Adaptive Replacement Cache
Encyclopedia
Adaptive Replacement Cache (ARC) is a page replacement algorithm
with
better performance than LRU
(Least Recently Used) developed at the IBM Almaden Research Center
. This is accomplished by keeping track of both Frequently Used and Recently Used pages plus a recent eviction history for both. In 2004, IBM submitted a patent application for the adaptive replacement cache policy.
ARC improves the basic LRU strategy by splitting the cache directory into two lists, T1 and T2, for recently and frequently referenced entries. In turn, each of these is extended with a ghost list (B1 or B2), which is attached to the bottom of the two lists. These ghost lists act as scorecards by keeping track of the history of recently evicted cache entries, and the algorithm uses ghost hits to adapt to recent change in resource usage. Note that the ghost lists only contain metadata (keys for the entries) and not the resource data itself, i.e. as an entry is evicted into a ghost list its data is discarded. The combined cache directory is organised in four LRU lists:
T1 and B1 together are referred to as L1, a combined history of recent single references.
Similarly, L2 is the combination of T2 and B2.
The whole cache directory can be visualised in a single line:
. . . [ B1 <-[ T1 <-!-> T2 ]-> B2 ] . .
[ . . . . [ . . . . . . ! . .^. . . . ] . . . . ]
[ fixed cache size (c) ]
The inner [ ] brackets indicate actual cache, which although fixed in size, can move freely across the B1 and B2 history.
L1 is now displayed from right to left, starting at the top, indicated by the ! marker. ^ indicates the target size for T1, and may be equal to, smaller than, or larger than the actual size (as indicated by !).
Sun Microsystems
's scalable file system ZFS
uses a variant of ARC as an alternative to the traditional Solaris
filesystem page cache in virtual memory. It has been modified to allow for locked pages that are currently in use and cannot be vacated.
PostgreSQL
used ARC in its buffer manager for a brief time (version 8.0.0), but quickly replaced it with another algorithm,
citing concerns over an IBM patent on ARC.
Page replacement algorithm
In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out when a page of memory needs to be allocated...
with
better performance than LRU
Cache algorithms
In computing, cache algorithms are optimizing instructions – algorithms – that a computer program or a hardware-maintained structure can follow to manage a cache of information stored on the computer...
(Least Recently Used) developed at the IBM Almaden Research Center
Almaden Research Center
The IBM Almaden Research Center is in San Jose, California, and is one of IBM's nine worldwide research labs. Its scientists perform basic and applied research in computer science, services, storage systems, physical sciences, and materials science and technology. The center opened in 1986, and...
. This is accomplished by keeping track of both Frequently Used and Recently Used pages plus a recent eviction history for both. In 2004, IBM submitted a patent application for the adaptive replacement cache policy.
Summary
Basic LRU maintains an ordered list (the cache directory) of resource entries in the cache, with the sort order based on the time of most recent access. New entries are added at the top of the list, after the bottom entry has been evicted. Cache hits move to the top, pushing all other entries down.ARC improves the basic LRU strategy by splitting the cache directory into two lists, T1 and T2, for recently and frequently referenced entries. In turn, each of these is extended with a ghost list (B1 or B2), which is attached to the bottom of the two lists. These ghost lists act as scorecards by keeping track of the history of recently evicted cache entries, and the algorithm uses ghost hits to adapt to recent change in resource usage. Note that the ghost lists only contain metadata (keys for the entries) and not the resource data itself, i.e. as an entry is evicted into a ghost list its data is discarded. The combined cache directory is organised in four LRU lists:
- T1, for recent cache entries.
- T2, for frequent entries, referenced at least twice.
- B1, ghost entries recently evicted from the T1 cache, but are still tracked.
- B2, similar ghost entries, but evicted from T2.
T1 and B1 together are referred to as L1, a combined history of recent single references.
Similarly, L2 is the combination of T2 and B2.
The whole cache directory can be visualised in a single line:
. . . [ B1 <-[ T1 <-!-> T2 ]-> B2 ] . .
[ . . . . [ . . . . . . ! . .^. . . . ] . . . . ]
[ fixed cache size (c) ]
The inner [ ] brackets indicate actual cache, which although fixed in size, can move freely across the B1 and B2 history.
L1 is now displayed from right to left, starting at the top, indicated by the ! marker. ^ indicates the target size for T1, and may be equal to, smaller than, or larger than the actual size (as indicated by !).
- New entries enter T1, to the left of !, and are gradually pushed to the left, eventually being evicted from T1 into B1, and finally dropped out altogether.
- Any entry in L1 that gets referenced once more, gets another chance, and enters L2, just to the right of the central ! marker. From there, it is again pushed outward, from T2 into B2. Entries in L2 that get another hit can repeat this indefinitely, until they finally drop out on the far right of B2.
Replacement
Entries (re-)entering the cache (T1,T2) will cause ! to move towards the target marker ^. If no free space exists in the cache, this marker also determines whether either T1 or T2 will evict an entry.- Hits in B1 will increase the size of T1, pushing ^ to the right. The last entry in T2 is evicted into B2.
- Hits in B2 will shrink T1, pushing ^ back to the left. The last entry in T1 is now evicted into B1.
- A cache miss will not affect ^, but the ! boundary will move closer to ^.
Deployment
ARC is currently deployed in IBM's DS6000/DS8000 storage controllers.Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
's scalable file system ZFS
ZFS
In computing, ZFS is a combined file system and logical volume manager designed by Sun Microsystems. The features of ZFS include data integrity verification against data corruption modes , support for high storage capacities, integration of the concepts of filesystem and volume management,...
uses a variant of ARC as an alternative to the traditional Solaris
Solaris Operating System
Solaris is a Unix operating system originally developed by Sun Microsystems. It superseded their earlier SunOS in 1993. Oracle Solaris, as it is now known, has been owned by Oracle Corporation since Oracle's acquisition of Sun in January 2010....
filesystem page cache in virtual memory. It has been modified to allow for locked pages that are currently in use and cannot be vacated.
PostgreSQL
PostgreSQL
PostgreSQL, often simply Postgres, is an object-relational database management system available for many platforms including Linux, FreeBSD, Solaris, MS Windows and Mac OS X. It is released under the PostgreSQL License, which is an MIT-style license, and is thus free and open source software...
used ARC in its buffer manager for a brief time (version 8.0.0), but quickly replaced it with another algorithm,
citing concerns over an IBM patent on ARC.