Incomputing,kernel same-page merging (KSM), also known askernel shared memory,memory merging,memory deduplication, andpage deduplication is akernel feature that makes it possible for ahypervisor system to sharememory pages that have identical contents between multipleprocesses orvirtualized guests. While not directly linked,Kernel-based Virtual Machine (KVM) can use KSM to merge memory pages occupied byvirtual machines.
KSM performs memory deduplication by scanning throughmain memory for physical pages that have identical content, and identifies the virtual pages that are mapped to those physical pages. It leaves one page unchanged, and re-maps each duplicate page to point to the same physical page, after which it releases the extra physical pages for re-use. It also marks both virtual pages as "copy-on-write" (COW), so that kernel will automatically remap a virtual page back to having its own separate physical page as soon as any process begins to write to it.[1]
KSM was originally intended to run more virtual machines on one host bysharing memory between processes as well as virtual machines. An experimental implementation of KSM by Red Hat found that 52 virtual instances of Windows XP with 1 GB of memory, could run on a host computer that had only 16 GB of RAM.[2] Upon itsimplementation, users found KSM to also be useful for non-virtualized environments in which memory is at a premium.[3][4]
KSM was merged into theLinux kernel mainline in version 2.6.32, which was released on December 3, 2009.[3][2] To be effective, the operating system kernel must find identical memory pages held by different processes. The kernel also needs to predict whether the pages are likely to update infrequently enough that the merging would be an efficient use of processor resources.[4] A concern is that although memory usage is reduced, CPU usage is increased, thus negating potential increases in performance.[1]
Security is also a concern:
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