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Page Cache System for Pc/370

IP.com Disclosure Number: IPCOM000034204D
Original Publication Date: 1989-Jan-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Erhard, JJ: AUTHOR [+2]

Abstract

The purpose of any cache is to increase computing system performance by decreasing data access time. The IBM Virtual Machine/Personal Computer (VM/PC) Page Cache employs an area of PC-AT RAM to cache I/O to the VM/PC page file. This reduces the run time of local IBM System/370 (S/370) jobs that page. The slow hard disk access time of the PC has been shown to increase job times significantly when paging increases. The page cache system described here is transparent to the S/370 operating system (and therefore to the S/370 user). No modification is needed to VM/PC Conversational Monitoring System (CMS) or the VM/PC local session application program. All of the page cache changes are in the CP/370 and control program input/output (CPIO) portions of VM/PC. The page cache system requires PC AT extended memory.

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Page Cache System for Pc/370

The purpose of any cache is to increase computing system performance by decreasing data access time. The IBM Virtual Machine/Personal Computer (VM/PC) Page Cache employs an area of PC-AT RAM to cache I/O to the VM/PC page file. This reduces the run time of local IBM System/370 (S/370) jobs that page. The slow hard disk access time of the PC has been shown to increase job times significantly when paging increases. The page cache system described here is transparent to the S/370 operating system (and therefore to the S/370 user). No modification is needed to VM/PC Conversational Monitoring System (CMS) or the VM/PC local session application program. All of the page cache changes are in the CP/370 and control program input/output (CPIO) portions of VM/PC. The page cache system requires PC AT extended memory. This memory, which resides above the 1 MB address boundary, is available as an option on the PC AT/370.

This memory is not normally used by PC-DOS applications, as it is only accessible when the 80286 processor executes in protected mode.

The page cache software uses a special ROM BIOS routine to access the extended memory. The VM/PC paging problems are caused by two factors: a small amount of 370 physical memory and a relatively slow paging disk. These two problems are compounded by the single-threaded nature of VM/PC. Each time a page fault occurs, the 370 processor has no option but to wait for its resolution. The page cache built for VM/PC is managed using a least recently used (LRU) algorithm.

Page usage is defined as moving a page to or from 370 physical storage. The cache manager keeps track of the order of page usage and attempts to keep the more recently used pages in the cache. LRU pages reside on disk. Each time VM/PC attempts to write a page to disk, or read a page from disk, the cache manager gains control. The manager immediately attempts to place the outgoing page in the page cache. If the page cache is full, the cache manager selects the LRU cache-resident page, moves it to disk, and places the outgoing page in the freed cache area. If the cache is not full, no disk write is required. In any case, the outgoing page is placed in the cache and marked as most recently used. The cache manager can sometimes take advantage of special situations that arise during...