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Documentation for /proc/sys/vm/*        kernel version 2.2.10

        (c) 1998, 1999,  Rik van Riel 

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

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

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

- min_unmapped_ratio

- min_slab_ratio

- panic_on_oom

- oom_kill_allocating_task

- mmap_min_address

- numa_zonelist_order

- nr_hugepages

- nr_overcommit_hugepages

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

dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,

dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode,

block_dump, swap_token_timeout, drop-caches,

hugepages_treat_as_movable:

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.

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

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.

Some minimal ammount of memory is needed to satisfy PF_MEMALLOC

allocations; if you set this to lower than 1024KB, your system will

become subtly broken, and prone to deadlock under high loads.

Setting this too high will OOM your machine instantly.

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

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 someone to set more or less aggressive 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

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.

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

min_unmapped_ratio:

This is available only on NUMA kernels.

A percentage of the total pages in each zone.  Zone reclaim will only

occur if more than this percentage of pages are file backed and unmapped.

This is to insure that a minimal amount of local pages is still available for

file I/O even if the node is overallocated.

The default is 1 percent.

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

min_slab_ratio:

This is available only on NUMA kernels.

A percentage of the total pages in each zone.  On Zone reclaim

(fallback from the local zone occurs) slabs will be reclaimed if more

than this percentage of pages in a zone are reclaimable slab pages.

This insures that the slab growth stays under control even in NUMA

systems that rarely perform global reclaim.

The default is 5 percent.

Note that slab reclaim is triggered in a per zone / node fashion.

The process of reclaiming slab memory is currently not node specific

and may not be fast.

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

panic_on_oom

This enables or disables panic on out-of-memory feature.

If this is set to 0, the kernel will kill some rogue process,

called oom_killer.  Usually, oom_killer can kill rogue processes and

system will survive.

If this is set to 1, the kernel panics when out-of-memory happens.

However, if a process limits using nodes by mempolicy/cpusets,

and those nodes become memory exhaustion status, one process

may be killed by oom-killer. No panic occurs in this case.

Because other nodes' memory may be free. This means system total status

may be not fatal yet.

If this is set to 2, the kernel panics compulsorily even on the

above-mentioned.

The default value is 0.

1 and 2 are for failover of clustering. Please select either

according to your policy of failover.

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

oom_kill_allocating_task

This enables or disables killing the OOM-triggering task in

out-of-memory situations.

If this is set to zero, the OOM killer will scan through the entire

tasklist and select a task based on heuristics to kill.  This normally

selects a rogue memory-hogging task that frees up a large amount of

memory when killed.

If this is set to non-zero, the OOM killer simply kills the task that

triggered the out-of-memory condition.  This avoids the expensive

tasklist scan.

If panic_on_oom is selected, it takes precedence over whatever value

is used in oom_kill_allocating_task.

The default value is 0.

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

mmap_min_addr

This file indicates the amount of address space  which a user process will

be restricted from mmaping.  Since kernel null dereference bugs could

accidentally operate based on the information in the first couple of pages

of memory userspace processes should not be allowed to write to them.  By

default this value is set to 0 and no protections will be enforced by the

security module.  Setting this value to something like 64k will allow the

vast majority of applications to work correctly and provide defense in depth

against future potential kernel bugs.

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

numa_zonelist_order

This sysctl is only for NUMA.

'where the memory is allocated from' is controlled by zonelists.

(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.

 you may be able to read ZONE_DMA as ZONE_DMA32...)

In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.

ZONE_NORMAL -> ZONE_DMA

This means that a memory allocation request for GFP_KERNEL will

get memory from ZONE_DMA only when ZONE_NORMAL is not available.

In NUMA case, you can think of following 2 types of order.

Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL

(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL

(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.

Type(A) offers the best locality for processes on Node(0), but ZONE_DMA

will be used before ZONE_NORMAL exhaustion. This increases possibility of

out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.

Type(B) cannot offer the best locality but is more robust against OOM of

the DMA zone.

Type(A) is called as "Node" order. Type (B) is "Zone" order.

"Node order" orders the zonelists by node, then by zone within each node.

Specify "[Nn]ode" for zone order

"Zone Order" orders the zonelists by zone type, then by node within each

zone.  Specify "[Zz]one"for zode order.

Specify "[Dd]efault" to request automatic configuration.  Autoconfiguration

will select "node" order in following case.

(1) if the DMA zone does not exist or

(2) if the DMA zone comprises greater than 50% of the available memory or

(3) if any node's DMA zone comprises greater than 60% of its local memory and

    the amount of local memory is big enough.

Otherwise, "zone" order will be selected. Default order is recommended unless

this is causing problems for your system/application.

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

nr_hugepages

Change the minimum size of the hugepage pool.

See Documentation/vm/hugetlbpage.txt

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

nr_overcommit_hugepages

Change the maximum size of the hugepage pool. The maximum is

nr_hugepages + nr_overcommit_hugepages.

See Documentation/vm/hugetlbpage.txt