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Swap-Out Candidate Selection for Storage Management

IP.com Disclosure Number: IPCOM000060061D
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Childs, AD: AUTHOR [+4]

Abstract

All implemented storage management strategies are approximations of the optimum strategy: to release those pages whose use will be requested furthest in the future. The optimum has never been implemented because systems are unable to precisely determine future events. Implemented strategies include random page selection, first-in/first-out, least recently used, least frequently used, not recently used, and combinations and permutations of these. They generally involve some evaluation of past events to predict those of the future. The history evaluation is driven by the specific swapping objectives defined by the system environment. Items selected for swap out are pages within a virtual area. A virtual area is a program or a virtual work space.

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Swap-Out Candidate Selection for Storage Management

All implemented storage management strategies are approximations of the optimum strategy: to release those pages whose use will be requested furthest in the future. The optimum has never been implemented because systems are unable to precisely determine future events. Implemented strategies include random page selection, first-in/first-out, least recently used, least frequently used, not recently used, and combinations and permutations of these. They generally involve some evaluation of past events to predict those of the future. The history evaluation is driven by the specific swapping objectives defined by the system environment. Items selected for swap out are pages within a virtual area. A virtual area is a program or a virtual work space. Each virtual area has a priority associated with it, which will be called virtual-area priority (VAP). The swap-out candidate (SOC) selection algorithm is driven by the relationship between the VAP and the swap-in task's priority (SIP). Calculation of the VAP is based on whether the virtual area is shared by multiple tasks or used by only one task. If a virtual area is used by only one task, then its VAP is equal to the priority of the task which is using it. If the virtual area is shared, then its priority is equal to a base value plus its demand over time. The demand for an area increases as requests for the area are made, and decreases on a fixed periodic basis. The SOC selection algorithm defines four preference classes for potential SOCs. 1. An area whose use is not expected for a long time, due to waiting for display- station input. The operating system can identify that a task is waiting for display- station input. The virtual areas associated with the waiting task are marked to be taken first, because they will not be needed until a display-station operator requests system service. Given a choice of several areas waiting for display station i...