The patch titled ksm: add some documentation has been added to the -mm tree. Its filename is ksm-add-some-documentation.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: ksm: add some documentation From: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx> Add Documentation/vm/ksm.txt: how to use the Kernel Samepage Merging feature Signed-off-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxxxxxxx> Cc: Randy Dunlap <randy.dunlap@xxxxxxxxxx> Acked-by: Izik Eidus <ieidus@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/vm/00-INDEX | 2 Documentation/vm/ksm.txt | 89 ++++++++++++++++++++++++++++++++++++ mm/Kconfig | 1 3 files changed, 92 insertions(+) diff -puN Documentation/vm/00-INDEX~ksm-add-some-documentation Documentation/vm/00-INDEX --- a/Documentation/vm/00-INDEX~ksm-add-some-documentation +++ a/Documentation/vm/00-INDEX @@ -6,6 +6,8 @@ balance - various information on memory balancing. hugetlbpage.txt - a brief summary of hugetlbpage support in the Linux kernel. +ksm.txt + - how to use the Kernel Samepage Merging feature. locking - info on how locking and synchronization is done in the Linux vm code. numa diff -puN /dev/null Documentation/vm/ksm.txt --- /dev/null +++ a/Documentation/vm/ksm.txt @@ -0,0 +1,89 @@ +How to use the Kernel Samepage Merging feature +---------------------------------------------- + +KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, +added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation, +and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/ + +The KSM daemon ksmd periodically scans those areas of user memory which +have been registered with it, looking for pages of identical content which +can be replaced by a single write-protected page (which is automatically +copied if a process later wants to update its content). + +KSM was originally developed for use with KVM (where it was known as +Kernel Shared Memory), to fit more virtual machines into physical memory, +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 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. + +If KSM is not configured into the running kernel, madvise MADV_MERGEABLE +and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was +built with CONFIG_KSM=y, those calls will normally succeed: even if the +the KSM daemon is not currently running, MADV_MERGEABLE still registers +the range for whenever the KSM daemon is started; even if the range +cannot contain any pages which KSM could actually merge; even if +MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. + +Like other madvise calls, they are intended for use on mapped areas of +the user address space: they will report ENOMEM if the specified range +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. + +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 2000 > /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: 2000 (chosen for demonstration purposes) + +pages_to_scan - how many present pages to scan before ksmd goes to sleep + e.g. "echo 200 > /sys/kernel/mm/ksm/pages_to_scan" + Default: 200 (chosen for demonstration purposes) + +sleep_millisecs - how many milliseconds ksmd should sleep before next scan + e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs" + Default: 20 (chosen for demonstration purposes) + +run - set 0 to stop ksmd from running but keep merged pages, + set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run", + set 2 to stop ksmd and unmerge all pages currently merged, + but leave mergeable areas registered for next run + Default: 1 (for immediate use by apps which register) + +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_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 + +A high ratio of pages_sharing to pages_shared indicates good sharing, but +a high ratio of pages_unshared to pages_sharing indicates wasted effort. +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, 30 July 2009 diff -puN mm/Kconfig~ksm-add-some-documentation mm/Kconfig --- a/mm/Kconfig~ksm-add-some-documentation +++ a/mm/Kconfig @@ -224,6 +224,7 @@ config KSM the many instances by a single resident page with that content, so saving memory until one or another app needs to modify the content. Recommended for use with KVM, or with other duplicative applications. + See Documentation/vm/ksm.txt for more information. config DEFAULT_MMAP_MIN_ADDR int "Low address space to protect from user allocation" _ Patches currently in -mm which might be from hugh.dickins@xxxxxxxxxxxxx are acpi-fix-null-bug-for-hid-uid-string.patch mm-oom-analysis-add-shmem-vmstat.patch ksm-add-mmu_notifier-set_pte_at_notify.patch ksm-first-tidy-up-madvise_vma.patch ksm-define-madv_mergeable-and-madv_unmergeable.patch ksm-the-mm-interface-to-ksm.patch ksm-no-debug-in-page_dup_rmap.patch ksm-identify-pageksm-pages.patch ksm-kernel-samepage-merging.patch ksm-prevent-mremap-move-poisoning.patch ksm-change-copyright-message.patch ksm-change-ksm-nice-level-to-be-5.patch ksm-rename-kernel_pages_allocated.patch ksm-move-pages_sharing-updates.patch ksm-pages_unshared-and-pages_volatile.patch ksm-break-cow-once-unshared.patch ksm-keep-quiet-while-list-empty.patch ksm-five-little-cleanups.patch ksm-fix-endless-loop-on-oom.patch ksm-distribute-remove_mm_from_lists.patch ksm-fix-oom-deadlock.patch ksm-sysfs-and-defaults.patch ksm-add-some-documentation.patch ksm-remove-vm_mergeable_flags.patch getrusage-fill-ru_maxrss-value.patch getrusage-fill-ru_maxrss-value-update.patch ramfs-move-ramfs_magic-to-include-linux-magich.patch memory-controller-soft-limit-organize-cgroups-v9-fix.patch prio_tree-debugging-patch.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html