Virtual memory compression: Difference between revisions

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'''Virtual memory compression''' is a [[memory management]] technique that utilizes [[data compression]] to reduce the size or number of [[paging]] requests to/from [[auxiliary memory]]. Virtual memory compression is distinct from [[garbage collection]] systems which remove unused memory blocks and in some cases consolidate used memory regions and reduce fragmentation for efficiency. Virtual memory compression is also distinct from [[context switching]] systems, such as Connectix’s RAM Doubler and Apple OS 7.1, in which inactive processes are suspended and then compressed.<ref name="PAT-5559978"/><ref name="PAT-5785474"/>

In a virtual memory compression system, paging requests are compressed and stored in [[primary storage]] (usually [[RAM]]) or sent as compressed to auxiliary storage. In both cases the original memory is marked inaccessibleaccessible. The memory footprint of the memory being paged is reduced by the compression process; and, In the first instance, the freed memory is returned to the general memory pool, while the compressed portion is kept in RAM; in the second, the compressed data is sent to auxiliary storage but the resulting I/O operation is smaller and thus takes less time. Attempts to access a compressed page result in a reversal of the process—the compressed data is optionally retrieved from auxiliary storage, and then decompressed.

In some implementations such as in [[zswap]], [[zram]] and [[Helix Software Company]]’s Hurricane, the entire process is managed by the operating system. In other systems such as IBM’s MXT, the compression process occurs in a dedicated processor which handles transfers between a local cache and primary storage.