modified list before it is removed from physical memory.  A page referenced this way will be removed from the Free or Modified list and placed back into a process working set.  The Modified Page List additionally provides an opportunity to write pages out to backing store in groups of more than one page, increasing efficiency.  These pages can then be placed on the Free Page List.  The sequence of pages that works its way to the head of the Free Page List resembles the results of a LRU or NRU mechanism and the overall effect has similarities to the Second-Chance algorithm described earlier.
 
Another example is used by the [[Linux kernel]] on [[ARM architecture|ARM]]. The lack of hardware functionality is made up for by providing two page tables - the processor-native page tables, with neither referenced bits nor [[dirty bit]]s, and software-maintained page tables with the required bits present. The emulated bits in the software-maintained table are set by page faults. In order to get the page faults, clearing emulated bits in the second table revokes some of the access rights to the corresponding page, which is implemented by altering the native table.
 
== Working set ==
{{main|Working set}}
The working set of a process is the set of pages expected to be used by that process during some time interval.
 
The "working set model" isn't a page replacement algorithm in the strict sense (it's actually a kind of [[scheduling (computing)#Medium-term_scheduling|medium-term scheduler]]){{Clarify|date=August 2011}}
 
== References ==
{{reflist|30em}}
 
===See also===
* K. Y. Wong, "[http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1580916 Web Cache Replacement Policies: A Pragmatic Approach]" IEEE Network, vol. 20, iss. 2, Jan-Feb. 2006, pp.&nbsp;28–34.
* Aho, Denning and Ullman, ''[http://portal.acm.org/citation.cfm?id=321632&coll=portal&dl=ACM Principles of Optimal Page Replacement]'', Journal of the ACM, Vol. 18, Issue 1,January 1971, pp 80–93
* Rhodehamel, Michael W. "[http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=63392 The Bus Interface and Paging Units of the i860(tm) Microprocessor]". In Proc. IEEE International Conference on Computer Design, p.&nbsp;380-384, 1989.
* Tanenbaum, Andrew S. ''Operating Systems: Design and Implementation (Second Edition)''. New Jersey: Prentice-Hall 1997.
* Tanenbaum, Andrew S. ''Modern Operating Systems (Second Edition)''. New Jersey: Prentice-Hall 2001. Online excerpt on page replacement algorithms: [http://www.informit.com/articles/article.aspx?p=25260 Page Replacement Algorithms].
* Johnson and Shasha, ''{{PDFlink|[http://www.vldb.org/conf/1994/P439.PDF 2Q: A Low Overhead High Performance Buffer Management Replacement Algorithm]|1.01&nbsp;MB}}'' [http://www.vldb.org/dblp/db/conf/vldb/vldb94-439.html abstract]
* Gideon Glass and Pei Cao [http://portal.acm.org/citation.cfm?id=258681&jmp=abstract&coll=portal&dl=ACM&CFID=12125227&CFTOKEN=21656990#abstract  Adaptive-Page-Replacement-Based-on-Memory-Reference-Behavior].  Also available in extended form as [http://www.cs.wisc.edu/techreports/viewreport.php?report=1338 Technical Report 1338] at www.cs.wisc.edu
* Jongmin Kim and others, ''{{PDFlink|[http://ssrnet.snu.ac.kr/~choijm/paper/IC-2000-OSDI-UBM.pdf A Low-Overhead High-Performance Unified Buffer Management Scheme that Exploits Sequential and Looping References]|4.14&nbsp;MB}}'', [http://www.usenix.org/events/osdi2000/ Usenix Symposium on Operating System Design and Implementation (OSDI'2000)], San Diego, CA,  17–21 October 2000
* Sorav Bansal and Dharmendra S. Modha, ''{{PDFlink|[http://www.almaden.ibm.com/cs/people/dmodha/clockfast.pdf CAR: Clock with Adaptive Replacement]|212&nbsp;KB}}''
* Smaragdakis and others, ''{{PDFlink|[http://www.cs.amherst.edu/~sfkaplan/courses/spring-2004/cs40/papers/SKW:EELRUSEAPR.pdf EELRU: Simple and Effective Adaptive Page Replacement]|1.55&nbsp;MB}}''
* Song Jiang and Xiaodong Zhang, ''{{PDFlink|[http://www.cse.ohio-state.edu/hpcs/WWW/HTML/publications/papers/TR-02-6.pdf LIRS: a Low Inter Reference recency Set replacement]|283&nbsp;KB}}'', SIGMETRICS 2002
* D. Lee and others, ''[http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/proceedings/euromicro/1997/8215/00/8215toc.xml&DOI=10.1109/EMSCNT.1997.658446  Implementation and Performance Evaluation of the LRFU Replacement Policy]'', p.&nbsp;0106,  23rd [http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/euromicro/&toc=comp/proceedings/euromicro/1997/8215/00/8215toc.xml Euromicro Conference: New Frontiers of Information Technology-Short Contributions], 1997
* Elizabeth J. O'Neil and others, ''{{PDFlink|[http://www.cs.cmu.edu/~christos/courses/721-resources/p297-o_neil.pdf The LRU-K page replacement algorithm for database disk buffering]|1.17&nbsp;MB}}'',  [http://portal.acm.org/citation.cfm?id=170036.170081 ACM SIGMOD Conf.], pp.&nbsp;297–306, 1993.
* Y. Zhou and J.F. Philbin, ''[http://www.usenix.org/events/usenix01/full_papers/zhou/ The Multi-Queue Replacement Algorithm for Second-Level Buffer Caches]'', Proc. Usenix Ann. Tech. Conf. (Usenix 2001), pp.&nbsp;91–104.
 
{{DEFAULTSORT:Page Replacement Algorithm}}
[[Category:Virtual memory]]
[[Category:Memory management algorithms]]
[[Category:Online algorithms]]
 
[[ar:تبديل الصفحات]]
[[es:Algoritmos de reemplazo de páginas]]
[[eu:Orriak ordezkatzeko algoritmoak]]
[[ko:페이지 교체 알고리즘]]
[[ja:ページ置換アルゴリズム]]
[[mk:Алгоритми за преместување на мемориски страници]]
[[pt:Algoritmo de troca de página]]
[[simple:Page replacement algorithm]]
[[tr:Sayfa yer değiştirme algoritması]]
[[uk:Задача заміщення сторінок]]

Revision as of 17:39, 12 August 2014

30 year-old Entertainer or Range Artist Wesley from Drumheller, really loves vehicle, property developers properties for sale in singapore singapore and horse racing. Finds inspiration by traveling to Works of Antoni Gaudí. 29 yr old Orthopaedic Surgeon Grippo from Saint-Paul, spends time with interests including model railways, top property developers in singapore developers in singapore and dolls. Finished a cruise ship experience that included passing by Runic Stones and Church. In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a page fault occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.

When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.

The page replacing problem is a typical online problem from the competitive analysis perspective in the sense that optimal deterministic algorithm is known.

History

Page replacement algorithms were a hot topic of research and debate in the 1960s and 1970s. That mostly ended with the development of sophisticated LRU approximations and working set algorithms. Since then, some basic assumptions made by the traditional page replacement algorithms were invalidated, resulting in a revival of research. In particular, the following trends in the behavior of underlying hardware and user-level software have affected the performance of page replacement algorithms:

  • Size of primary storage has increased by multiple orders of magnitude. With several gigabytes of primary memory, algorithms that require a periodic check of each and every memory frame are becoming less and less practical.
  • Memory hierarchies have grown taller. The cost of a CPU cache miss is far more expensive. This exacerbates the previous problem.
  • Locality of reference of user software has weakened. This is mostly attributed to the spread of object-oriented programming techniques that favor large numbers of small functions, use of sophisticated data structures like trees and hash tables that tend to result in chaotic memory reference patterns, and the advent of garbage collection that drastically changed memory access behavior of applications.

Requirements for page replacement algorithms have changed due to differences in operating system kernel architectures. In particular, most modern OS kernels have unified virtual memory and file system caches, requiring the page replacement algorithm to select a page from among the pages of both user program virtual address spaces and cached files. The latter pages have specific properties. For example, they can be locked, or can have write ordering requirements imposed by journaling. Moreover, as the goal of page replacement is to minimize total time waiting for memory, it has to take into account memory requirements imposed by other kernel sub-systems that allocate memory. As a result, page replacement in modern kernels (Linux, FreeBSD, and Solaris) tends to work at the level of a general purpose kernel memory allocator, rather than at the higher level of a virtual memory subsystem.

Local vs. global replacement

Replacement algorithms can be local or global.

When a process incurs a page fault, a local page replacement algorithm selects for replacement some page that belongs to that same process (or a group of processes sharing a memory partition). A global replacement algorithm is free to select any page in memory.

Local page replacement assumes some form of memory partitioning that determines how many pages are to be assigned to a given process or a group of processes. Most popular forms of partitioning are fixed partitioning and balanced set algorithms based on the working set model. The advantage of local page replacement is its scalability: each process can handle its page faults independently without contending for some shared global data structure.

Precleaning

Most replacement algorithms simply return the target page as their result. This means that if target page is dirty (that is, contains data that have to be written to the stable storage before page can be reclaimed), I/O has to be initiated to send that page to the stable storage (to clean the page). In the early days of virtual memory, time spent on cleaning was not of much concern, because virtual memory was first implemented on systems with full duplex channels to the stable storage, and cleaning was customarily overlapped with paging. Contemporary commodity hardware, on the other hand, does not support full duplex transfers, and cleaning of target pages becomes an issue.

To deal with this situation, various precleaning policies are implemented. Precleaning is the mechanism that starts I/O on dirty pages that are (likely) to be replaced soon. The idea is that by the time the precleaned page is actually selected for the replacement, the I/O will complete and the page will be clean. Precleaning assumes that it is possible to identify pages that will be replaced next. Precleaning that is too eager can waste I/O bandwidth by writing pages that manage to get re-dirtied before being selected for replacement.

Anticipatory paging

Fitter (General ) Cameron Broadbent from Stevensville, spends time with hobbies including metal detection, property developers in singapore and psychology. Finds encouragement by making vacation to Tomb of Askia.

Have a look at my page; condo for Sale Some systems use demand paging -- waiting until a page is actually requested before loading it into RAM.

Other systems attempt to reduce latency by guessing which pages not in RAM are likely to be needed soon, and pre-loading such pages into RAM, before that page is requested. (This is often in combination with pre-cleaning, which guesses which pages currently in RAM are not likely to be needed soon, and pre-writing them out to storage).

When a page fault occurs, "anticipatory paging" systems will not only bring in the referenced page, but also the next few consecutive pages (analogous to a prefetch input queue in a CPU).

The swap prefetch mechanism goes even further in loading pages (even if they are not consecutive) that are likely to be needed soon.

The (h,k)-Paging problem

The (h,k)-Paging problem is a generalization of the model of paging problem: Let h,k be positive integers that hk. We measure the performance of an algorithm with cache of size hk relative to the theoretically optimal page replacement algorithm. If h<k we provide the optimal page replacement algorithm with strictly less resource.

The (h,k)-Paging problem is a way to measure how an online algorithm performs by comparing it with the performance of the optimal algorithm, specifically, separately parameterizing the cache size of the online algorithm and optimal algorithm.

Marking algorithms

Marking algorithms is a general class of paging algorithms. For each page, we associate it with a bit called its mark. Initially, we set all pages as unmarked. During a stage of page requests, we mark a page when it is first requested in this stage. A marking algorithm is such an algorithm that never pages out a marked page.

If ALG is a marking algorithm with a cache of size k, and OPT is the optimal algorithm with a cache of hk. Then ALG is kkh+1-competitive. So every marking algorithm attains the kkh+1-competitive ratio.

LRU and CLOCK are marking algorithms while FIFO is not a marking algorithm.

Conservative algorithms

An algorithm is conservative, if on any consecutive request sequence containing k or fewer distinct page references, the algorithm will incur k or fewer page faults.

If ALG is a conservative algorithm with a cache of size k, and OPT is the optimal algorithm with a cache of hk. Then ALG is kkh+1-competitive. So every conservative algorithm attains the kkh+1-competitive ratio.

LRU, FIFO and CLOCK are conservative algorithms.


Page replacement algorithms

There are a variety of page replacement algorithms [1]:

The theoretically optimal page replacement algorithm

The theoretically optimal page replacement algorithm (also known as OPT, clairvoyant replacement algorithm, or Bélády's optimal page replacement policy)[2][3][4] is an algorithm that works as follows: when a page needs to be swapped in, the operating system swaps out the page whose next use will occur farthest in the future. For example, a page that is not going to be used for the next 6 seconds will be swapped out over a page that is going to be used within the next 0.4 seconds.

This algorithm cannot be implemented in the general purpose operating system because it is impossible to compute reliably how long it will be before a page is going to be used, except when all software that will run on a system is either known beforehand and is amenable to the static analysis of its memory reference patterns, or only a class of applications allowing run-time analysis. Despite this limitation, algorithms existPotter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park. that can offer near-optimal performance — the operating system keeps track of all pages referenced by the program, and it uses those data to decide which pages to swap in and out on subsequent runs. This algorithm can offer near-optimal performance, but not on the first run of a program, and only if the program's memory reference pattern is relatively consistent each time it runs.

Analysis of the paging problem has also been done in the field of online algorithms. Efficiency of randomized online algorithms for the paging problem is measured using amortized analysis.

Not recently used

The not recently used (NRU), sometimes known as the Least Recently Used (LRU), page replacement algorithm is an algorithm that favours keeping pages in memory that have been recently used. This algorithm works on the following principle: when a page is referenced, a referenced bit is set for that page, marking it as referenced. Similarly, when a page is modified (written to), a modified bit is set. The setting of the bits is usually done by the hardware, although it is possible to do so on the software level as well.

At a certain fixed time interval, the clock interrupt triggers and clears the referenced bit of all the pages, so only pages referenced within the current clock interval are marked with a referenced bit. When a page needs to be replaced, the operating system divides the pages into four classes:

  1. not referenced, not modified
  2. not referenced, modified
  3. referenced, not modified
  4. referenced, modified

Although it does not seem possible for a page to be not referenced yet modified, this happens when a class 3 page has its referenced bit cleared by the clock interrupt. The NRU algorithm picks a random page from the lowest category for removal. Note that this algorithm implies that a modified (within clock interval) but not referenced page is less important than a not modified page that is intensely referenced.

NRU is a marking algorithm, so it is kkh+1-competitive.

First-in, first-out

The simplest page-replacement algorithm is a FIFO algorithm. The first-in, first-out (FIFO) page replacement algorithm is a low-overhead algorithm that requires little book-keeping on the part of the operating system. The idea is obvious from the name - the operating system keeps track of all the pages in memory in a queue, with the most recent arrival at the back, and the earliest arrival in front. When a page needs to be replaced, the page at the front of the queue (the oldest page) is selected. While FIFO is cheap and intuitive, it performs poorly in practical application. Thus, it is rarely used in its unmodified form. This algorithm experiences Bélády's anomaly.

FIFO page replacement algorithm is used by the VAX/VMS operating system, with some modifications.[5] Partial second chance is provided by skipping a limited number of entries with valid translation table references,[6] and additionally, pages are displaced from process working set to a systemwide pool from which they can be recovered if not already re-used.

FIFO is a conservative algorithm, so it is kkh+1-competitive.

Second-chance

A modified form of the FIFO page replacement algorithm, known as the Second-chance page replacement algorithm, fares relatively better than FIFO at little cost for the improvement. It works by looking at the front of the queue as FIFO does, but instead of immediately paging out that page, it checks to see if its referenced bit is set. If it is not set, the page is swapped out. Otherwise, the referenced bit is cleared, the page is inserted at the back of the queue (as if it were a new page) and this process is repeated. This can also be thought of as a circular queue. If all the pages have their referenced bit set, on the second encounter of the first page in the list, that page will be swapped out, as it now has its referenced bit cleared. If all the pages have their reference bit set then second chance algorithm degenerates into pure FIFO.

As its name suggests, Second-chance gives every page a "second-chance" - an old page that has been referenced is probably in use, and should not be swapped out over a new page that has not been referenced.

Clock

Clock is a more efficient version of FIFO than Second-chance because pages don't have to be constantly pushed to the back of the list, but it performs the same general function as Second-Chance. The clock algorithm keeps a circular list of pages in memory, with the "hand" (iterator) pointing to the last examined page frame in the list. When a page fault occurs and no empty frames exist, then the R (referenced) bit is inspected at the hand's location. If R is 0, the new page is put in place of the page the "hand" points to, otherwise the R bit is cleared. Then, the clock hand is incremented and the process is repeated until a page is replaced.[7]

Variants on Clock

  • GCLOCK: Generalized clock page replacement algorithm.[8]
  • Clock-Pro keeps a circular list of information about recently-referenced pages, including all M pages in memory as well as the most recent M pages that have been paged out. This extra information on paged-out pages, like the similar information maintained by ARC, helps it work better than LRU on large loops and one-time scans.[9]
  • WSclock.[10] The "aging" algorithm and the "WSClock" algorithm are probably the most important page replacement algorithms in practice.[11][12]
  • Clock with Adaptive Replacement (CAR) is a page replacement algorithm that has performance comparable to ARC, and substantially outperforms both LRU and CLOCK[13]. The algorithm CAR is self-tuning and requires no user-specified magic parameters.

CLOCK is a conservative algorithm, so it is kkh+1-competitive.

Least recently used

The least recently used page (LRU) replacement algorithm, though similar in name to NRU, differs in the fact that LRU keeps track of page usage over a short period of time, while NRU just looks at the usage in the last clock interval. LRU works on the idea that pages that have been most heavily used in the past few instructions are most likely to be used heavily in the next few instructions too. While LRU can provide near-optimal performance in theory (almost as good as Adaptive Replacement Cache), it is rather expensive to implement in practice. There are a few implementation methods for this algorithm that try to reduce the cost yet keep as much of the performance as possible.

The most expensive method is the linked list method, which uses a linked list containing all the pages in memory. At the back of this list is the least recently used page, and at the front is the most recently used page. The cost of this implementation lies in the fact that items in the list will have to be moved about every memory reference, which is a very time-consuming process.

Another method that requires hardware support is as follows: suppose the hardware has a 64-bit counter that is incremented at every instruction. Whenever a page is accessed, it gains a value equal to the counter at the time of page access. Whenever a page needs to be replaced, the operating system selects the page with the lowest counter and swaps it out. With present hardware, this is not feasible because the OS needs to examine the counter for every page in memory.

Because of implementation costs, one may consider algorithms (like those that follow) that are similar to LRU, but which offer cheaper implementations.

One important advantage of the LRU algorithm is that it is amenable to full statistical analysis. It has been proven, for example, that LRU can never result in more than N-times more page faults than OPT algorithm, where N is proportional to the number of pages in the managed pool.

On the other hand, LRU's weakness is that its performance tends to degenerate under many quite common reference patterns. For example, if there are N pages in the LRU pool, an application executing a loop over array of N + 1 pages will cause a page fault on each and every access. As loops over large arrays are common, much effort has been put into modifying LRU to work better in such situations. Many of the proposed LRU modifications try to detect looping reference patterns and to switch into suitable replacement algorithm, like Most Recently Used (MRU).

Variants on LRU

  1. LRU-K improves greatly on LRU with regard to locality in time. It's also known as LRU-2, for the case that K=2. LRU-1 (i.e. K=1) is the same as normal LRU.
  2. The ARC[14] algorithm extends LRU by maintaining a history of recently evicted pages and uses this to change preference to recent or frequent access. It is particularly resistant to sequential scans.

A comparison of ARC with other algorithms (LRU,MQ,2Q,LRU-2,LRFU,LIRS) can be found in Megiddo & Modha.[15]

LRU is a marking algorithm, so it is kkh+1-competitive.

Random

Random replacement algorithm replaces a random page in memory. This eliminates the overhead cost of tracking page references. Usually it fares better than FIFO, and for looping memory references it is better than LRU, although generally LRU performs better in practice. OS/390 uses global LRU approximation and falls back to random replacement when LRU performance degenerates, and the Intel i860 processor used a random replacement policy (Rhodehamel 1989).

Not frequently used

The not frequently used (NFU) page replacement algorithm requires a counter, and every page has one counter of its own which is initially set to 0. At each clock interval, all pages that have been referenced within that interval will have their counter incremented by 1. In effect, the counters keep track of how frequently a page has been used. Thus, the page with the lowest counter can be swapped out when necessary.

The main problem with NFU is that it keeps track of the frequency of use without regard to the time span of use. Thus, in a multi-pass compiler, pages which were heavily used during the first pass, but are not needed in the second pass will be favoured over pages which are comparably lightly used in the second pass, as they have higher frequency counters. This results in poor performance. Other common scenarios exist where NFU will perform similarly, such as an OS boot-up. Thankfully, a similar and better algorithm exists, and its description follows.

The not frequently used page-replacement algorithm generates fewer page faults than the least recently used page replacement algorithm when the page table contains null pointer values.

Aging

The aging algorithm is a descendant of the NFU algorithm, with modifications to make it aware of the time span of use. Instead of just incrementing the counters of pages referenced, putting equal emphasis on page references regardless of the time, the reference counter on a page is first shifted right (divided by 2), before adding the referenced bit to the left of that binary number. For instance, if a page has referenced bits 1,0,0,1,1,0 in the past 6 clock ticks, its referenced counter will look like this: 10000000, 01000000, 00100000, 10010000, 11001000, 01100100. Page references closer to the present time have more impact than page references long ago. This ensures that pages referenced more recently, though less frequently referenced, will have higher priority over pages more frequently referenced in the past. Thus, when a page needs to be swapped out, the page with the lowest counter will be chosen.

Note that aging differs from LRU in the sense that aging can only keep track of the references in the latest 16/32 (depending on the bit size of the processor's integers) time intervals. Consequently, two pages may have referenced counters of 00000000, even though one page was referenced 9 intervals ago and the other 1000 intervals ago. Generally speaking, knowing the usage within the past 16 intervals is sufficient for making a good decision as to which page to swap out. Thus, aging can offer near-optimal performance for a moderate price.

Techniques for hardware with no reference bit

Many of the techniques discussed above assume the presence of a reference bit associated with each page. Some hardware has no such bit, so its efficient use requires techniques that operate well without one.

One notable example is VAX hardware running OpenVMS. This system knows if a page has been modified, but not necessarily if a page has been read. Its approach is known as Secondary Page Caching. Pages removed from working sets (process-private memory, generally) are placed on special-purpose lists while remaining in physical memory for some time. Removing a page from a working set is not technically a page-replacement operation, but effectively identifies that page as a candidate. A page whose backing store is still valid (whose contents are not dirty, or otherwise do not need to be preserved) is placed on the tail of the Free Page List. A page that requires writing to backing store will be placed on the Modified Page List. These actions are typically triggered when the size of the Free Page List falls below an adjustable threshold.

Pages may be selected for working set removal in an essentially random fashion, with the expectation that if a poor choice is made, a future reference may retrieve that page from the Free or Modified list before it is removed from physical memory. A page referenced this way will be removed from the Free or Modified list and placed back into a process working set. The Modified Page List additionally provides an opportunity to write pages out to backing store in groups of more than one page, increasing efficiency. These pages can then be placed on the Free Page List. The sequence of pages that works its way to the head of the Free Page List resembles the results of a LRU or NRU mechanism and the overall effect has similarities to the Second-Chance algorithm described earlier.

Another example is used by the Linux kernel on ARM. The lack of hardware functionality is made up for by providing two page tables - the processor-native page tables, with neither referenced bits nor dirty bits, and software-maintained page tables with the required bits present. The emulated bits in the software-maintained table are set by page faults. In order to get the page faults, clearing emulated bits in the second table revokes some of the access rights to the corresponding page, which is implemented by altering the native table.

Working set

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. The working set of a process is the set of pages expected to be used by that process during some time interval.

The "working set model" isn't a page replacement algorithm in the strict sense (it's actually a kind of medium-term scheduler)Template:Clarify

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

See also

  • K. Y. Wong, "Web Cache Replacement Policies: A Pragmatic Approach" IEEE Network, vol. 20, iss. 2, Jan-Feb. 2006, pp. 28–34.
  • Aho, Denning and Ullman, Principles of Optimal Page Replacement, Journal of the ACM, Vol. 18, Issue 1,January 1971, pp 80–93
  • Rhodehamel, Michael W. "The Bus Interface and Paging Units of the i860(tm) Microprocessor". In Proc. IEEE International Conference on Computer Design, p. 380-384, 1989.
  • Tanenbaum, Andrew S. Operating Systems: Design and Implementation (Second Edition). New Jersey: Prentice-Hall 1997.
  • Tanenbaum, Andrew S. Modern Operating Systems (Second Edition). New Jersey: Prentice-Hall 2001. Online excerpt on page replacement algorithms: Page Replacement Algorithms.
  • Johnson and Shasha, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
    abstract
  • Gideon Glass and Pei Cao Adaptive-Page-Replacement-Based-on-Memory-Reference-Behavior. Also available in extended form as Technical Report 1338 at www.cs.wisc.edu
  • Jongmin Kim and others, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
    , Usenix Symposium on Operating System Design and Implementation (OSDI'2000), San Diego, CA, 17–21 October 2000
  • Sorav Bansal and Dharmendra S. Modha, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
  • Smaragdakis and others, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
  • Song Jiang and Xiaodong Zhang, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
    , SIGMETRICS 2002
  • D. Lee and others, Implementation and Performance Evaluation of the LRFU Replacement Policy, p. 0106, 23rd Euromicro Conference: New Frontiers of Information Technology-Short Contributions, 1997
  • Elizabeth J. O'Neil and others, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
    , ACM SIGMOD Conf., pp. 297–306, 1993.
  • Y. Zhou and J.F. Philbin, The Multi-Queue Replacement Algorithm for Second-Level Buffer Caches, Proc. Usenix Ann. Tech. Conf. (Usenix 2001), pp. 91–104.

ar:تبديل الصفحات es:Algoritmos de reemplazo de páginas eu:Orriak ordezkatzeko algoritmoak ko:페이지 교체 알고리즘 ja:ページ置換アルゴリズム mk:Алгоритми за преместување на мемориски страници pt:Algoritmo de troca de página simple:Page replacement algorithm tr:Sayfa yer değiştirme algoritması uk:Задача заміщення сторінок

  1. "Lecture Notes" by Douglas W. Jones 1995
  2. 2006fall:notes:lec11 [CS111]
  3. Characterization of Web reference behavior revisited: Evidence for Dichotomized Cache management
  4. 22C:116, Notes, 8 September 1995
  5. Abraham Silberschatz, Peter Baer Galvin, Greg Gagne. Operating Systems Concepts (Seventh Edition).: Wiley 2005. p. 339.
  6. VMS Help
  7. Andrew S. Tanenbaum. Modern Operating Systems (Second Edition). pp. 218 (4.4.5). 2001.
  8. Sequentiality and prefetching in database systems
  9. "CLOCK-Pro: An Effective Improvement of the CLOCK Replacement" by Song Jiang, Feng Chen, and Xiaodong Zhang, 2005
  10. "WSCLOCK—a simple and effective algorithm for virtual memory management" by Richard W. Carr and John L. Hennessy, 1981 [1] [2]
  11. "WSClock" by Allan Gottlieb
  12. "Page Replacement Algorithms" by Andrew S. Tanenbaum 2002
  13. 55 years old Systems Administrator Antony from Clarence Creek, really loves learning, PC Software and aerobics. Likes to travel and was inspired after making a journey to Historic Ensemble of the Potala Palace.

    You can view that web-site... ccleaner free download
  14. Megiddo & Modha, ARC: A Self-tuning, low overhead replacement cache
  15. Nimrod Megiddo & Dharmendra S. Modha, Is a upcoming condominium situated within the neighbourhood of Hillview The Singapore authorities has put in place a brand new subsidised pancreas transplant programme (Thinkstock picture). In the latest effort to spur funding in blighted areas of one of the city's poorest neighborhoods, Chicago plans to promote several hundred properties for the worth of a sweet bar.

    When clients ask concerning the subsequent batch of items for launch, gross sales representatives might hint that subsequent phases will supply items with poorer view or lower high quality finishings, even if they are offered at greater prices. Tempted patrons are often unaware that the value of those goodies can easily be offset by a slight drop in the property's market worth. It is due to this fact more sensible to get an immediate discount off the listing worth. Ask the gross sales consultant the estimated value of that branded equipment or furnishing package, then request for a direct deduction from the unit's asking price in lieu of the developer's goodie. December 4, 2013 by iskandarinsider November four, 2013 by iskandarinsider October 23, 2013 by iskandarinsider

    With elevated accessibility from the northern a part of Singapore via the two new MRT stations, Woodlands North MRT Station and Woodlands South MRT Station , along with the longer term North South Expressway NSE, that will link Woodlands, Sembawang and Yishun to the city, journey could be made easier. To qualify for the "Giant Lot Program," applicants should already personal property on the identical block as the lot they wish to buy; they have to also be present on property taxes, have no financial obligations to the town (like water bills or parking tickets) and should tell the city how they plan to make use of the property, DNAinfo Chicago experiences. New Condo Trilinq @ Clementi New Condominium La fiesta @ Sengkang MRT Expertise Guru Property's tantalizing new property launch! And the Web site

    The Brilliant Hill Drive condominium is an exciting upcoming development in Higher Thomson space by UVD Pte Ltd, a joint venture between Singapore Land and UOL Group Restricted. Located alongside Vibrant Hill Drive, this site is well located with close proximity to Thomson Plaza and the longer term Upper Thomson MRT. Get pleasure from a inexperienced and wholesome life-style with visits to MacRitchie Reservoir, Lower Peirce Reservoir, Bishan Park and Singapore Island Golf Course which are simply minutes' drive away

    Whereas every effort has been made to ensure that all information displayed herein are correct and full, the information are indicative somewhat than definitive. Thus its accuracy, whether or not express or implicit, isn't assured and to the fullest extent permitted by relevant legal guidelines. The Creator/Developer/Huttons Actual Estate Group does not accept duty for any errors, inaccuracies, omissions or for any loss which could result immediately or indirectly from reliance on the content herein. The Creator also reserves the right to right or replace the content at any time with out prior notification. Customers are advised to contact the advertiser for any clarifications or newest updates.

    MAS agrees that it is crucial for banks to make use of valuations which are reflective of precise property values. We anticipate banks to adopt sound valuation processes. These embrace partaking independent valuers from corporations that aren't concerned within the property transaction as sales brokers or consultants, allocating valuation assignments randomly or on a rotational foundation, obtaining multiple valuations for every property, and checking that the valuations are cheap.

    residences and penthouses. Fashionable interiors and quality designer fittings. Excellent recreational amenities. You can start by having a correct dialogue with individuals near you about investing on a family. This can normally lead you to 2 most important questions; what are the benefits of investing on a new apartment and the way you are going to find the perfect individual that can assist you in investing. The Venue Residences to Stay, Store and Dine, a luxurious mixeddevelopment by CDL in preparation for launch now. Looking for registrationof curiosity. TheMidtown is a mixed improvement near Hougang MRT. It's ex-Hougang Plazaand will comprise of residences constructed above business shops, duplexrestaurants and buying a property in singapore grocery store. VVIP Preview Quickly! Register Now. Late 2012, Probably 2013
    , IEEE Computer Magazine, pp. 58-65, April 2004.