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Peter W Morrealedb0fb182009-01-15 13:50:42 -08001Documentation for /proc/sys/vm/* kernel version 2.6.29
Linus Torvalds1da177e2005-04-16 15:20:36 -07002 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
Peter W Morrealedb0fb182009-01-15 13:50:42 -08003 (c) 2008 Peter W. Morreale <pmorreale@novell.com>
Linus Torvalds1da177e2005-04-16 15:20:36 -07004
5For general info and legal blurb, please look in README.
6
7==============================================================
8
9This file contains the documentation for the sysctl files in
Peter W Morrealedb0fb182009-01-15 13:50:42 -080010/proc/sys/vm and is valid for Linux kernel version 2.6.29.
Linus Torvalds1da177e2005-04-16 15:20:36 -070011
12The files in this directory can be used to tune the operation
13of the virtual memory (VM) subsystem of the Linux kernel and
14the writeout of dirty data to disk.
15
16Default values and initialization routines for most of these
17files can be found in mm/swap.c.
18
19Currently, these files are in /proc/sys/vm:
Peter W Morrealedb0fb182009-01-15 13:50:42 -080020
Andrew Shewmaker4eeab4f2013-04-29 15:08:11 -070021- admin_reserve_kbytes
Peter W Morrealedb0fb182009-01-15 13:50:42 -080022- block_dump
Mel Gorman76ab0f52010-05-24 14:32:28 -070023- compact_memory
Eric B Munson5bbe3542015-04-15 16:13:20 -070024- compact_unevictable_allowed
Peter W Morrealedb0fb182009-01-15 13:50:42 -080025- dirty_background_bytes
Linus Torvalds1da177e2005-04-16 15:20:36 -070026- dirty_background_ratio
Peter W Morrealedb0fb182009-01-15 13:50:42 -080027- dirty_bytes
Linus Torvalds1da177e2005-04-16 15:20:36 -070028- dirty_expire_centisecs
Peter W Morrealedb0fb182009-01-15 13:50:42 -080029- dirty_ratio
Linus Torvalds1da177e2005-04-16 15:20:36 -070030- dirty_writeback_centisecs
Peter W Morrealedb0fb182009-01-15 13:50:42 -080031- drop_caches
Mel Gorman5e771902010-05-24 14:32:31 -070032- extfrag_threshold
Peter W Morrealedb0fb182009-01-15 13:50:42 -080033- hugepages_treat_as_movable
34- hugetlb_shm_group
35- laptop_mode
36- legacy_va_layout
37- lowmem_reserve_ratio
Linus Torvalds1da177e2005-04-16 15:20:36 -070038- max_map_count
Andi Kleen6a460792009-09-16 11:50:15 +020039- memory_failure_early_kill
40- memory_failure_recovery
Linus Torvalds1da177e2005-04-16 15:20:36 -070041- min_free_kbytes
Christoph Lameter0ff38492006-09-25 23:31:52 -070042- min_slab_ratio
Peter W Morrealedb0fb182009-01-15 13:50:42 -080043- min_unmapped_ratio
44- mmap_min_addr
Daniel Cashmand07e2252016-01-14 15:19:53 -080045- mmap_rnd_bits
46- mmap_rnd_compat_bits
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -080047- nr_hugepages
48- nr_overcommit_hugepages
Peter W Morrealedb0fb182009-01-15 13:50:42 -080049- nr_trim_pages (only if CONFIG_MMU=n)
50- numa_zonelist_order
51- oom_dump_tasks
52- oom_kill_allocating_task
Jerome Marchand49f0ce52014-01-21 15:49:14 -080053- overcommit_kbytes
Peter W Morrealedb0fb182009-01-15 13:50:42 -080054- overcommit_memory
55- overcommit_ratio
56- page-cluster
57- panic_on_oom
58- percpu_pagelist_fraction
59- stat_interval
Hugh Dickins52b6f462016-05-19 17:12:50 -070060- stat_refresh
Peter W Morrealedb0fb182009-01-15 13:50:42 -080061- swappiness
Andrew Shewmakerc9b1d092013-04-29 15:08:10 -070062- user_reserve_kbytes
Peter W Morrealedb0fb182009-01-15 13:50:42 -080063- vfs_cache_pressure
Jerome Marchande6507a02016-07-12 12:05:59 +020064- watermark_scale_factor
Peter W Morrealedb0fb182009-01-15 13:50:42 -080065- zone_reclaim_mode
66
Linus Torvalds1da177e2005-04-16 15:20:36 -070067==============================================================
68
Andrew Shewmaker4eeab4f2013-04-29 15:08:11 -070069admin_reserve_kbytes
70
71The amount of free memory in the system that should be reserved for users
72with the capability cap_sys_admin.
73
74admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
75
76That should provide enough for the admin to log in and kill a process,
77if necessary, under the default overcommit 'guess' mode.
78
79Systems running under overcommit 'never' should increase this to account
80for the full Virtual Memory Size of programs used to recover. Otherwise,
81root may not be able to log in to recover the system.
82
83How do you calculate a minimum useful reserve?
84
85sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
86
87For overcommit 'guess', we can sum resident set sizes (RSS).
88On x86_64 this is about 8MB.
89
90For overcommit 'never', we can take the max of their virtual sizes (VSZ)
91and add the sum of their RSS.
92On x86_64 this is about 128MB.
93
94Changing this takes effect whenever an application requests memory.
95
96==============================================================
97
Peter W Morrealedb0fb182009-01-15 13:50:42 -080098block_dump
Linus Torvalds1da177e2005-04-16 15:20:36 -070099
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800100block_dump enables block I/O debugging when set to a nonzero value. More
101information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700102
103==============================================================
104
Mel Gorman76ab0f52010-05-24 14:32:28 -0700105compact_memory
106
107Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
108all zones are compacted such that free memory is available in contiguous
109blocks where possible. This can be important for example in the allocation of
110huge pages although processes will also directly compact memory as required.
111
112==============================================================
113
Eric B Munson5bbe3542015-04-15 16:13:20 -0700114compact_unevictable_allowed
115
116Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
117allowed to examine the unevictable lru (mlocked pages) for pages to compact.
118This should be used on systems where stalls for minor page faults are an
119acceptable trade for large contiguous free memory. Set to 0 to prevent
120compaction from moving pages that are unevictable. Default value is 1.
121
122==============================================================
123
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800124dirty_background_bytes
Linus Torvalds1da177e2005-04-16 15:20:36 -0700125
Artem Bityutskiy6601fac2012-07-25 18:12:01 +0300126Contains the amount of dirty memory at which the background kernel
127flusher threads will start writeback.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700128
Andrea Righiabffc022010-10-27 15:33:31 -0700129Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
130one of them may be specified at a time. When one sysctl is written it is
131immediately taken into account to evaluate the dirty memory limits and the
132other appears as 0 when read.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700133
134==============================================================
135
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800136dirty_background_ratio
Linus Torvalds1da177e2005-04-16 15:20:36 -0700137
Zheng Liu715ea412013-11-12 15:08:30 -0800138Contains, as a percentage of total available memory that contains free pages
139and reclaimable pages, the number of pages at which the background kernel
140flusher threads will start writing out dirty data.
141
Chris Dunlopd83e2a42015-09-18 16:10:55 +1000142The total available memory is not equal to total system memory.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143
144==============================================================
145
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800146dirty_bytes
Linus Torvalds1da177e2005-04-16 15:20:36 -0700147
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800148Contains the amount of dirty memory at which a process generating disk writes
149will itself start writeback.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700150
Andrea Righiabffc022010-10-27 15:33:31 -0700151Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
152specified at a time. When one sysctl is written it is immediately taken into
153account to evaluate the dirty memory limits and the other appears as 0 when
154read.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800155
Andrea Righi9e4a5bd2009-04-30 15:08:57 -0700156Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
157value lower than this limit will be ignored and the old configuration will be
158retained.
159
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800160==============================================================
161
162dirty_expire_centisecs
163
164This tunable is used to define when dirty data is old enough to be eligible
Artem Bityutskiy6601fac2012-07-25 18:12:01 +0300165for writeout by the kernel flusher threads. It is expressed in 100'ths
166of a second. Data which has been dirty in-memory for longer than this
167interval will be written out next time a flusher thread wakes up.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800168
169==============================================================
170
171dirty_ratio
172
Zheng Liu715ea412013-11-12 15:08:30 -0800173Contains, as a percentage of total available memory that contains free pages
174and reclaimable pages, the number of pages at which a process which is
175generating disk writes will itself start writing out dirty data.
176
Chris Dunlopd83e2a42015-09-18 16:10:55 +1000177The total available memory is not equal to total system memory.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800178
179==============================================================
180
181dirty_writeback_centisecs
182
Artem Bityutskiy6601fac2012-07-25 18:12:01 +0300183The kernel flusher threads will periodically wake up and write `old' data
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800184out to disk. This tunable expresses the interval between those wakeups, in
185100'ths of a second.
186
187Setting this to zero disables periodic writeback altogether.
188
189==============================================================
190
191drop_caches
192
Dave Hansen5509a5d2014-04-03 14:48:19 -0700193Writing to this will cause the kernel to drop clean caches, as well as
194reclaimable slab objects like dentries and inodes. Once dropped, their
195memory becomes free.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800196
197To free pagecache:
198 echo 1 > /proc/sys/vm/drop_caches
Dave Hansen5509a5d2014-04-03 14:48:19 -0700199To free reclaimable slab objects (includes dentries and inodes):
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800200 echo 2 > /proc/sys/vm/drop_caches
Dave Hansen5509a5d2014-04-03 14:48:19 -0700201To free slab objects and pagecache:
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800202 echo 3 > /proc/sys/vm/drop_caches
203
Dave Hansen5509a5d2014-04-03 14:48:19 -0700204This is a non-destructive operation and will not free any dirty objects.
205To increase the number of objects freed by this operation, the user may run
206`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
207number of dirty objects on the system and create more candidates to be
208dropped.
209
210This file is not a means to control the growth of the various kernel caches
211(inodes, dentries, pagecache, etc...) These objects are automatically
212reclaimed by the kernel when memory is needed elsewhere on the system.
213
214Use of this file can cause performance problems. Since it discards cached
215objects, it may cost a significant amount of I/O and CPU to recreate the
216dropped objects, especially if they were under heavy use. Because of this,
217use outside of a testing or debugging environment is not recommended.
218
219You may see informational messages in your kernel log when this file is
220used:
221
222 cat (1234): drop_caches: 3
223
224These are informational only. They do not mean that anything is wrong
225with your system. To disable them, echo 4 (bit 3) into drop_caches.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800226
227==============================================================
228
Mel Gorman5e771902010-05-24 14:32:31 -0700229extfrag_threshold
230
231This parameter affects whether the kernel will compact memory or direct
Rabin Vincenta10726b2015-07-14 07:35:11 +0200232reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
233debugfs shows what the fragmentation index for each order is in each zone in
234the system. Values tending towards 0 imply allocations would fail due to lack
235of memory, values towards 1000 imply failures are due to fragmentation and -1
236implies that the allocation will succeed as long as watermarks are met.
Mel Gorman5e771902010-05-24 14:32:31 -0700237
238The kernel will not compact memory in a zone if the
239fragmentation index is <= extfrag_threshold. The default value is 500.
240
241==============================================================
242
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800243hugepages_treat_as_movable
244
Naoya Horiguchi86cdb462013-09-11 14:22:13 -0700245This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
246or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
247ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
248so this parameter has no effect if used without kernelcore=.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800249
Naoya Horiguchi86cdb462013-09-11 14:22:13 -0700250Hugepage migration is now available in some situations which depend on the
251architecture and/or the hugepage size. If a hugepage supports migration,
252allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
253of the value of this parameter.
254IOW, this parameter affects only non-migratable hugepages.
255
256Assuming that hugepages are not migratable in your system, one usecase of
257this parameter is that users can make hugepage pool more extensible by
258enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
259page reclaim/migration/compaction work more and you can get contiguous
260memory more likely. Note that using ZONE_MOVABLE for non-migratable
261hugepages can do harm to other features like memory hotremove (because
262memory hotremove expects that memory blocks on ZONE_MOVABLE are always
263removable,) so it's a trade-off responsible for the users.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800264
265==============================================================
266
267hugetlb_shm_group
268
269hugetlb_shm_group contains group id that is allowed to create SysV
270shared memory segment using hugetlb page.
271
272==============================================================
273
274laptop_mode
275
276laptop_mode is a knob that controls "laptop mode". All the things that are
277controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
278
279==============================================================
280
281legacy_va_layout
282
Kulikov Vasiliy2174efb2010-06-28 13:59:28 +0200283If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800284will use the legacy (2.4) layout for all processes.
285
286==============================================================
287
288lowmem_reserve_ratio
289
290For some specialised workloads on highmem machines it is dangerous for
291the kernel to allow process memory to be allocated from the "lowmem"
292zone. This is because that memory could then be pinned via the mlock()
293system call, or by unavailability of swapspace.
294
295And on large highmem machines this lack of reclaimable lowmem memory
296can be fatal.
297
298So the Linux page allocator has a mechanism which prevents allocations
299which _could_ use highmem from using too much lowmem. This means that
300a certain amount of lowmem is defended from the possibility of being
301captured into pinned user memory.
302
303(The same argument applies to the old 16 megabyte ISA DMA region. This
304mechanism will also defend that region from allocations which could use
305highmem or lowmem).
306
307The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
308in defending these lower zones.
309
310If you have a machine which uses highmem or ISA DMA and your
311applications are using mlock(), or if you are running with no swap then
312you probably should change the lowmem_reserve_ratio setting.
313
314The lowmem_reserve_ratio is an array. You can see them by reading this file.
315-
316% cat /proc/sys/vm/lowmem_reserve_ratio
317256 256 32
318-
319Note: # of this elements is one fewer than number of zones. Because the highest
320 zone's value is not necessary for following calculation.
321
322But, these values are not used directly. The kernel calculates # of protection
323pages for each zones from them. These are shown as array of protection pages
324in /proc/zoneinfo like followings. (This is an example of x86-64 box).
325Each zone has an array of protection pages like this.
326
327-
328Node 0, zone DMA
329 pages free 1355
330 min 3
331 low 3
332 high 4
333 :
334 :
335 numa_other 0
336 protection: (0, 2004, 2004, 2004)
337 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
338 pagesets
339 cpu: 0 pcp: 0
340 :
341-
342These protections are added to score to judge whether this zone should be used
343for page allocation or should be reclaimed.
344
345In this example, if normal pages (index=2) are required to this DMA zone and
Mel Gorman41858962009-06-16 15:32:12 -0700346watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
347not be used because pages_free(1355) is smaller than watermark + protection[2]
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800348(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
349normal page requirement. If requirement is DMA zone(index=0), protection[0]
350(=0) is used.
351
352zone[i]'s protection[j] is calculated by following expression.
353
354(i < j):
355 zone[i]->protection[j]
Yaowei Bai013110a2015-09-08 15:04:10 -0700356 = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800357 / lowmem_reserve_ratio[i];
358(i = j):
359 (should not be protected. = 0;
360(i > j):
361 (not necessary, but looks 0)
362
363The default values of lowmem_reserve_ratio[i] are
364 256 (if zone[i] means DMA or DMA32 zone)
365 32 (others).
366As above expression, they are reciprocal number of ratio.
Yaowei Bai013110a2015-09-08 15:04:10 -0700367256 means 1/256. # of protection pages becomes about "0.39%" of total managed
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800368pages of higher zones on the node.
369
370If you would like to protect more pages, smaller values are effective.
371The minimum value is 1 (1/1 -> 100%).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700372
373==============================================================
374
375max_map_count:
376
377This file contains the maximum number of memory map areas a process
378may have. Memory map areas are used as a side-effect of calling
379malloc, directly by mmap and mprotect, and also when loading shared
380libraries.
381
382While most applications need less than a thousand maps, certain
383programs, particularly malloc debuggers, may consume lots of them,
384e.g., up to one or two maps per allocation.
385
386The default value is 65536.
387
Andi Kleen6a460792009-09-16 11:50:15 +0200388=============================================================
389
390memory_failure_early_kill:
391
392Control how to kill processes when uncorrected memory error (typically
393a 2bit error in a memory module) is detected in the background by hardware
394that cannot be handled by the kernel. In some cases (like the page
395still having a valid copy on disk) the kernel will handle the failure
396transparently without affecting any applications. But if there is
397no other uptodate copy of the data it will kill to prevent any data
398corruptions from propagating.
399
4001: Kill all processes that have the corrupted and not reloadable page mapped
401as soon as the corruption is detected. Note this is not supported
402for a few types of pages, like kernel internally allocated data or
403the swap cache, but works for the majority of user pages.
404
4050: Only unmap the corrupted page from all processes and only kill a process
406who tries to access it.
407
408The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
409handle this if they want to.
410
411This is only active on architectures/platforms with advanced machine
412check handling and depends on the hardware capabilities.
413
414Applications can override this setting individually with the PR_MCE_KILL prctl
415
416==============================================================
417
418memory_failure_recovery
419
420Enable memory failure recovery (when supported by the platform)
421
4221: Attempt recovery.
423
4240: Always panic on a memory failure.
425
Linus Torvalds1da177e2005-04-16 15:20:36 -0700426==============================================================
427
428min_free_kbytes:
429
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800430This is used to force the Linux VM to keep a minimum number
Mel Gorman41858962009-06-16 15:32:12 -0700431of kilobytes free. The VM uses this number to compute a
432watermark[WMARK_MIN] value for each lowmem zone in the system.
433Each lowmem zone gets a number of reserved free pages based
434proportionally on its size.
Rohit Seth8ad4b1f2006-01-08 01:00:40 -0800435
Matt LaPlanted9195882008-07-25 19:45:33 -0700436Some minimal amount of memory is needed to satisfy PF_MEMALLOC
Pavel Machek24950892007-10-16 23:31:28 -0700437allocations; if you set this to lower than 1024KB, your system will
438become subtly broken, and prone to deadlock under high loads.
439
440Setting this too high will OOM your machine instantly.
441
Christoph Lameter96146342006-07-03 00:24:13 -0700442=============================================================
443
Christoph Lameter0ff38492006-09-25 23:31:52 -0700444min_slab_ratio:
445
446This is available only on NUMA kernels.
447
448A percentage of the total pages in each zone. On Zone reclaim
449(fallback from the local zone occurs) slabs will be reclaimed if more
450than this percentage of pages in a zone are reclaimable slab pages.
451This insures that the slab growth stays under control even in NUMA
452systems that rarely perform global reclaim.
453
454The default is 5 percent.
455
456Note that slab reclaim is triggered in a per zone / node fashion.
457The process of reclaiming slab memory is currently not node specific
458and may not be fast.
459
460=============================================================
461
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800462min_unmapped_ratio:
KAMEZAWA Hiroyukifadd8fb2006-06-23 02:03:13 -0700463
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800464This is available only on NUMA kernels.
Yasunori Goto2b744c02007-05-06 14:49:59 -0700465
Mel Gorman90afa5d2009-06-16 15:33:20 -0700466This is a percentage of the total pages in each zone. Zone reclaim will
467only occur if more than this percentage of pages are in a state that
468zone_reclaim_mode allows to be reclaimed.
469
470If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
471against all file-backed unmapped pages including swapcache pages and tmpfs
472files. Otherwise, only unmapped pages backed by normal files but not tmpfs
473files and similar are considered.
Yasunori Goto2b744c02007-05-06 14:49:59 -0700474
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800475The default is 1 percent.
David Rientjesfe071d72007-10-16 23:25:56 -0700476
Eric Parised032182007-06-28 15:55:21 -0400477==============================================================
478
479mmap_min_addr
480
481This file indicates the amount of address space which a user process will
André Goddard Rosaaf901ca2009-11-14 13:09:05 -0200482be restricted from mmapping. Since kernel null dereference bugs could
Eric Parised032182007-06-28 15:55:21 -0400483accidentally operate based on the information in the first couple of pages
484of memory userspace processes should not be allowed to write to them. By
485default this value is set to 0 and no protections will be enforced by the
486security module. Setting this value to something like 64k will allow the
487vast majority of applications to work correctly and provide defense in depth
488against future potential kernel bugs.
489
KAMEZAWA Hiroyukif0c0b2b2007-07-15 23:38:01 -0700490==============================================================
491
Daniel Cashmand07e2252016-01-14 15:19:53 -0800492mmap_rnd_bits:
493
494This value can be used to select the number of bits to use to
495determine the random offset to the base address of vma regions
496resulting from mmap allocations on architectures which support
497tuning address space randomization. This value will be bounded
498by the architecture's minimum and maximum supported values.
499
500This value can be changed after boot using the
501/proc/sys/vm/mmap_rnd_bits tunable
502
503==============================================================
504
505mmap_rnd_compat_bits:
506
507This value can be used to select the number of bits to use to
508determine the random offset to the base address of vma regions
509resulting from mmap allocations for applications run in
510compatibility mode on architectures which support tuning address
511space randomization. This value will be bounded by the
512architecture's minimum and maximum supported values.
513
514This value can be changed after boot using the
515/proc/sys/vm/mmap_rnd_compat_bits tunable
516
517==============================================================
518
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800519nr_hugepages
520
521Change the minimum size of the hugepage pool.
522
523See Documentation/vm/hugetlbpage.txt
524
525==============================================================
526
527nr_overcommit_hugepages
528
529Change the maximum size of the hugepage pool. The maximum is
530nr_hugepages + nr_overcommit_hugepages.
531
532See Documentation/vm/hugetlbpage.txt
533
534==============================================================
535
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800536nr_trim_pages
537
538This is available only on NOMMU kernels.
539
540This value adjusts the excess page trimming behaviour of power-of-2 aligned
541NOMMU mmap allocations.
542
543A value of 0 disables trimming of allocations entirely, while a value of 1
544trims excess pages aggressively. Any value >= 1 acts as the watermark where
545trimming of allocations is initiated.
546
547The default value is 1.
548
549See Documentation/nommu-mmap.txt for more information.
550
551==============================================================
552
KAMEZAWA Hiroyukif0c0b2b2007-07-15 23:38:01 -0700553numa_zonelist_order
554
555This sysctl is only for NUMA.
556'where the memory is allocated from' is controlled by zonelists.
557(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
558 you may be able to read ZONE_DMA as ZONE_DMA32...)
559
560In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
561ZONE_NORMAL -> ZONE_DMA
562This means that a memory allocation request for GFP_KERNEL will
563get memory from ZONE_DMA only when ZONE_NORMAL is not available.
564
565In NUMA case, you can think of following 2 types of order.
566Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
567
568(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
569(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
570
571Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
572will be used before ZONE_NORMAL exhaustion. This increases possibility of
573out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
574
575Type(B) cannot offer the best locality but is more robust against OOM of
576the DMA zone.
577
578Type(A) is called as "Node" order. Type (B) is "Zone" order.
579
580"Node order" orders the zonelists by node, then by zone within each node.
Paul Bolle5a3016a2011-04-06 11:09:55 +0200581Specify "[Nn]ode" for node order
KAMEZAWA Hiroyukif0c0b2b2007-07-15 23:38:01 -0700582
583"Zone Order" orders the zonelists by zone type, then by node within each
Paul Bolle5a3016a2011-04-06 11:09:55 +0200584zone. Specify "[Zz]one" for zone order.
KAMEZAWA Hiroyukif0c0b2b2007-07-15 23:38:01 -0700585
Xishi Qiu7c88a292016-04-28 16:19:11 -0700586Specify "[Dd]efault" to request automatic configuration.
KAMEZAWA Hiroyukif0c0b2b2007-07-15 23:38:01 -0700587
Xishi Qiu7c88a292016-04-28 16:19:11 -0700588On 32-bit, the Normal zone needs to be preserved for allocations accessible
589by the kernel, so "zone" order will be selected.
590
591On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
592order will be selected.
593
594Default order is recommended unless this is causing problems for your
595system/application.
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800596
597==============================================================
598
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800599oom_dump_tasks
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800600
Kirill A. Shutemovdc6c9a32015-02-11 15:26:50 -0800601Enables a system-wide task dump (excluding kernel threads) to be produced
602when the kernel performs an OOM-killing and includes such information as
603pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj
604score, and name. This is helpful to determine why the OOM killer was
605invoked, to identify the rogue task that caused it, and to determine why
606the OOM killer chose the task it did to kill.
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800607
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800608If this is set to zero, this information is suppressed. On very
609large systems with thousands of tasks it may not be feasible to dump
610the memory state information for each one. Such systems should not
611be forced to incur a performance penalty in OOM conditions when the
612information may not be desired.
613
614If this is set to non-zero, this information is shown whenever the
615OOM killer actually kills a memory-hogging task.
616
David Rientjesad915c42010-08-09 17:18:53 -0700617The default value is 1 (enabled).
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800618
619==============================================================
620
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800621oom_kill_allocating_task
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800622
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800623This enables or disables killing the OOM-triggering task in
624out-of-memory situations.
Nishanth Aravamudand5dbac82007-12-17 16:20:25 -0800625
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800626If this is set to zero, the OOM killer will scan through the entire
627tasklist and select a task based on heuristics to kill. This normally
628selects a rogue memory-hogging task that frees up a large amount of
629memory when killed.
630
631If this is set to non-zero, the OOM killer simply kills the task that
632triggered the out-of-memory condition. This avoids the expensive
633tasklist scan.
634
635If panic_on_oom is selected, it takes precedence over whatever value
636is used in oom_kill_allocating_task.
637
638The default value is 0.
Paul Mundtdd8632a2009-01-08 12:04:47 +0000639
640==============================================================
641
Jerome Marchand49f0ce52014-01-21 15:49:14 -0800642overcommit_kbytes:
643
644When overcommit_memory is set to 2, the committed address space is not
645permitted to exceed swap plus this amount of physical RAM. See below.
646
647Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
648of them may be specified at a time. Setting one disables the other (which
649then appears as 0 when read).
650
651==============================================================
652
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800653overcommit_memory:
Paul Mundtdd8632a2009-01-08 12:04:47 +0000654
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800655This value contains a flag that enables memory overcommitment.
Paul Mundtdd8632a2009-01-08 12:04:47 +0000656
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800657When this flag is 0, the kernel attempts to estimate the amount
658of free memory left when userspace requests more memory.
Paul Mundtdd8632a2009-01-08 12:04:47 +0000659
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800660When this flag is 1, the kernel pretends there is always enough
661memory until it actually runs out.
Paul Mundtdd8632a2009-01-08 12:04:47 +0000662
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800663When this flag is 2, the kernel uses a "never overcommit"
664policy that attempts to prevent any overcommit of memory.
Andrew Shewmakerc9b1d092013-04-29 15:08:10 -0700665Note that user_reserve_kbytes affects this policy.
Paul Mundtdd8632a2009-01-08 12:04:47 +0000666
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800667This feature can be very useful because there are a lot of
668programs that malloc() huge amounts of memory "just-in-case"
669and don't use much of it.
670
671The default value is 0.
672
673See Documentation/vm/overcommit-accounting and
Chun Chenc56050c2015-11-09 14:58:15 -0800674mm/mmap.c::__vm_enough_memory() for more information.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800675
676==============================================================
677
678overcommit_ratio:
679
680When overcommit_memory is set to 2, the committed address
681space is not permitted to exceed swap plus this percentage
682of physical RAM. See above.
683
684==============================================================
685
686page-cluster
687
Christian Ehrhardtdf858fa2012-07-31 16:41:46 -0700688page-cluster controls the number of pages up to which consecutive pages
689are read in from swap in a single attempt. This is the swap counterpart
690to page cache readahead.
691The mentioned consecutivity is not in terms of virtual/physical addresses,
692but consecutive on swap space - that means they were swapped out together.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800693
694It is a logarithmic value - setting it to zero means "1 page", setting
695it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
Christian Ehrhardtdf858fa2012-07-31 16:41:46 -0700696Zero disables swap readahead completely.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800697
698The default value is three (eight pages at a time). There may be some
699small benefits in tuning this to a different value if your workload is
700swap-intensive.
701
Christian Ehrhardtdf858fa2012-07-31 16:41:46 -0700702Lower values mean lower latencies for initial faults, but at the same time
703extra faults and I/O delays for following faults if they would have been part of
704that consecutive pages readahead would have brought in.
705
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800706=============================================================
707
708panic_on_oom
709
710This enables or disables panic on out-of-memory feature.
711
712If this is set to 0, the kernel will kill some rogue process,
713called oom_killer. Usually, oom_killer can kill rogue processes and
714system will survive.
715
716If this is set to 1, the kernel panics when out-of-memory happens.
717However, if a process limits using nodes by mempolicy/cpusets,
718and those nodes become memory exhaustion status, one process
719may be killed by oom-killer. No panic occurs in this case.
720Because other nodes' memory may be free. This means system total status
721may be not fatal yet.
722
723If this is set to 2, the kernel panics compulsorily even on the
KAMEZAWA Hiroyukidaaf1e62010-03-10 15:22:32 -0800724above-mentioned. Even oom happens under memory cgroup, the whole
725system panics.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800726
727The default value is 0.
7281 and 2 are for failover of clustering. Please select either
729according to your policy of failover.
KAMEZAWA Hiroyukidaaf1e62010-03-10 15:22:32 -0800730panic_on_oom=2+kdump gives you very strong tool to investigate
731why oom happens. You can get snapshot.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800732
733=============================================================
734
735percpu_pagelist_fraction
736
737This is the fraction of pages at most (high mark pcp->high) in each zone that
738are allocated for each per cpu page list. The min value for this is 8. It
739means that we don't allow more than 1/8th of pages in each zone to be
740allocated in any single per_cpu_pagelist. This entry only changes the value
741of hot per cpu pagelists. User can specify a number like 100 to allocate
7421/100th of each zone to each per cpu page list.
743
744The batch value of each per cpu pagelist is also updated as a result. It is
745set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
746
747The initial value is zero. Kernel does not use this value at boot time to set
David Rientjes7cd2b0a2014-06-23 13:22:04 -0700748the high water marks for each per cpu page list. If the user writes '0' to this
749sysctl, it will revert to this default behavior.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800750
751==============================================================
752
753stat_interval
754
755The time interval between which vm statistics are updated. The default
756is 1 second.
757
758==============================================================
759
Hugh Dickins52b6f462016-05-19 17:12:50 -0700760stat_refresh
761
762Any read or write (by root only) flushes all the per-cpu vm statistics
763into their global totals, for more accurate reports when testing
764e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
765
766As a side-effect, it also checks for negative totals (elsewhere reported
767as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
768(At time of writing, a few stats are known sometimes to be found negative,
769with no ill effects: errors and warnings on these stats are suppressed.)
770
771==============================================================
772
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800773swappiness
774
775This control is used to define how aggressive the kernel will swap
776memory pages. Higher values will increase agressiveness, lower values
Aaron Tomlin8582cb92014-01-29 14:05:38 -0800777decrease the amount of swap. A value of 0 instructs the kernel not to
778initiate swap until the amount of free and file-backed pages is less
779than the high water mark in a zone.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800780
781The default value is 60.
782
783==============================================================
784
Andrew Shewmakerc9b1d092013-04-29 15:08:10 -0700785- user_reserve_kbytes
786
Masanari Iida633708a2015-01-02 12:03:19 +0900787When overcommit_memory is set to 2, "never overcommit" mode, reserve
Andrew Shewmakerc9b1d092013-04-29 15:08:10 -0700788min(3% of current process size, user_reserve_kbytes) of free memory.
789This is intended to prevent a user from starting a single memory hogging
790process, such that they cannot recover (kill the hog).
791
792user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
793
794If this is reduced to zero, then the user will be allowed to allocate
795all free memory with a single process, minus admin_reserve_kbytes.
796Any subsequent attempts to execute a command will result in
797"fork: Cannot allocate memory".
798
799Changing this takes effect whenever an application requests memory.
800
801==============================================================
802
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800803vfs_cache_pressure
804------------------
805
Denys Vlasenko4a0da712014-06-04 16:11:03 -0700806This percentage value controls the tendency of the kernel to reclaim
807the memory which is used for caching of directory and inode objects.
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800808
809At the default value of vfs_cache_pressure=100 the kernel will attempt to
810reclaim dentries and inodes at a "fair" rate with respect to pagecache and
811swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer
Jan Kara55c37a82009-09-21 17:01:40 -0700812to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
813never reclaim dentries and inodes due to memory pressure and this can easily
814lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800815causes the kernel to prefer to reclaim dentries and inodes.
816
Denys Vlasenko4a0da712014-06-04 16:11:03 -0700817Increasing vfs_cache_pressure significantly beyond 100 may have negative
818performance impact. Reclaim code needs to take various locks to find freeable
819directory and inode objects. With vfs_cache_pressure=1000, it will look for
820ten times more freeable objects than there are.
821
Johannes Weiner795ae7a2016-03-17 14:19:14 -0700822=============================================================
823
824watermark_scale_factor:
825
826This factor controls the aggressiveness of kswapd. It defines the
827amount of memory left in a node/system before kswapd is woken up and
828how much memory needs to be free before kswapd goes back to sleep.
829
830The unit is in fractions of 10,000. The default value of 10 means the
831distances between watermarks are 0.1% of the available memory in the
832node/system. The maximum value is 1000, or 10% of memory.
833
834A high rate of threads entering direct reclaim (allocstall) or kswapd
835going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
836that the number of free pages kswapd maintains for latency reasons is
837too small for the allocation bursts occurring in the system. This knob
838can then be used to tune kswapd aggressiveness accordingly.
839
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800840==============================================================
841
842zone_reclaim_mode:
843
844Zone_reclaim_mode allows someone to set more or less aggressive approaches to
845reclaim memory when a zone runs out of memory. If it is set to zero then no
846zone reclaim occurs. Allocations will be satisfied from other zones / nodes
847in the system.
848
849This is value ORed together of
850
8511 = Zone reclaim on
8522 = Zone reclaim writes dirty pages out
8534 = Zone reclaim swaps pages
854
Mel Gorman4f9b16a2014-06-04 16:07:14 -0700855zone_reclaim_mode is disabled by default. For file servers or workloads
856that benefit from having their data cached, zone_reclaim_mode should be
857left disabled as the caching effect is likely to be more important than
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800858data locality.
859
Mel Gorman4f9b16a2014-06-04 16:07:14 -0700860zone_reclaim may be enabled if it's known that the workload is partitioned
861such that each partition fits within a NUMA node and that accessing remote
862memory would cause a measurable performance reduction. The page allocator
863will then reclaim easily reusable pages (those page cache pages that are
864currently not used) before allocating off node pages.
865
Peter W Morrealedb0fb182009-01-15 13:50:42 -0800866Allowing zone reclaim to write out pages stops processes that are
867writing large amounts of data from dirtying pages on other nodes. Zone
868reclaim will write out dirty pages if a zone fills up and so effectively
869throttle the process. This may decrease the performance of a single process
870since it cannot use all of system memory to buffer the outgoing writes
871anymore but it preserve the memory on other nodes so that the performance
872of other processes running on other nodes will not be affected.
873
874Allowing regular swap effectively restricts allocations to the local
875node unless explicitly overridden by memory policies or cpuset
876configurations.
877
878============ End of Document =================================