linux/Documentation/sysctl/vm.txt

Documentation for /proc/sys/vm/*	kernel version 2.2.10
	(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

For general info and legal blurb, please look in README.

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This file contains the documentation for the sysctl files in
/proc/sys/vm and is valid for Linux kernel version 2.2.

The files in this directory can be used to tune the operation
of the virtual memory (VM) subsystem of the Linux kernel and
the writeout of dirty data to disk.

Default values and initialization routines for most of these
files can be found in mm/swap.c.

Currently, these files are in /proc/sys/vm:
- overcommit_memory
- page-cluster
- dirty_ratio
- dirty_background_ratio
- dirty_expire_centisecs
- dirty_writeback_centisecs
- max_map_count
- min_free_kbytes
- laptop_mode
- block_dump
- drop-caches
- zone_reclaim_mode
- zone_reclaim_interval

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dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode,
block_dump, swap_token_timeout, drop-caches:

See Documentation/filesystems/proc.txt

==============================================================

overcommit_memory:

This value contains a flag that enables memory overcommitment.

When this flag is 0, the kernel attempts to estimate the amount
of free memory left when userspace requests more memory.

When this flag is 1, the kernel pretends there is always enough
memory until it actually runs out.

When this flag is 2, the kernel uses a "never overcommit"
policy that attempts to prevent any overcommit of memory.  

This feature can be very useful because there are a lot of
programs that malloc() huge amounts of memory "just-in-case"
and don't use much of it.

The default value is 0.

See Documentation/vm/overcommit-accounting and
security/commoncap.c::cap_vm_enough_memory() for more information.

==============================================================

overcommit_ratio:

When overcommit_memory is set to 2, the committed address
space is not permitted to exceed swap plus this percentage
of physical RAM.  See above.

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page-cluster:

The Linux VM subsystem avoids excessive disk seeks by reading
multiple pages on a page fault. The number of pages it reads
is dependent on the amount of memory in your machine.

The number of pages the kernel reads in at once is equal to
2 ^ page-cluster. Values above 2 ^ 5 don't make much sense
for swap because we only cluster swap data in 32-page groups.

==============================================================

max_map_count:

This file contains the maximum number of memory map areas a process
may have. Memory map areas are used as a side-effect of calling
malloc, directly by mmap and mprotect, and also when loading shared
libraries.

While most applications need less than a thousand maps, certain
programs, particularly malloc debuggers, may consume lots of them,
e.g., up to one or two maps per allocation.

The default value is 65536.

==============================================================

min_free_kbytes:

This is used to force the Linux VM to keep a minimum number 
of kilobytes free.  The VM uses this number to compute a pages_min
value for each lowmem zone in the system.  Each lowmem zone gets 
a number of reserved free pages based proportionally on its size.

==============================================================

percpu_pagelist_fraction

This is the fraction of pages at most (high mark pcp->high) in each zone that
are allocated for each per cpu page list.  The min value for this is 8.  It
means that we don't allow more than 1/8th of pages in each zone to be
allocated in any single per_cpu_pagelist.  This entry only changes the value
of hot per cpu pagelists.  User can specify a number like 100 to allocate
1/100th of each zone to each per cpu page list.

The batch value of each per cpu pagelist is also updated as a result.  It is
set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)

The initial value is zero.  Kernel does not use this value at boot time to set
the high water marks for each per cpu page list.

===============================================================

zone_reclaim_mode:

Zone_reclaim_mode allows to set more or less agressive approaches to
reclaim memory when a zone runs out of memory. If it is set to zero then no
zone reclaim occurs. Allocations will be satisfied from other zones / nodes
in the system.

This is value ORed together of

1	= Zone reclaim on
2	= Zone reclaim writes dirty pages out
4	= Zone reclaim swaps pages
8	= Also do a global slab reclaim pass

zone_reclaim_mode is set during bootup to 1 if it is determined that pages
from remote zones will cause a measurable performance reduction. The
page allocator will then reclaim easily reusable pages (those page
cache pages that are currently not used) before allocating off node pages.

It may be beneficial to switch off zone reclaim if the system is
used for a file server and all of memory should be used for caching files
from disk. In that case the caching effect is more important than
data locality.

Allowing zone reclaim to write out pages stops processes that are
writing large amounts of data from dirtying pages on other nodes. Zone
reclaim will write out dirty pages if a zone fills up and so effectively
throttle the process. This may decrease the performance of a single process
since it cannot use all of system memory to buffer the outgoing writes
anymore but it preserve the memory on other nodes so that the performance
of other processes running on other nodes will not be affected.

Allowing regular swap effectively restricts allocations to the local
node unless explicitly overridden by memory policies or cpuset
configurations.

It may be advisable to allow slab reclaim if the system makes heavy
use of files and builds up large slab caches. However, the slab
shrink operation is global, may take a long time and free slabs
in all nodes of the system.

================================================================

zone_reclaim_interval:

The time allowed for off node allocations after zone reclaim
has failed to reclaim enough pages to allow a local allocation.

Time is set in seconds and set by default to 30 seconds.

Reduce the interval if undesired off node allocations occur. However, too
frequent scans will have a negative impact onoff node allocation performance.