ksm: add some documentation

	- 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

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

	  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"