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.

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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:

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

The user can override this setting. 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.

It may be beneficial to switch this on if one wants to do zone
reclaim regardless of the numa distances in 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.