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|
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>
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 2.6.29.
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:
- admin_reserve_kbytes
- block_dump
- compact_memory
- compact_unevictable_allowed
- dirty_background_bytes
- dirty_background_ratio
- dirty_bytes
- dirty_expire_centisecs
- dirty_ratio
- dirty_writeback_centisecs
- drop_caches
- extfrag_threshold
- hugepages_treat_as_movable
- hugetlb_shm_group
- laptop_mode
- legacy_va_layout
- lowmem_reserve_ratio
- max_map_count
- memory_failure_early_kill
- memory_failure_recovery
- min_free_kbytes
- min_slab_ratio
- min_unmapped_ratio
- mmap_min_addr
- mmap_rnd_bits
- mmap_rnd_compat_bits
- nr_hugepages
- nr_overcommit_hugepages
- nr_trim_pages (only if CONFIG_MMU=n)
- numa_zonelist_order
- oom_dump_tasks
- oom_kill_allocating_task
- overcommit_kbytes
- overcommit_memory
- overcommit_ratio
- page-cluster
- panic_on_oom
- percpu_pagelist_fraction
- stat_interval
- stat_refresh
- numa_stat
- swappiness
- user_reserve_kbytes
- vfs_cache_pressure
- 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.
admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
That should provide enough for the admin to log in and kill a process,
if necessary, under the default overcommit 'guess' mode.
Systems running under overcommit 'never' should increase this to account
for the full Virtual Memory Size of programs used to recover. Otherwise,
root may not be able to log in to recover the system.
How do you calculate a minimum useful reserve?
sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
For overcommit 'guess', we can sum resident set sizes (RSS).
On x86_64 this is about 8MB.
For overcommit 'never', we can take the max of their virtual sizes (VSZ)
and add the sum of their RSS.
On x86_64 this is about 128MB.
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.txt.
==============================================================
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.
This should be used on systems where stalls for minor page faults are an
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.
==============================================================
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
flusher threads will start writing out dirty data.
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.
Note: dirty_bytes is the counterpart of dirty_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: the minimum value allowed for dirty_bytes is two pages (in bytes); any
value lower than this limit will be ignored and the old configuration will be
retained.
Note: the value of dirty_bytes also must be set greater than
dirty_background_bytes or the amount of memory corresponding to
dirty_background_ratio.
==============================================================
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
generating disk writes will itself start writing out dirty data.
The total available memory is not equal to total system memory.
Note: dirty_ratio must be set greater than dirty_background_ratio or
ratio corresponding to dirty_background_bytes.
==============================================================
dirty_writeback_centisecs
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:
echo 1 > /proc/sys/vm/drop_caches
To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
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
number of dirty objects on the system and create more candidates to be
dropped.
This file is not a means to control the growth of the various kernel caches
(inodes, dentries, pagecache, etc...) These objects are automatically
reclaimed by the kernel when memory is needed elsewhere on the system.
Use of this file can cause performance problems. Since it discards cached
objects, it may cost a significant amount of I/O and CPU to recreate the
dropped objects, especially if they were under heavy use. Because of this,
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:
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 3) 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
debugfs shows what the fragmentation index for each order is in each zone in
the system. Values tending towards 0 imply allocations would fail due to lack
of memory, values towards 1000 imply failures are due to fragmentation and -1
implies that the allocation will succeed as long as watermarks are met.
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).
This parameter controls whether the high memory is considered for dirty
writers throttling. This is not the case by default which means that
only the amount of memory directly visible/usable by the kernel can
be dirtied. As a result, on systems with a large amount of memory and
lowmem basically depleted writers might be throttled too early and
streaming writes can get very slow.
Changing the value to non zero would allow more memory to be dirtied
and thus allow writers to write more data which can be flushed to the
storage more effectively. Note this also comes with a risk of pre-mature
OOM killer because some writers (e.g. direct block device writes) can
only use the low memory and they can fill it up with dirty data without
any throttling.
==============================================================
hugepages_treat_as_movable
This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
so this parameter has no effect if used without kernelcore=.
Hugepage migration is now available in some situations which depend on the
architecture and/or the hugepage size. If a hugepage supports migration,
allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
of the value of this parameter.
IOW, this parameter affects only non-migratable hugepages.
Assuming that hugepages are not migratable in your system, one usecase of
this parameter is that users can make hugepage pool more extensible by
enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
page reclaim/migration/compaction work more and you can get contiguous
memory more likely. Note that using ZONE_MOVABLE for non-migratable
hugepages can do harm to other features like memory hotremove (because
memory hotremove expects that memory blocks on ZONE_MOVABLE are always
removable,) so it's a trade-off responsible for the users.
==============================================================
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.txt.
==============================================================
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"
zone. This is because that memory could then be pinned via the mlock()
system call, or by unavailability of swapspace.
And on large highmem machines this lack of reclaimable lowmem memory
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
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.
(The same argument applies to the old 16 megabyte ISA DMA region. This
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
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
-
Note: # of this elements is one fewer than number of zones. Because the highest
zone's value is not necessary for following calculation.
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
:
:
numa_other 0
protection: (0, 2004, 2004, 2004)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.
In this example, if normal pages (index=2) are required to this DMA zone and
watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
not be used because pages_free(1355) is smaller than watermark + protection[2]
(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
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.
(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).
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.
If you would like to protect more pages, smaller values are effective.
The minimum value is 1 (1/1 -> 100%).
==============================================================
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, mprotect, and madvise, 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.
=============================================================
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
that cannot be handled by the kernel. In some cases (like the page
still having a valid copy on disk) the kernel will handle the failure
transparently without affecting any applications. But if there is
no other uptodate copy of the data it will kill to prevent any data
corruptions from propagating.
1: Kill all processes that have the corrupted and not reloadable page mapped
as soon as the corruption is detected. Note this is not supported
for a few types of pages, like kernel internally allocated data or
the swap cache, but works for the majority of user pages.
0: Only unmap the corrupted page from all processes and only kill a process
who tries to access it.
The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
handle this if they want to.
This is only active on architectures/platforms with advanced machine
check handling and depends on the hardware capabilities.
Applications can override this setting individually with the PR_MCE_KILL prctl
==============================================================
memory_failure_recovery
Enable memory failure recovery (when supported by the platform)
1: Attempt recovery.
0: Always panic on a memory failure.
==============================================================
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
watermark[WMARK_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 amount 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.
=============================================================
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.
=============================================================
min_unmapped_ratio:
This is available only on NUMA kernels.
This is a percentage of the total pages in each zone. Zone reclaim will
only occur if more than this percentage of pages are in a state that
zone_reclaim_mode allows to be reclaimed.
If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
against all file-backed unmapped pages including swapcache pages and tmpfs
files. Otherwise, only unmapped pages backed by normal files but not tmpfs
files and similar are considered.
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
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.
==============================================================
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
resulting from mmap allocations on architectures which support
tuning address space randomization. This value will be bounded
by the architecture's minimum and maximum supported values.
This value can be changed after boot using the
/proc/sys/vm/mmap_rnd_bits tunable
==============================================================
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
resulting from mmap allocations for applications run in
compatibility mode on architectures which support tuning address
space randomization. This value will be bounded by the
architecture's minimum and maximum supported values.
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/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
==============================================================
nr_trim_pages
This is available only on NOMMU kernels.
This value adjusts the excess page trimming behaviour of power-of-2 aligned
NOMMU mmap allocations.
A value of 0 disables trimming of allocations entirely, while a value of 1
trims excess pages aggressively. Any value >= 1 acts as the watermark where
trimming of allocations is initiated.
The default value is 1.
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...)
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 node order
"Zone Order" orders the zonelists by zone type, then by node within each
zone. Specify "[Zz]one" for zone order.
Specify "[Dd]efault" to request automatic configuration.
On 32-bit, the Normal zone needs to be preserved for allocations accessible
by the kernel, so "zone" order will be selected.
On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
order will be selected.
Default order is recommended unless this is causing problems for your
system/application.
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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
pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj
score, and name. This is helpful to determine why the OOM killer was
invoked, to identify the rogue task that caused it, and to determine why
the OOM killer chose the task it did to kill.
If this is set to zero, this information is suppressed. On very
large systems with thousands of tasks it may not be feasible to dump
the memory state information for each one. Such systems should not
be forced to incur a performance penalty in OOM conditions when the
information may not be desired.
If this is set to non-zero, this information is shown whenever the
OOM killer actually kills a memory-hogging task.
The default value is 1 (enabled).
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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.
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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.
Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
of them may be specified at a time. Setting one disables the other (which
then appears as 0 when read).
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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.
Note that user_reserve_kbytes affects this policy.
This feature can be very useful because there are
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