Browse Prior Art Database

Page Replacement Algorithm

IP.com Disclosure Number: IPCOM000080685D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Hoffman, PR: AUTHOR [+2]

Abstract

In VS2/R2 (Virtual System 2/Release 2) page stealing is not normally used as a means of remedying real frame shortages. Instead, page stealing is normally only used to pare away pages which no longer belong to a user's working set. The frames that are freed in this process add to the amount of available real storage and in a main storage bound system, this should improve system throughput. This performance profit is realized, however, only if a predictable percentage of the steals are "good", in the sense that they do not result in a page fault within a predefined period after the actual steal takes place.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 62% of the total text.

Page 1 of 1

Page Replacement Algorithm

In VS2/R2 (Virtual System 2/Release 2) page stealing is not normally used as a means of remedying real frame shortages. Instead, page stealing is normally only used to pare away pages which no longer belong to a user's working set. The frames that are freed in this process add to the amount of available real storage and in a main storage bound system, this should improve system throughput. This performance profit is realized, however, only if a predictable percentage of the steals are "good", in the sense that they do not result in a page fault within a predefined period after the actual steal takes place.

To insure an acceptable percentage of "good" steals, this algorithm monitors the page referencing activity of individual users (address spaces) and selects a page to be stolen, only if it has gone unreferenced for a predefined amount of that address space's CPU execution time. This disuse criteria prevents the stealing of active working set pages that go unreferenced for periods of elapsed time, simply because the address space temporarily did not get dispatched.

This algorithm has been implemented in VS2/R2 by supporting a disuse count in each PCB (Page Control Block). Periodically, the Page Replacement algorithm examines the reference bits in all the PCB's associated with an address space. If the page has not been referenced, the disuse count is incremented, otherwise it is reset to 0. By scheduling these examinations only after the...