-Documentation for /proc/sys/vm/* kernel version 2.6.29
- (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
- (c) 2008 Peter W. Morreale <pmorreale@novell.com>
+===============================
+Documentation for /proc/sys/vm/
+===============================
-For general info and legal blurb, please look in README.
+kernel version 2.6.29
-==============================================================
+Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
+
+Copyright (c) 2008 Peter W. Morreale <pmorreale@novell.com>
+
+For general info and legal blurb, please look in index.rst.
+
+------------------------------------------------------------------------------
This file contains the documentation for the sysctl files in
/proc/sys/vm and is valid for Linux kernel version 2.6.29.
- watermark_scale_factor
- zone_reclaim_mode
-==============================================================
admin_reserve_kbytes
+====================
The amount of free memory in the system that should be reserved for users
with the capability cap_sys_admin.
Changing this takes effect whenever an application requests memory.
-==============================================================
block_dump
+==========
block_dump enables block I/O debugging when set to a nonzero value. More
information on block I/O debugging is in Documentation/laptops/laptop-mode.rst.
-==============================================================
compact_memory
+==============
Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
all zones are compacted such that free memory is available in contiguous
blocks where possible. This can be important for example in the allocation of
huge pages although processes will also directly compact memory as required.
-==============================================================
compact_unevictable_allowed
+===========================
Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
allowed to examine the unevictable lru (mlocked pages) for pages to compact.
acceptable trade for large contiguous free memory. Set to 0 to prevent
compaction from moving pages that are unevictable. Default value is 1.
-==============================================================
dirty_background_bytes
+======================
Contains the amount of dirty memory at which the background kernel
flusher threads will start writeback.
-Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
-one of them may be specified at a time. When one sysctl is written it is
-immediately taken into account to evaluate the dirty memory limits and the
-other appears as 0 when read.
+Note:
+ dirty_background_bytes is the counterpart of dirty_background_ratio. Only
+ one of them may be specified at a time. When one sysctl is written it is
+ immediately taken into account to evaluate the dirty memory limits and the
+ other appears as 0 when read.
-==============================================================
dirty_background_ratio
+======================
Contains, as a percentage of total available memory that contains free pages
and reclaimable pages, the number of pages at which the background kernel
The total available memory is not equal to total system memory.
-==============================================================
dirty_bytes
+===========
Contains the amount of dirty memory at which a process generating disk writes
will itself start writeback.
value lower than this limit will be ignored and the old configuration will be
retained.
-==============================================================
dirty_expire_centisecs
+======================
This tunable is used to define when dirty data is old enough to be eligible
for writeout by the kernel flusher threads. It is expressed in 100'ths
of a second. Data which has been dirty in-memory for longer than this
interval will be written out next time a flusher thread wakes up.
-==============================================================
dirty_ratio
+===========
Contains, as a percentage of total available memory that contains free pages
and reclaimable pages, the number of pages at which a process which is
The total available memory is not equal to total system memory.
-==============================================================
dirtytime_expire_seconds
+========================
When a lazytime inode is constantly having its pages dirtied, the inode with
an updated timestamp will never get chance to be written out. And, if the
inode is old enough to be eligible for writeback by the kernel flusher threads.
And, it is also used as the interval to wakeup dirtytime_writeback thread.
-==============================================================
dirty_writeback_centisecs
+=========================
-The kernel flusher threads will periodically wake up and write `old' data
+The kernel flusher threads will periodically wake up and write `old` data
out to disk. This tunable expresses the interval between those wakeups, in
100'ths of a second.
Setting this to zero disables periodic writeback altogether.
-==============================================================
drop_caches
+===========
Writing to this will cause the kernel to drop clean caches, as well as
reclaimable slab objects like dentries and inodes. Once dropped, their
memory becomes free.
-To free pagecache:
+To free pagecache::
+
echo 1 > /proc/sys/vm/drop_caches
-To free reclaimable slab objects (includes dentries and inodes):
+
+To free reclaimable slab objects (includes dentries and inodes)::
+
echo 2 > /proc/sys/vm/drop_caches
-To free slab objects and pagecache:
+
+To free slab objects and pagecache::
+
echo 3 > /proc/sys/vm/drop_caches
This is a non-destructive operation and will not free any dirty objects.
To increase the number of objects freed by this operation, the user may run
-`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
+`sync` prior to writing to /proc/sys/vm/drop_caches. This will minimize the
number of dirty objects on the system and create more candidates to be
dropped.
use outside of a testing or debugging environment is not recommended.
You may see informational messages in your kernel log when this file is
-used:
+used::
cat (1234): drop_caches: 3
These are informational only. They do not mean that anything is wrong
with your system. To disable them, echo 4 (bit 2) into drop_caches.
-==============================================================
extfrag_threshold
+=================
This parameter affects whether the kernel will compact memory or direct
reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
The kernel will not compact memory in a zone if the
fragmentation index is <= extfrag_threshold. The default value is 500.
-==============================================================
highmem_is_dirtyable
+====================
Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
only use the low memory and they can fill it up with dirty data without
any throttling.
-==============================================================
hugetlb_shm_group
+=================
hugetlb_shm_group contains group id that is allowed to create SysV
shared memory segment using hugetlb page.
-==============================================================
laptop_mode
+===========
laptop_mode is a knob that controls "laptop mode". All the things that are
controlled by this knob are discussed in Documentation/laptops/laptop-mode.rst.
-==============================================================
legacy_va_layout
+================
If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
will use the legacy (2.4) layout for all processes.
-==============================================================
lowmem_reserve_ratio
+====================
For some specialised workloads on highmem machines it is dangerous for
the kernel to allow process memory to be allocated from the "lowmem"
can be fatal.
So the Linux page allocator has a mechanism which prevents allocations
-which _could_ use highmem from using too much lowmem. This means that
+which *could* use highmem from using too much lowmem. This means that
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.
mechanism will also defend that region from allocations which could use
highmem or lowmem).
-The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
+The `lowmem_reserve_ratio` tunable determines how aggressive the kernel is
in defending these lower zones.
If you have a machine which uses highmem or ISA DMA and your
applications are using mlock(), or if you are running with no swap then
you probably should change the lowmem_reserve_ratio setting.
-The lowmem_reserve_ratio is an array. You can see them by reading this file.
--
-% cat /proc/sys/vm/lowmem_reserve_ratio
-256 256 32
--
+The lowmem_reserve_ratio is an array. You can see them by reading this file::
+
+ % cat /proc/sys/vm/lowmem_reserve_ratio
+ 256 256 32
But, these values are not used directly. The kernel calculates # of protection
pages for each zones from them. These are shown as array of protection pages
in /proc/zoneinfo like followings. (This is an example of x86-64 box).
-Each zone has an array of protection pages like this.
-
--
-Node 0, zone DMA
- pages free 1355
- min 3
- low 3
- high 4
+Each zone has an array of protection pages like this::
+
+ Node 0, zone DMA
+ pages free 1355
+ min 3
+ low 3
+ high 4
:
:
- numa_other 0
- protection: (0, 2004, 2004, 2004)
+ numa_other 0
+ protection: (0, 2004, 2004, 2004)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- pagesets
- cpu: 0 pcp: 0
- :
--
+ pagesets
+ cpu: 0 pcp: 0
+ :
+
These protections are added to score to judge whether this zone should be used
for page allocation or should be reclaimed.
normal page requirement. If requirement is DMA zone(index=0), protection[0]
(=0) is used.
-zone[i]'s protection[j] is calculated by following expression.
+zone[i]'s protection[j] is calculated by following expression::
-(i < j):
- zone[i]->protection[j]
- = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
- / lowmem_reserve_ratio[i];
-(i = j):
- (should not be protected. = 0;
-(i > j):
- (not necessary, but looks 0)
+ (i < j):
+ zone[i]->protection[j]
+ = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
+ / lowmem_reserve_ratio[i];
+ (i = j):
+ (should not be protected. = 0;
+ (i > j):
+ (not necessary, but looks 0)
The default values of lowmem_reserve_ratio[i] are
+
+ === ====================================
256 (if zone[i] means DMA or DMA32 zone)
- 32 (others).
+ 32 (others)
+ === ====================================
+
As above expression, they are reciprocal number of ratio.
256 means 1/256. # of protection pages becomes about "0.39%" of total managed
pages of higher zones on the node.
The minimum value is 1 (1/1 -> 100%). The value less than 1 completely
disables protection of the pages.
-==============================================================
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
The default value is 65536.
-=============================================================
memory_failure_early_kill:
+==========================
Control how to kill processes when uncorrected memory error (typically
a 2bit error in a memory module) is detected in the background by hardware
Applications can override this setting individually with the PR_MCE_KILL prctl
-==============================================================
memory_failure_recovery
+=======================
Enable memory failure recovery (when supported by the platform)
0: Always panic on a memory failure.
-==============================================================
-min_free_kbytes:
+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
Setting this too high will OOM your machine instantly.
-=============================================================
-min_slab_ratio:
+min_slab_ratio
+==============
This is available only on NUMA kernels.
The process of reclaiming slab memory is currently not node specific
and may not be fast.
-=============================================================
-min_unmapped_ratio:
+min_unmapped_ratio
+==================
This is available only on NUMA kernels.
The default is 1 percent.
-==============================================================
mmap_min_addr
+=============
This file indicates the amount of address space which a user process will
be restricted from mmapping. Since kernel null dereference bugs could
vast majority of applications to work correctly and provide defense in depth
against future potential kernel bugs.
-==============================================================
-mmap_rnd_bits:
+mmap_rnd_bits
+=============
This value can be used to select the number of bits to use to
determine the random offset to the base address of vma regions
This value can be changed after boot using the
/proc/sys/vm/mmap_rnd_bits tunable
-==============================================================
-mmap_rnd_compat_bits:
+mmap_rnd_compat_bits
+====================
This value can be used to select the number of bits to use to
determine the random offset to the base address of vma regions
This value can be changed after boot using the
/proc/sys/vm/mmap_rnd_compat_bits tunable
-==============================================================
nr_hugepages
+============
Change the minimum size of the hugepage pool.
See Documentation/admin-guide/mm/hugetlbpage.rst
-==============================================================
nr_hugepages_mempolicy
+======================
Change the size of the hugepage pool at run-time on a specific
set of NUMA nodes.
See Documentation/admin-guide/mm/hugetlbpage.rst
-==============================================================
nr_overcommit_hugepages
+=======================
Change the maximum size of the hugepage pool. The maximum is
nr_hugepages + nr_overcommit_hugepages.
See Documentation/admin-guide/mm/hugetlbpage.rst
-==============================================================
nr_trim_pages
+=============
This is available only on NOMMU kernels.
See Documentation/nommu-mmap.txt for more information.
-==============================================================
numa_zonelist_order
+===================
This sysctl is only for NUMA and it is deprecated. Anything but
Node order will fail!
'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...)
+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
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
+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.
+ (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
Default order is recommended unless this is causing problems for your
system/application.
-==============================================================
oom_dump_tasks
+==============
Enables a system-wide task dump (excluding kernel threads) to be produced
when the kernel performs an OOM-killing and includes such information as
The default value is 1 (enabled).
-==============================================================
oom_kill_allocating_task
+========================
This enables or disables killing the OOM-triggering task in
out-of-memory situations.
The default value is 0.
-==============================================================
-overcommit_kbytes:
+overcommit_kbytes
+=================
When overcommit_memory is set to 2, the committed address space is not
permitted to exceed swap plus this amount of physical RAM. See below.
of them may be specified at a time. Setting one disables the other (which
then appears as 0 when read).
-==============================================================
-overcommit_memory:
+overcommit_memory
+=================
This value contains a flag that enables memory overcommitment.
See Documentation/vm/overcommit-accounting.rst and
mm/util.c::__vm_enough_memory() for more information.
-==============================================================
-overcommit_ratio:
+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
+============
page-cluster controls the number of pages up to which consecutive pages
are read in from swap in a single attempt. This is the swap counterpart
extra faults and I/O delays for following faults if they would have been part of
that consecutive pages readahead would have brought in.
-=============================================================
panic_on_oom
+============
This enables or disables panic on out-of-memory feature.
system panics.
The default value is 0.
+
1 and 2 are for failover of clustering. Please select either
according to your policy of failover.
+
panic_on_oom=2+kdump gives you very strong tool to investigate
why oom happens. You can get snapshot.
-=============================================================
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
the high water marks for each per cpu page list. If the user writes '0' to this
sysctl, it will revert to this default behavior.
-==============================================================
stat_interval
+=============
The time interval between which vm statistics are updated. The default
is 1 second.
-==============================================================
stat_refresh
+============
Any read or write (by root only) flushes all the per-cpu vm statistics
into their global totals, for more accurate reports when testing
(At time of writing, a few stats are known sometimes to be found negative,
with no ill effects: errors and warnings on these stats are suppressed.)
-==============================================================
numa_stat
+=========
This interface allows runtime configuration of numa statistics.
When page allocation performance becomes a bottleneck and you can tolerate
some possible tool breakage and decreased numa counter precision, you can
-do:
+do::
+
echo 0 > /proc/sys/vm/numa_stat
When page allocation performance is not a bottleneck and you want all
-tooling to work, you can do:
+tooling to work, you can do::
+
echo 1 > /proc/sys/vm/numa_stat
-==============================================================
swappiness
+==========
This control is used to define how aggressive the kernel will swap
memory pages. Higher values will increase aggressiveness, lower values
The default value is 60.
-==============================================================
unprivileged_userfaultfd
+========================
This flag controls whether unprivileged users can use the userfaultfd
system calls. Set this to 1 to allow unprivileged users to use the
The default value is 1.
-==============================================================
-- user_reserve_kbytes
+user_reserve_kbytes
+===================
When overcommit_memory is set to 2, "never overcommit" mode, reserve
min(3% of current process size, user_reserve_kbytes) of free memory.
Changing this takes effect whenever an application requests memory.
-==============================================================
vfs_cache_pressure
-------------------
+==================
This percentage value controls the tendency of the kernel to reclaim
the memory which is used for caching of directory and inode objects.
directory and inode objects. With vfs_cache_pressure=1000, it will look for
ten times more freeable objects than there are.
-=============================================================
-watermark_boost_factor:
+watermark_boost_factor
+======================
This factor controls the level of reclaim when memory is being fragmented.
It defines the percentage of the high watermark of a zone that will be
smaller than a pageblock then a pageblocks worth of pages will be reclaimed
(e.g. 2MB on 64-bit x86). A boost factor of 0 will disable the feature.
-=============================================================
-watermark_scale_factor:
+watermark_scale_factor
+======================
This factor controls the aggressiveness of kswapd. It defines the
amount of memory left in a node/system before kswapd is woken up and
too small for the allocation bursts occurring in the system. This knob
can then be used to tune kswapd aggressiveness accordingly.
-==============================================================
-zone_reclaim_mode:
+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
+This is value OR'ed together of
-1 = Zone reclaim on
-2 = Zone reclaim writes dirty pages out
-4 = Zone reclaim swaps pages
+= ===================================
+1 Zone reclaim on
+2 Zone reclaim writes dirty pages out
+4 Zone reclaim swaps pages
+= ===================================
zone_reclaim_mode is disabled by default. For file servers or workloads
that benefit from having their data cached, zone_reclaim_mode should be
Allowing regular swap effectively restricts allocations to the local
node unless explicitly overridden by memory policies or cpuset
configurations.
-
-============ End of Document =================================
git.samba.org / sfrench / cifs-2.6.git / blobdiff