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1===============================
2Documentation for /proc/sys/vm/
3===============================
4
5kernel version 2.6.29
6
7Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
8
9Copyright (c) 2008 Peter W. Morreale <pmorreale@novell.com>
10
11For general info and legal blurb, please look in index.rst.
12
13------------------------------------------------------------------------------
14
15This file contains the documentation for the sysctl files in
16/proc/sys/vm and is valid for Linux kernel version 2.6.29.
17
18The files in this directory can be used to tune the operation
19of the virtual memory (VM) subsystem of the Linux kernel and
20the writeout of dirty data to disk.
21
22Default values and initialization routines for most of these
23files can be found in mm/swap.c.
24
25Currently, these files are in /proc/sys/vm:
26
27- admin_reserve_kbytes
28- compact_memory
29- compaction_proactiveness
30- compact_unevictable_allowed
31- dirty_background_bytes
32- dirty_background_ratio
33- dirty_bytes
34- dirty_expire_centisecs
35- dirty_ratio
36- dirtytime_expire_seconds
37- dirty_writeback_centisecs
38- drop_caches
39- extfrag_threshold
40- highmem_is_dirtyable
41- hugetlb_shm_group
42- laptop_mode
43- legacy_va_layout
44- lowmem_reserve_ratio
45- max_map_count
46- memory_failure_early_kill
47- memory_failure_recovery
48- min_free_kbytes
49- min_slab_ratio
50- min_unmapped_ratio
51- mmap_min_addr
52- mmap_rnd_bits
53- mmap_rnd_compat_bits
54- nr_hugepages
55- nr_hugepages_mempolicy
56- nr_overcommit_hugepages
57- nr_trim_pages (only if CONFIG_MMU=n)
58- numa_zonelist_order
59- oom_dump_tasks
60- oom_kill_allocating_task
61- overcommit_kbytes
62- overcommit_memory
63- overcommit_ratio
64- page-cluster
65- page_lock_unfairness
66- panic_on_oom
67- percpu_pagelist_high_fraction
68- stat_interval
69- stat_refresh
70- numa_stat
71- swappiness
72- unprivileged_userfaultfd
73- user_reserve_kbytes
74- vfs_cache_pressure
75- watermark_boost_factor
76- watermark_scale_factor
77- zone_reclaim_mode
78
79
80admin_reserve_kbytes
81====================
82
83The amount of free memory in the system that should be reserved for users
84with the capability cap_sys_admin.
85
86admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
87
88That should provide enough for the admin to log in and kill a process,
89if necessary, under the default overcommit 'guess' mode.
90
91Systems running under overcommit 'never' should increase this to account
92for the full Virtual Memory Size of programs used to recover. Otherwise,
93root may not be able to log in to recover the system.
94
95How do you calculate a minimum useful reserve?
96
97sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
98
99For overcommit 'guess', we can sum resident set sizes (RSS).
100On x86_64 this is about 8MB.
101
102For overcommit 'never', we can take the max of their virtual sizes (VSZ)
103and add the sum of their RSS.
104On x86_64 this is about 128MB.
105
106Changing this takes effect whenever an application requests memory.
107
108
109compact_memory
110==============
111
112Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
113all zones are compacted such that free memory is available in contiguous
114blocks where possible. This can be important for example in the allocation of
115huge pages although processes will also directly compact memory as required.
116
117compaction_proactiveness
118========================
119
120This tunable takes a value in the range [0, 100] with a default value of
12120. This tunable determines how aggressively compaction is done in the
122background. Write of a non zero value to this tunable will immediately
123trigger the proactive compaction. Setting it to 0 disables proactive compaction.
124
125Note that compaction has a non-trivial system-wide impact as pages
126belonging to different processes are moved around, which could also lead
127to latency spikes in unsuspecting applications. The kernel employs
128various heuristics to avoid wasting CPU cycles if it detects that
129proactive compaction is not being effective.
130
131Be careful when setting it to extreme values like 100, as that may
132cause excessive background compaction activity.
133
134compact_unevictable_allowed
135===========================
136
137Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
138allowed to examine the unevictable lru (mlocked pages) for pages to compact.
139This should be used on systems where stalls for minor page faults are an
140acceptable trade for large contiguous free memory. Set to 0 to prevent
141compaction from moving pages that are unevictable. Default value is 1.
142On CONFIG_PREEMPT_RT the default value is 0 in order to avoid a page fault, due
143to compaction, which would block the task from becoming active until the fault
144is resolved.
145
146
147dirty_background_bytes
148======================
149
150Contains the amount of dirty memory at which the background kernel
151flusher threads will start writeback.
152
153Note:
154 dirty_background_bytes is the counterpart of dirty_background_ratio. Only
155 one of them may be specified at a time. When one sysctl is written it is
156 immediately taken into account to evaluate the dirty memory limits and the
157 other appears as 0 when read.
158
159
160dirty_background_ratio
161======================
162
163Contains, as a percentage of total available memory that contains free pages
164and reclaimable pages, the number of pages at which the background kernel
165flusher threads will start writing out dirty data.
166
167The total available memory is not equal to total system memory.
168
169
170dirty_bytes
171===========
172
173Contains the amount of dirty memory at which a process generating disk writes
174will itself start writeback.
175
176Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
177specified at a time. When one sysctl is written it is immediately taken into
178account to evaluate the dirty memory limits and the other appears as 0 when
179read.
180
181Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
182value lower than this limit will be ignored and the old configuration will be
183retained.
184
185
186dirty_expire_centisecs
187======================
188
189This tunable is used to define when dirty data is old enough to be eligible
190for writeout by the kernel flusher threads. It is expressed in 100'ths
191of a second. Data which has been dirty in-memory for longer than this
192interval will be written out next time a flusher thread wakes up.
193
194
195dirty_ratio
196===========
197
198Contains, as a percentage of total available memory that contains free pages
199and reclaimable pages, the number of pages at which a process which is
200generating disk writes will itself start writing out dirty data.
201
202The total available memory is not equal to total system memory.
203
204
205dirtytime_expire_seconds
206========================
207
208When a lazytime inode is constantly having its pages dirtied, the inode with
209an updated timestamp will never get chance to be written out. And, if the
210only thing that has happened on the file system is a dirtytime inode caused
211by an atime update, a worker will be scheduled to make sure that inode
212eventually gets pushed out to disk. This tunable is used to define when dirty
213inode is old enough to be eligible for writeback by the kernel flusher threads.
214And, it is also used as the interval to wakeup dirtytime_writeback thread.
215
216
217dirty_writeback_centisecs
218=========================
219
220The kernel flusher threads will periodically wake up and write `old` data
221out to disk. This tunable expresses the interval between those wakeups, in
222100'ths of a second.
223
224Setting this to zero disables periodic writeback altogether.
225
226
227drop_caches
228===========
229
230Writing to this will cause the kernel to drop clean caches, as well as
231reclaimable slab objects like dentries and inodes. Once dropped, their
232memory becomes free.
233
234To free pagecache::
235
236 echo 1 > /proc/sys/vm/drop_caches
237
238To free reclaimable slab objects (includes dentries and inodes)::
239
240 echo 2 > /proc/sys/vm/drop_caches
241
242To free slab objects and pagecache::
243
244 echo 3 > /proc/sys/vm/drop_caches
245
246This is a non-destructive operation and will not free any dirty objects.
247To increase the number of objects freed by this operation, the user may run
248`sync` prior to writing to /proc/sys/vm/drop_caches. This will minimize the
249number of dirty objects on the system and create more candidates to be
250dropped.
251
252This file is not a means to control the growth of the various kernel caches
253(inodes, dentries, pagecache, etc...) These objects are automatically
254reclaimed by the kernel when memory is needed elsewhere on the system.
255
256Use of this file can cause performance problems. Since it discards cached
257objects, it may cost a significant amount of I/O and CPU to recreate the
258dropped objects, especially if they were under heavy use. Because of this,
259use outside of a testing or debugging environment is not recommended.
260
261You may see informational messages in your kernel log when this file is
262used::
263
264 cat (1234): drop_caches: 3
265
266These are informational only. They do not mean that anything is wrong
267with your system. To disable them, echo 4 (bit 2) into drop_caches.
268
269
270extfrag_threshold
271=================
272
273This parameter affects whether the kernel will compact memory or direct
274reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
275debugfs shows what the fragmentation index for each order is in each zone in
276the system. Values tending towards 0 imply allocations would fail due to lack
277of memory, values towards 1000 imply failures are due to fragmentation and -1
278implies that the allocation will succeed as long as watermarks are met.
279
280The kernel will not compact memory in a zone if the
281fragmentation index is <= extfrag_threshold. The default value is 500.
282
283
284highmem_is_dirtyable
285====================
286
287Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
288
289This parameter controls whether the high memory is considered for dirty
290writers throttling. This is not the case by default which means that
291only the amount of memory directly visible/usable by the kernel can
292be dirtied. As a result, on systems with a large amount of memory and
293lowmem basically depleted writers might be throttled too early and
294streaming writes can get very slow.
295
296Changing the value to non zero would allow more memory to be dirtied
297and thus allow writers to write more data which can be flushed to the
298storage more effectively. Note this also comes with a risk of pre-mature
299OOM killer because some writers (e.g. direct block device writes) can
300only use the low memory and they can fill it up with dirty data without
301any throttling.
302
303
304hugetlb_shm_group
305=================
306
307hugetlb_shm_group contains group id that is allowed to create SysV
308shared memory segment using hugetlb page.
309
310
311laptop_mode
312===========
313
314laptop_mode is a knob that controls "laptop mode". All the things that are
315controlled by this knob are discussed in Documentation/admin-guide/laptops/laptop-mode.rst.
316
317
318legacy_va_layout
319================
320
321If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
322will use the legacy (2.4) layout for all processes.
323
324
325lowmem_reserve_ratio
326====================
327
328For some specialised workloads on highmem machines it is dangerous for
329the kernel to allow process memory to be allocated from the "lowmem"
330zone. This is because that memory could then be pinned via the mlock()
331system call, or by unavailability of swapspace.
332
333And on large highmem machines this lack of reclaimable lowmem memory
334can be fatal.
335
336So the Linux page allocator has a mechanism which prevents allocations
337which *could* use highmem from using too much lowmem. This means that
338a certain amount of lowmem is defended from the possibility of being
339captured into pinned user memory.
340
341(The same argument applies to the old 16 megabyte ISA DMA region. This
342mechanism will also defend that region from allocations which could use
343highmem or lowmem).
344
345The `lowmem_reserve_ratio` tunable determines how aggressive the kernel is
346in defending these lower zones.
347
348If you have a machine which uses highmem or ISA DMA and your
349applications are using mlock(), or if you are running with no swap then
350you probably should change the lowmem_reserve_ratio setting.
351
352The lowmem_reserve_ratio is an array. You can see them by reading this file::
353
354 % cat /proc/sys/vm/lowmem_reserve_ratio
355 256 256 32
356
357But, these values are not used directly. The kernel calculates # of protection
358pages for each zones from them. These are shown as array of protection pages
359in /proc/zoneinfo like followings. (This is an example of x86-64 box).
360Each zone has an array of protection pages like this::
361
362 Node 0, zone DMA
363 pages free 1355
364 min 3
365 low 3
366 high 4
367 :
368 :
369 numa_other 0
370 protection: (0, 2004, 2004, 2004)
371 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
372 pagesets
373 cpu: 0 pcp: 0
374 :
375
376These protections are added to score to judge whether this zone should be used
377for page allocation or should be reclaimed.
378
379In this example, if normal pages (index=2) are required to this DMA zone and
380watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
381not be used because pages_free(1355) is smaller than watermark + protection[2]
382(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
383normal page requirement. If requirement is DMA zone(index=0), protection[0]
384(=0) is used.
385
386zone[i]'s protection[j] is calculated by following expression::
387
388 (i < j):
389 zone[i]->protection[j]
390 = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
391 / lowmem_reserve_ratio[i];
392 (i = j):
393 (should not be protected. = 0;
394 (i > j):
395 (not necessary, but looks 0)
396
397The default values of lowmem_reserve_ratio[i] are
398
399 === ====================================
400 256 (if zone[i] means DMA or DMA32 zone)
401 32 (others)
402 === ====================================
403
404As above expression, they are reciprocal number of ratio.
405256 means 1/256. # of protection pages becomes about "0.39%" of total managed
406pages of higher zones on the node.
407
408If you would like to protect more pages, smaller values are effective.
409The minimum value is 1 (1/1 -> 100%). The value less than 1 completely
410disables protection of the pages.
411
412
413max_map_count:
414==============
415
416This file contains the maximum number of memory map areas a process
417may have. Memory map areas are used as a side-effect of calling
418malloc, directly by mmap, mprotect, and madvise, and also when loading
419shared libraries.
420
421While most applications need less than a thousand maps, certain
422programs, particularly malloc debuggers, may consume lots of them,
423e.g., up to one or two maps per allocation.
424
425The default value is 65530.
426
427
428memory_failure_early_kill:
429==========================
430
431Control how to kill processes when uncorrected memory error (typically
432a 2bit error in a memory module) is detected in the background by hardware
433that cannot be handled by the kernel. In some cases (like the page
434still having a valid copy on disk) the kernel will handle the failure
435transparently without affecting any applications. But if there is
436no other uptodate copy of the data it will kill to prevent any data
437corruptions from propagating.
438
4391: Kill all processes that have the corrupted and not reloadable page mapped
440as soon as the corruption is detected. Note this is not supported
441for a few types of pages, like kernel internally allocated data or
442the swap cache, but works for the majority of user pages.
443
4440: Only unmap the corrupted page from all processes and only kill a process
445who tries to access it.
446
447The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
448handle this if they want to.
449
450This is only active on architectures/platforms with advanced machine
451check handling and depends on the hardware capabilities.
452
453Applications can override this setting individually with the PR_MCE_KILL prctl
454
455
456memory_failure_recovery
457=======================
458
459Enable memory failure recovery (when supported by the platform)
460
4611: Attempt recovery.
462
4630: Always panic on a memory failure.
464
465
466min_free_kbytes
467===============
468
469This is used to force the Linux VM to keep a minimum number
470of kilobytes free. The VM uses this number to compute a
471watermark[WMARK_MIN] value for each lowmem zone in the system.
472Each lowmem zone gets a number of reserved free pages based
473proportionally on its size.
474
475Some minimal amount of memory is needed to satisfy PF_MEMALLOC
476allocations; if you set this to lower than 1024KB, your system will
477become subtly broken, and prone to deadlock under high loads.
478
479Setting this too high will OOM your machine instantly.
480
481
482min_slab_ratio
483==============
484
485This is available only on NUMA kernels.
486
487A percentage of the total pages in each zone. On Zone reclaim
488(fallback from the local zone occurs) slabs will be reclaimed if more
489than this percentage of pages in a zone are reclaimable slab pages.
490This insures that the slab growth stays under control even in NUMA
491systems that rarely perform global reclaim.
492
493The default is 5 percent.
494
495Note that slab reclaim is triggered in a per zone / node fashion.
496The process of reclaiming slab memory is currently not node specific
497and may not be fast.
498
499
500min_unmapped_ratio
501==================
502
503This is available only on NUMA kernels.
504
505This is a percentage of the total pages in each zone. Zone reclaim will
506only occur if more than this percentage of pages are in a state that
507zone_reclaim_mode allows to be reclaimed.
508
509If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
510against all file-backed unmapped pages including swapcache pages and tmpfs
511files. Otherwise, only unmapped pages backed by normal files but not tmpfs
512files and similar are considered.
513
514The default is 1 percent.
515
516
517mmap_min_addr
518=============
519
520This file indicates the amount of address space which a user process will
521be restricted from mmapping. Since kernel null dereference bugs could
522accidentally operate based on the information in the first couple of pages
523of memory userspace processes should not be allowed to write to them. By
524default this value is set to 0 and no protections will be enforced by the
525security module. Setting this value to something like 64k will allow the
526vast majority of applications to work correctly and provide defense in depth
527against future potential kernel bugs.
528
529
530mmap_rnd_bits
531=============
532
533This value can be used to select the number of bits to use to
534determine the random offset to the base address of vma regions
535resulting from mmap allocations on architectures which support
536tuning address space randomization. This value will be bounded
537by the architecture's minimum and maximum supported values.
538
539This value can be changed after boot using the
540/proc/sys/vm/mmap_rnd_bits tunable
541
542
543mmap_rnd_compat_bits
544====================
545
546This value can be used to select the number of bits to use to
547determine the random offset to the base address of vma regions
548resulting from mmap allocations for applications run in
549compatibility mode on architectures which support tuning address
550space randomization. This value will be bounded by the
551architecture's minimum and maximum supported values.
552
553This value can be changed after boot using the
554/proc/sys/vm/mmap_rnd_compat_bits tunable
555
556
557nr_hugepages
558============
559
560Change the minimum size of the hugepage pool.
561
562See Documentation/admin-guide/mm/hugetlbpage.rst
563
564
565hugetlb_optimize_vmemmap
566========================
567
568This knob is not available when the size of 'struct page' (a structure defined
569in include/linux/mm_types.h) is not power of two (an unusual system config could
570result in this).
571
572Enable (set to 1) or disable (set to 0) HugeTLB Vmemmap Optimization (HVO).
573
574Once enabled, the vmemmap pages of subsequent allocation of HugeTLB pages from
575buddy allocator will be optimized (7 pages per 2MB HugeTLB page and 4095 pages
576per 1GB HugeTLB page), whereas already allocated HugeTLB pages will not be
577optimized. When those optimized HugeTLB pages are freed from the HugeTLB pool
578to the buddy allocator, the vmemmap pages representing that range needs to be
579remapped again and the vmemmap pages discarded earlier need to be rellocated
580again. If your use case is that HugeTLB pages are allocated 'on the fly' (e.g.
581never explicitly allocating HugeTLB pages with 'nr_hugepages' but only set
582'nr_overcommit_hugepages', those overcommitted HugeTLB pages are allocated 'on
583the fly') instead of being pulled from the HugeTLB pool, you should weigh the
584benefits of memory savings against the more overhead (~2x slower than before)
585of allocation or freeing HugeTLB pages between the HugeTLB pool and the buddy
586allocator. Another behavior to note is that if the system is under heavy memory
587pressure, it could prevent the user from freeing HugeTLB pages from the HugeTLB
588pool to the buddy allocator since the allocation of vmemmap pages could be
589failed, you have to retry later if your system encounter this situation.
590
591Once disabled, the vmemmap pages of subsequent allocation of HugeTLB pages from
592buddy allocator will not be optimized meaning the extra overhead at allocation
593time from buddy allocator disappears, whereas already optimized HugeTLB pages
594will not be affected. If you want to make sure there are no optimized HugeTLB
595pages, you can set "nr_hugepages" to 0 first and then disable this. Note that
596writing 0 to nr_hugepages will make any "in use" HugeTLB pages become surplus
597pages. So, those surplus pages are still optimized until they are no longer
598in use. You would need to wait for those surplus pages to be released before
599there are no optimized pages in the system.
600
601
602nr_hugepages_mempolicy
603======================
604
605Change the size of the hugepage pool at run-time on a specific
606set of NUMA nodes.
607
608See Documentation/admin-guide/mm/hugetlbpage.rst
609
610
611nr_overcommit_hugepages
612=======================
613
614Change the maximum size of the hugepage pool. The maximum is
615nr_hugepages + nr_overcommit_hugepages.
616
617See Documentation/admin-guide/mm/hugetlbpage.rst
618
619
620nr_trim_pages
621=============
622
623This is available only on NOMMU kernels.
624
625This value adjusts the excess page trimming behaviour of power-of-2 aligned
626NOMMU mmap allocations.
627
628A value of 0 disables trimming of allocations entirely, while a value of 1
629trims excess pages aggressively. Any value >= 1 acts as the watermark where
630trimming of allocations is initiated.
631
632The default value is 1.
633
634See Documentation/admin-guide/mm/nommu-mmap.rst for more information.
635
636
637numa_zonelist_order
638===================
639
640This sysctl is only for NUMA and it is deprecated. Anything but
641Node order will fail!
642
643'where the memory is allocated from' is controlled by zonelists.
644
645(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
646you may be able to read ZONE_DMA as ZONE_DMA32...)
647
648In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
649ZONE_NORMAL -> ZONE_DMA
650This means that a memory allocation request for GFP_KERNEL will
651get memory from ZONE_DMA only when ZONE_NORMAL is not available.
652
653In NUMA case, you can think of following 2 types of order.
654Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL::
655
656 (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
657 (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
658
659Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
660will be used before ZONE_NORMAL exhaustion. This increases possibility of
661out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
662
663Type(B) cannot offer the best locality but is more robust against OOM of
664the DMA zone.
665
666Type(A) is called as "Node" order. Type (B) is "Zone" order.
667
668"Node order" orders the zonelists by node, then by zone within each node.
669Specify "[Nn]ode" for node order
670
671"Zone Order" orders the zonelists by zone type, then by node within each
672zone. Specify "[Zz]one" for zone order.
673
674Specify "[Dd]efault" to request automatic configuration.
675
676On 32-bit, the Normal zone needs to be preserved for allocations accessible
677by the kernel, so "zone" order will be selected.
678
679On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
680order will be selected.
681
682Default order is recommended unless this is causing problems for your
683system/application.
684
685
686oom_dump_tasks
687==============
688
689Enables a system-wide task dump (excluding kernel threads) to be produced
690when the kernel performs an OOM-killing and includes such information as
691pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj
692score, and name. This is helpful to determine why the OOM killer was
693invoked, to identify the rogue task that caused it, and to determine why
694the OOM killer chose the task it did to kill.
695
696If this is set to zero, this information is suppressed. On very
697large systems with thousands of tasks it may not be feasible to dump
698the memory state information for each one. Such systems should not
699be forced to incur a performance penalty in OOM conditions when the
700information may not be desired.
701
702If this is set to non-zero, this information is shown whenever the
703OOM killer actually kills a memory-hogging task.
704
705The default value is 1 (enabled).
706
707
708oom_kill_allocating_task
709========================
710
711This enables or disables killing the OOM-triggering task in
712out-of-memory situations.
713
714If this is set to zero, the OOM killer will scan through the entire
715tasklist and select a task based on heuristics to kill. This normally
716selects a rogue memory-hogging task that frees up a large amount of
717memory when killed.
718
719If this is set to non-zero, the OOM killer simply kills the task that
720triggered the out-of-memory condition. This avoids the expensive
721tasklist scan.
722
723If panic_on_oom is selected, it takes precedence over whatever value
724is used in oom_kill_allocating_task.
725
726The default value is 0.
727
728
729overcommit_kbytes
730=================
731
732When overcommit_memory is set to 2, the committed address space is not
733permitted to exceed swap plus this amount of physical RAM. See below.
734
735Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
736of them may be specified at a time. Setting one disables the other (which
737then appears as 0 when read).
738
739
740overcommit_memory
741=================
742
743This value contains a flag that enables memory overcommitment.
744
745When this flag is 0, the kernel attempts to estimate the amount
746of free memory left when userspace requests more memory.
747
748When this flag is 1, the kernel pretends there is always enough
749memory until it actually runs out.
750
751When this flag is 2, the kernel uses a "never overcommit"
752policy that attempts to prevent any overcommit of memory.
753Note that user_reserve_kbytes affects this policy.
754
755This feature can be very useful because there are a lot of
756programs that malloc() huge amounts of memory "just-in-case"
757and don't use much of it.
758
759The default value is 0.
760
761See Documentation/mm/overcommit-accounting.rst and
762mm/util.c::__vm_enough_memory() for more information.
763
764
765overcommit_ratio
766================
767
768When overcommit_memory is set to 2, the committed address
769space is not permitted to exceed swap plus this percentage
770of physical RAM. See above.
771
772
773page-cluster
774============
775
776page-cluster controls the number of pages up to which consecutive pages
777are read in from swap in a single attempt. This is the swap counterpart
778to page cache readahead.
779The mentioned consecutivity is not in terms of virtual/physical addresses,
780but consecutive on swap space - that means they were swapped out together.
781
782It is a logarithmic value - setting it to zero means "1 page", setting
783it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
784Zero disables swap readahead completely.
785
786The default value is three (eight pages at a time). There may be some
787small benefits in tuning this to a different value if your workload is
788swap-intensive.
789
790Lower values mean lower latencies for initial faults, but at the same time
791extra faults and I/O delays for following faults if they would have been part of
792that consecutive pages readahead would have brought in.
793
794
795page_lock_unfairness
796====================
797
798This value determines the number of times that the page lock can be
799stolen from under a waiter. After the lock is stolen the number of times
800specified in this file (default is 5), the "fair lock handoff" semantics
801will apply, and the waiter will only be awakened if the lock can be taken.
802
803panic_on_oom
804============
805
806This enables or disables panic on out-of-memory feature.
807
808If this is set to 0, the kernel will kill some rogue process,
809called oom_killer. Usually, oom_killer can kill rogue processes and
810system will survive.
811
812If this is set to 1, the kernel panics when out-of-memory happens.
813However, if a process limits using nodes by mempolicy/cpusets,
814and those nodes become memory exhaustion status, one process
815may be killed by oom-killer. No panic occurs in this case.
816Because other nodes' memory may be free. This means system total status
817may be not fatal yet.
818
819If this is set to 2, the kernel panics compulsorily even on the
820above-mentioned. Even oom happens under memory cgroup, the whole
821system panics.
822
823The default value is 0.
824
8251 and 2 are for failover of clustering. Please select either
826according to your policy of failover.
827
828panic_on_oom=2+kdump gives you very strong tool to investigate
829why oom happens. You can get snapshot.
830
831
832percpu_pagelist_high_fraction
833=============================
834
835This is the fraction of pages in each zone that are can be stored to
836per-cpu page lists. It is an upper boundary that is divided depending
837on the number of online CPUs. The min value for this is 8 which means
838that we do not allow more than 1/8th of pages in each zone to be stored
839on per-cpu page lists. This entry only changes the value of hot per-cpu
840page lists. A user can specify a number like 100 to allocate 1/100th of
841each zone between per-cpu lists.
842
843The batch value of each per-cpu page list remains the same regardless of
844the value of the high fraction so allocation latencies are unaffected.
845
846The initial value is zero. Kernel uses this value to set the high pcp->high
847mark based on the low watermark for the zone and the number of local
848online CPUs. If the user writes '0' to this sysctl, it will revert to
849this default behavior.
850
851
852stat_interval
853=============
854
855The time interval between which vm statistics are updated. The default
856is 1 second.
857
858
859stat_refresh
860============
861
862Any read or write (by root only) flushes all the per-cpu vm statistics
863into their global totals, for more accurate reports when testing
864e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
865
866As a side-effect, it also checks for negative totals (elsewhere reported
867as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
868(At time of writing, a few stats are known sometimes to be found negative,
869with no ill effects: errors and warnings on these stats are suppressed.)
870
871
872numa_stat
873=========
874
875This interface allows runtime configuration of numa statistics.
876
877When page allocation performance becomes a bottleneck and you can tolerate
878some possible tool breakage and decreased numa counter precision, you can
879do::
880
881 echo 0 > /proc/sys/vm/numa_stat
882
883When page allocation performance is not a bottleneck and you want all
884tooling to work, you can do::
885
886 echo 1 > /proc/sys/vm/numa_stat
887
888
889swappiness
890==========
891
892This control is used to define the rough relative IO cost of swapping
893and filesystem paging, as a value between 0 and 200. At 100, the VM
894assumes equal IO cost and will thus apply memory pressure to the page
895cache and swap-backed pages equally; lower values signify more
896expensive swap IO, higher values indicates cheaper.
897
898Keep in mind that filesystem IO patterns under memory pressure tend to
899be more efficient than swap's random IO. An optimal value will require
900experimentation and will also be workload-dependent.
901
902The default value is 60.
903
904For in-memory swap, like zram or zswap, as well as hybrid setups that
905have swap on faster devices than the filesystem, values beyond 100 can
906be considered. For example, if the random IO against the swap device
907is on average 2x faster than IO from the filesystem, swappiness should
908be 133 (x + 2x = 200, 2x = 133.33).
909
910At 0, the kernel will not initiate swap until the amount of free and
911file-backed pages is less than the high watermark in a zone.
912
913
914unprivileged_userfaultfd
915========================
916
917This flag controls the mode in which unprivileged users can use the
918userfaultfd system calls. Set this to 0 to restrict unprivileged users
919to handle page faults in user mode only. In this case, users without
920SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to
921succeed. Prohibiting use of userfaultfd for handling faults from kernel
922mode may make certain vulnerabilities more difficult to exploit.
923
924Set this to 1 to allow unprivileged users to use the userfaultfd system
925calls without any restrictions.
926
927The default value is 0.
928
929Another way to control permissions for userfaultfd is to use
930/dev/userfaultfd instead of userfaultfd(2). See
931Documentation/admin-guide/mm/userfaultfd.rst.
932
933user_reserve_kbytes
934===================
935
936When overcommit_memory is set to 2, "never overcommit" mode, reserve
937min(3% of current process size, user_reserve_kbytes) of free memory.
938This is intended to prevent a user from starting a single memory hogging
939process, such that they cannot recover (kill the hog).
940
941user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
942
943If this is reduced to zero, then the user will be allowed to allocate
944all free memory with a single process, minus admin_reserve_kbytes.
945Any subsequent attempts to execute a command will result in
946"fork: Cannot allocate memory".
947
948Changing this takes effect whenever an application requests memory.
949
950
951vfs_cache_pressure
952==================
953
954This percentage value controls the tendency of the kernel to reclaim
955the memory which is used for caching of directory and inode objects.
956
957At the default value of vfs_cache_pressure=100 the kernel will attempt to
958reclaim dentries and inodes at a "fair" rate with respect to pagecache and
959swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer
960to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
961never reclaim dentries and inodes due to memory pressure and this can easily
962lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
963causes the kernel to prefer to reclaim dentries and inodes.
964
965Increasing vfs_cache_pressure significantly beyond 100 may have negative
966performance impact. Reclaim code needs to take various locks to find freeable
967directory and inode objects. With vfs_cache_pressure=1000, it will look for
968ten times more freeable objects than there are.
969
970
971watermark_boost_factor
972======================
973
974This factor controls the level of reclaim when memory is being fragmented.
975It defines the percentage of the high watermark of a zone that will be
976reclaimed if pages of different mobility are being mixed within pageblocks.
977The intent is that compaction has less work to do in the future and to
978increase the success rate of future high-order allocations such as SLUB
979allocations, THP and hugetlbfs pages.
980
981To make it sensible with respect to the watermark_scale_factor
982parameter, the unit is in fractions of 10,000. The default value of
98315,000 means that up to 150% of the high watermark will be reclaimed in the
984event of a pageblock being mixed due to fragmentation. The level of reclaim
985is determined by the number of fragmentation events that occurred in the
986recent past. If this value is smaller than a pageblock then a pageblocks
987worth of pages will be reclaimed (e.g. 2MB on 64-bit x86). A boost factor
988of 0 will disable the feature.
989
990
991watermark_scale_factor
992======================
993
994This factor controls the aggressiveness of kswapd. It defines the
995amount of memory left in a node/system before kswapd is woken up and
996how much memory needs to be free before kswapd goes back to sleep.
997
998The unit is in fractions of 10,000. The default value of 10 means the
999distances between watermarks are 0.1% of the available memory in the
1000node/system. The maximum value is 3000, or 30% of memory.
1001
1002A high rate of threads entering direct reclaim (allocstall) or kswapd
1003going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
1004that the number of free pages kswapd maintains for latency reasons is
1005too small for the allocation bursts occurring in the system. This knob
1006can then be used to tune kswapd aggressiveness accordingly.
1007
1008
1009zone_reclaim_mode
1010=================
1011
1012Zone_reclaim_mode allows someone to set more or less aggressive approaches to
1013reclaim memory when a zone runs out of memory. If it is set to zero then no
1014zone reclaim occurs. Allocations will be satisfied from other zones / nodes
1015in the system.
1016
1017This is value OR'ed together of
1018
1019= ===================================
10201 Zone reclaim on
10212 Zone reclaim writes dirty pages out
10224 Zone reclaim swaps pages
1023= ===================================
1024
1025zone_reclaim_mode is disabled by default. For file servers or workloads
1026that benefit from having their data cached, zone_reclaim_mode should be
1027left disabled as the caching effect is likely to be more important than
1028data locality.
1029
1030Consider enabling one or more zone_reclaim mode bits if it's known that the
1031workload is partitioned such that each partition fits within a NUMA node
1032and that accessing remote memory would cause a measurable performance
1033reduction. The page allocator will take additional actions before
1034allocating off node pages.
1035
1036Allowing zone reclaim to write out pages stops processes that are
1037writing large amounts of data from dirtying pages on other nodes. Zone
1038reclaim will write out dirty pages if a zone fills up and so effectively
1039throttle the process. This may decrease the performance of a single process
1040since it cannot use all of system memory to buffer the outgoing writes
1041anymore but it preserve the memory on other nodes so that the performance
1042of other processes running on other nodes will not be affected.
1043
1044Allowing regular swap effectively restricts allocations to the local
1045node unless explicitly overridden by memory policies or cpuset
1046configurations.