p87-carr - WSCLoca - A Simple and Effective Algorithm for...

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WSCLoca - A Simple and Effective Algorithm for Virtual Memory Management Richard W. Cart 1 Department of Computer Science Stanford University John L. Hennessy Computer Systems Laboratory Stanford University Abstract A new virtual memory management algorithm WSCLOCK has been synthesized from the local working set (WS) algorithm, the global CLOCK algorithm, and a new load control mechanism for auxiliary memory access. The new algorithm combines the most useful feature of WS-a natural and efti:ctive load control that prevents thrashing-with the simplicity and efficiency of CLOCK. Studies are presented to show that the performance of WS and WSCLOCK are equivalent, even if the savings in overhead are ignored. Introduction Modern memory management policies optimize performance by varying the space allocated to each task as its perceived need changes. Such policies also vary the load (i.e., the number of active tasks) to achieve high levels of multiprogramming while avoiding thrashing. Modern va,'iable-space, variable-load memory management policies have been divided into local policies and global ptflicies. Ideally. a local policy estimates the memory needs, or locality, of each task independently of other tasks and allocates sufficient main memory to hold the each active task's locality. A global policy correlates a task's memory allocation with its locality, but makes no explicit, independent measure of the locality, and does not necessarily allocate sufficient main memory for each active task's locality. This work was supported by the Departanent of Energy, Contract DE-AC03-76SF00515. IAuthor's current address: Tandem Computers Inc., 19333 Vallco Parkway, Cupertino, CA 95014 Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. Local policies are typified by the working set (WS) policy which was first defined by Denning [DI~N~68] and has been the object of much study [DENN72, RODR73, PRIE73, SMIT76, MARS79]. Global policies are typified by the global least-recently-used (LRU) approximation algorithm CLOCK that is used in MULTICS; studies of CLOCK have appeared infrequently [CORB68, EAST76]. Although a local policy, such as WS, isolates tasks from each other and may be better at preventing thrashing, a global policy is often used in real systems because it is simpler to implement and has less computational overhead. This paper presents a new policy WSCLOCK that combines the operational advantages of WS with the simplicity and efficiency of CLOCK.
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This note was uploaded on 11/02/2010 for the course CS 3224 taught by Professor Johnsterling during the Spring '10 term at NYU Poly.

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p87-carr - WSCLoca - A Simple and Effective Algorithm for...

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