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Modified Data Tracking to Reduce Unnecessary Data Movement

IP.com Disclosure Number: IPCOM000044403D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Dutton, PF: AUTHOR

Abstract

One technique for modifying data in a storage unit that contains a cache is to write to the page that is contained in the cache and then turn on a status bit that marks this page as modified. This method of storing data is faster than the store-through method, that not only alters the page of data in the cache, but also alters the master page that is in the basic storage module (BSM). However, one disadvantage of this approach is that eventually, when a cache search produces a miss, the page requested is in the BSM and not the cache. In that event, the cache least recently used (LRU) algorithm will sometimes direct that the page requested be fetched from the BSM and put into a location of the cache that contains modified data.

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Modified Data Tracking to Reduce Unnecessary Data Movement

One technique for modifying data in a storage unit that contains a cache is to write to the page that is contained in the cache and then turn on a status bit that marks this page as modified. This method of storing data is faster than the store- through method, that not only alters the page of data in the cache, but also alters the master page that is in the basic storage module (BSM). However, one disadvantage of this approach is that eventually, when a cache search produces a miss, the page requested is in the BSM and not the cache. In that event, the cache least recently used (LRU) algorithm will sometimes direct that the page requested be fetched from the BSM and put into a location of the cache that contains modified data. Before the requested page, page "A", is moved into the cache, the modified page, page "B", that is presently contained therein, must first be stored in some temporary storage area. Then, page "A" can be stored in the cache and page "B" can be moved into the BSM. This movement of data, called an outpage, takes a considerable amount of time. It is not uncommon that when a cache search produces a miss, the LRU will point to a modified page 20% to 25% of the time. When an outpage takes place, all of the modified page, page "B", is removed from the cache, X bytes of data per machine cycle. The data width X, a subpage, is determined by the dataflow width of the machine in question. It is not uncommon to move 4, 8 or 16 bytes per machine cycle. The size of a cache page will also vary with different...