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{{about|algorithms specific to paging|outline of general cache algorithms (e.g. processor, disk, database, web)|Cache algorithms}} | |||
{{Use dmy dates|date=August 2012}} | |||
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. | |||
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 the 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 [[Page replacement algorithm#Least_recently_used|LRU]] (Least Recently Used) 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 [[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. | |||
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. | |||
== 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 management (operating systems)#Partitioned allocation|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== | |||
{{see also|Paging}} | |||
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 [[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. | |||
{{confusing|date=January 2014}} | |||
==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 [[Page_replacement_algorithm#The_theoretically_optimal_page_replacement_algorithm|the theoretically optimal page replacement algorithm]]. If <math>h<k</math> 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 <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. | |||
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 <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. | |||
LRU, FIFO and CLOCK are conservative algorithms. | |||
==Page replacement algorithms== | |||
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> | |||
===The theoretically optimal page replacement algorithm=== | |||
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]<!-- 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. | |||
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 —<!-- 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. | |||
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: | |||
<br /> | |||
:3. referenced, modified | |||
:2. referenced, not modified | |||
:1. not referenced, modified | |||
:0. not referenced, not 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. So out of the above four pages, the NRU algorithm will replace the not referenced, not modified. Note that this algorithm implies that a modified but not referenced (within last clock interval) 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 oldest 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 = Bansal, Sorav and Modha, Dharmendra S. | |||
| 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 acquires the 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 the cache 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] evicts the page whose K-th most recent access is furthest in the past. For example, LRU-1 is simply LRU whereas LRU-2 evicts pages according to the time of their penultimate access. LRU-K improves greatly on LRU with regards to locality in time. | |||
# The [[Adaptive Replacement Cache|ARC]]<ref>Megiddo, Nimrod and Modha, Dharmendra S. (2003) ''[http://web.archive.org/web/20100208162647/http://www.almaden.ibm.com/cs/people/dmodha/arcfast.pdf ARC: A Self-tuning, low overhead replacement cache]''. Proceedings of FAST ’03: 2nd USENIX Conference on File and Storage Technologies</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, [[The LIRS caching algorithm|LIRS]]) can be found in Megiddo & Modha.<ref> | |||
{{cite journal|author=Megiddo, Nimrod and Modha, Dharmendra S. |url=http://dbs.uni-leipzig.de/file/ARC.pdf|title=Outperforming LRU with an Adaptive Replacement Cache Algorithm|doi=10.1109/MC.2004.1297303|year=2004|journal=Computer|volume=37|issue=4|pages=58}}</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. 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. 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 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 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 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 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 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. 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 MB}}'', [http://portal.acm.org/citation.cfm?id=170036.170081 ACM SIGMOD Conf.], pp. 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. 91–104. | |||
{{DEFAULTSORT:Page Replacement Algorithm}} | |||
[[Category:Virtual memory]] | |||
[[Category:Memory management algorithms]] | |||
[[Category:Online algorithms]] |
Latest revision as of 07:20, 23 March 2013
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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 the 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 (Least Recently Used) 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
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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.
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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 . We measure the performance of an algorithm with cache of size relative to the theoretically optimal page replacement algorithm. If 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 . Then ALG is -competitive. So every marking algorithm attains the -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 . Then ALG is -competitive. So every conservative algorithm attains the -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:
- 3. referenced, modified
- 2. referenced, not modified
- 1. not referenced, modified
- 0. not referenced, not 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. So out of the above four pages, the NRU algorithm will replace the not referenced, not modified. Note that this algorithm implies that a modified but not referenced (within last clock interval) page is less important than a not modified page that is intensely referenced.
NRU is a marking algorithm, so it is -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 oldest 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 -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 -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 acquires the 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 the cache 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
- LRU-K evicts the page whose K-th most recent access is furthest in the past. For example, LRU-1 is simply LRU whereas LRU-2 evicts pages according to the time of their penultimate access. LRU-K improves greatly on LRU with regards to locality in time.
- 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 -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
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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.
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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.
- ↑ "Lecture Notes" by Douglas W. Jones 1995
- ↑ 2006fall:notes:lec11 [CS111]
- ↑ Characterization of Web reference behavior revisited: Evidence for Dichotomized Cache management
- ↑ 22C:116, Notes, 8 September 1995
- ↑ Abraham Silberschatz, Peter Baer Galvin, Greg Gagne. Operating Systems Concepts (Seventh Edition).: Wiley 2005. p. 339.
- ↑ VMS Help
- ↑ Andrew S. Tanenbaum. Modern Operating Systems (Second Edition). pp. 218 (4.4.5). 2001.
- ↑ Sequentiality and prefetching in database systems
- ↑ "CLOCK-Pro: An Effective Improvement of the CLOCK Replacement" by Song Jiang, Feng Chen, and Xiaodong Zhang, 2005
- ↑ "WSCLOCK—a simple and effective algorithm for virtual memory management" by Richard W. Carr and John L. Hennessy, 1981 [1] [2]
- ↑ "WSClock" by Allan Gottlieb
- ↑ "Page Replacement Algorithms" by Andrew S. Tanenbaum 2002
- ↑ 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 - ↑ Megiddo, Nimrod and Modha, Dharmendra S. (2003) ARC: A Self-tuning, low overhead replacement cache. Proceedings of FAST ’03: 2nd USENIX Conference on File and Storage Technologies
- ↑
One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting
In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang
Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules
Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.
A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running
The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more
There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang