Virtual memory compression: Difference between revisions

Revision as of 07:21, 6 January 2015

Revision as of 07:24, 6 January 2015

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===Increased thrashing===

Less obvious is the fact that the memory used by the compression system reduces the available system memory and thus causes a corresponding increase in over-alloverall paging activity. As more primary storage is used to store compressed data, less primary storage is available to programs, causing the level of paging activity to increase, reducing the effectiveness of the compression system.

Since the relationship between paging activity and available memory is exponential, any gains in available memory tend to be offset by significant increases in [[thrashing]].<ref name="DENNING"/> Again, the more compressible the data, the more pronounced the performance improvement, because less primary storage is needed to hold the compressed data.

For example, in order to maximize the use of the primary storage cache of compressed pages, [[Helix Software Company]]’s Hurricane 2.0 providedprovides a user-settableconfigurable threshold whichthat allowedallows adjustment of athe rejection level for compression to be adjusted. The program would compress the first 256 to 512 bytes of a 4K4&nbsp;KiB page and if that small region achieved the designated level of compression, the rest of the page would be compressed and then retained in a primary storage buffer, while all others would be sent to auxiliary storage through the normal paging system. The default setting for this threshold was aan 8:1 compression ratio of 8:1.<ref name="PCMAG-HURR-2"/>

===Price/performance issues===