'''Virtual memory compression''' (also referred to as '''RAM compression''' and '''memory compression''') is a [[memory management]] technique that utilizes [[data compression]] to reduce the size or number of [[paging]] requests to and from the [[auxiliary storage]].<ref name ="CaseForCompressedCaching"/> In a virtual memory compression system, pagingpages requeststo be paged out of virtual memory are compressed and stored in [[physical memory]], which is usually [[random-access memory]] (RAM), or sent as compressed to auxiliary storage such as a [[hard disk drive]] (HDD) or [[solid-state drive]] (SSD). In both cases the [[virtual memory]] range, whose contents has been compressed during the paging request, is marked inaccessible so that attempts to access compressed pages can trigger [[page fault]]s and reversal of the process (retrieval from auxiliary storage and decompression). The footprint of the data being paged is reduced by the compression process; in the first instance, the freed RAM is returned to the available physical memory pool, while the compressed portion is kept in RAM. In the second instance, the compressed data is sent to auxiliary storage but the resulting I/O operation is smaller and therefore takes less time.<ref name="PAT-5559978"/><ref name="PAT-5785474"/>
In some implementations, including [[zswap]], [[zram]] and [[Helix Software Company]]’s [[Helix Hurricane|Hurricane]], the entire process is implemented in software. In other systems, such as IBM's MXT, the compression process occurs in a dedicated processor that handles transfers between a local [[Cache (computing)|cache]] and RAM.
