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Diffstat (limited to 'Documentation/vm/ksm.txt')
| -rw-r--r-- | Documentation/vm/ksm.txt | 22 |
1 files changed, 7 insertions, 15 deletions
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt index 262d8e6793a..b392e496f81 100644 --- a/Documentation/vm/ksm.txt +++ b/Documentation/vm/ksm.txt @@ -16,9 +16,9 @@ by sharing the data common between them. But it can be useful to any application which generates many instances of the same data. KSM only merges anonymous (private) pages, never pagecache (file) pages. -KSM's merged pages are at present locked into kernel memory for as long -as they are shared: so cannot be swapped out like the user pages they -replace (but swapping KSM pages should follow soon in a later release). +KSM's merged pages were originally locked into kernel memory, but can now +be swapped out just like other user pages (but sharing is broken when they +are swapped back in: ksmd must rediscover their identity and merge again). 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) @@ -44,20 +44,12 @@ includes unmapped gaps (though working on the intervening mapped areas), and might fail with EAGAIN if not enough memory for internal structures. Applications should be considerate in their use of MADV_MERGEABLE, -restricting its use to areas likely to benefit. KSM's scans may use -a lot of processing power, and its kernel-resident pages are a limited -resource. Some installations will disable KSM for these reasons. +restricting its use to areas likely to benefit. KSM's scans may use a lot +of processing power: some installations will disable KSM for that reason. The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/, readable by all but writable only by root: -max_kernel_pages - set to maximum number of kernel pages that KSM may use - e.g. "echo 100000 > /sys/kernel/mm/ksm/max_kernel_pages" - Value 0 imposes no limit on the kernel pages KSM may use; - but note that any process using MADV_MERGEABLE can cause - KSM to allocate these pages, unswappable until it exits. - Default: quarter of memory (chosen to not pin too much) - pages_to_scan - how many present pages to scan before ksmd goes to sleep e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan" Default: 100 (chosen for demonstration purposes) @@ -75,7 +67,7 @@ run - set 0 to stop ksmd from running but keep merged pages, The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/: -pages_shared - how many shared unswappable kernel pages KSM is using +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 @@ -87,4 +79,4 @@ pages_volatile embraces several different kinds of activity, but a high proportion there would also indicate poor use of madvise MADV_MERGEABLE. Izik Eidus, -Hugh Dickins, 24 Sept 2009 +Hugh Dickins, 17 Nov 2009 |