KSM

Kernel Samepage Merging

KSM is a memory-saving de-duplication feature, that merges anonymous (private) pages (not pagecache ones). Although it started this way, KSM is currently not suitable only for Virtual Machine use, but rather, it can be useful to any application which generates many instances of the same data

Enabling KSM

KSM only operates on those areas of address space which an application has advised to be likely candidates for merging, by using the madvise(2) system call: int madvise(addr, length, MADV_MERGEABLE).

The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel that advice and restore unshared pages: whereupon KSM unmerges whatever it merged in that range. Note: this unmerging call may suddenly require more memory than is available - possibly failing with EAGAIN, but more probably arousing the Out-Of-Memory killer.

Status

Merged in linux 2.6.32. One needs to enable it via CONFIG_KSM=y

Results

TBD

Testing

You can verify KSM is in action, by checking for the existence of some of its /sys files, under /sys/kernel/mm/ksm/

They are:

pages_shared	how many shared pages are being used
pages_sharing	how many more sites are sharing them i.e. how much saved
pages_unshared	how many pages unique but repeatedly checked for merging
pages_volatile	how many pages changing too fast to be placed in a tree
full_scans	how many times all mergeable areas have been scanned