From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id B3B256B006A for ; Mon, 3 Aug 2009 08:02:17 -0400 (EDT) Date: Mon, 3 Aug 2009 13:21:34 +0100 (BST) From: Hugh Dickins Subject: [PATCH 11/12] ksm: add some documentation In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Izik Eidus Cc: Andrea Arcangeli , Rik van Riel , Chris Wright , Nick Piggin , Andrew Morton , Michael Kerrisk , Randy Dunlap , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: Add Documentation/vm/ksm.txt: how to use the Kernel Samepage Merging feature Signed-off-by: Hugh Dickins Cc: Michael Kerrisk Cc: Randy Dunlap --- Documentation/vm/00-INDEX | 2 Documentation/vm/ksm.txt | 89 ++++++++++++++++++++++++++++++++++++ mm/Kconfig | 1 3 files changed, 92 insertions(+) --- ksm10/Documentation/vm/00-INDEX 2009-06-10 04:05:27.000000000 +0100 +++ ksm11/Documentation/vm/00-INDEX 2009-08-02 13:50:57.000000000 +0100 @@ -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 --- ksm10/Documentation/vm/ksm.txt 1970-01-01 01:00:00.000000000 +0100 +++ ksm11/Documentation/vm/ksm.txt 2009-08-02 13:50:57.000000000 +0100 @@ -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 --- ksm10/mm/Kconfig 2009-08-01 05:02:09.000000000 +0100 +++ ksm11/mm/Kconfig 2009-08-02 13:50:57.000000000 +0100 @@ -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" -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org