From: Izik Eidus Date: Tue, 22 Sep 2009 00:02:03 +0000 (-0700) Subject: ksm: Kernel SamePage Merging X-Git-Tag: v2.6.32-rc1~556 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=31dbd01f314364b70c2e026a5793a29a4da8a9dc;p=pandora-kernel.git ksm: Kernel SamePage Merging Ksm is code that allows merging of identical pages between one or more applications, in a way invisible to the applications that use it. Pages that are merged are marked as read-only, then COWed when any application tries to change them. Whereas fork() allows sharing anonymous pages between parent and child, ksm can share anonymous pages between unrelated processes. Ksm works by walking over the memory pages of the applications it scans, in order to find identical pages. It uses two sorted data structures, called the stable and unstable trees, to locate identical pages in an effective way. When ksm finds two identical pages, it marks them as readonly and merges them into a single page. After the pages have been marked as readonly and merged into one, Linux treats them as normal copy-on-write pages, copying to a fresh anonymous page if write access is required later. Ksm scans and merges anonymous pages only in those memory areas that have been registered with it by madvise(addr, length, MADV_MERGEABLE). The ksm scanner is controlled by sysfs files in /sys/kernel/mm/ksm/: max_kernel_pages - the maximum number of unswappable kernel pages which may be allocated by ksm (0 for unlimited). kernel_pages_allocated - how many ksm pages are currently allocated, sharing identical content between different processes (pages unswappable in this release). pages_shared - how many pages have been saved by sharing with ksm pages (kernel_pages_allocated being excluded from this count). pages_to_scan - how many pages ksm should scan before sleeping. sleep_millisecs - how many milliseconds ksm should sleep between scans. run - write 0 to disable ksm, read 0 while ksm is disabled (default), write 1 to run ksm, read 1 while ksm is running, write 2 to disable ksm and unmerge all its pages. Includes contributions by Andrea Arcangeli Chris Wright and Hugh Dickins. [hugh.dickins@tiscali.co.uk: fix rare page leak] Signed-off-by: Izik Eidus Signed-off-by: Hugh Dickins Signed-off-by: Chris Wright Cc: Andrea Arcangeli Cc: Rik van Riel Cc: Wu Fengguang Cc: Balbir Singh Cc: Hugh Dickins Cc: KAMEZAWA Hiroyuki Cc: Lee Schermerhorn Cc: Avi Kivity Cc: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed