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Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.

1	Documentation for /proc/sys/vm/*	kernel version 2.6.29
2		(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
3		(c) 2008         Peter W. Morreale <pmorreale@novell.com>
5	For general info and legal blurb, please look in README.
7	==============================================================
9	This file contains the documentation for the sysctl files in
10	/proc/sys/vm and is valid for Linux kernel version 2.6.29.
12	The files in this directory can be used to tune the operation
13	of the virtual memory (VM) subsystem of the Linux kernel and
14	the writeout of dirty data to disk.
16	Default values and initialization routines for most of these
17	files can be found in mm/swap.c.
19	Currently, these files are in /proc/sys/vm:
21	- admin_reserve_kbytes
22	- block_dump
23	- compact_memory
24	- compact_unevictable_allowed
25	- dirty_background_bytes
26	- dirty_background_ratio
27	- dirty_bytes
28	- dirty_expire_centisecs
29	- dirty_ratio
30	- dirty_writeback_centisecs
31	- drop_caches
32	- extfrag_threshold
33	- hugepages_treat_as_movable
34	- hugetlb_shm_group
35	- laptop_mode
36	- legacy_va_layout
37	- lowmem_reserve_ratio
38	- max_map_count
39	- memory_failure_early_kill
40	- memory_failure_recovery
41	- min_free_kbytes
42	- min_slab_ratio
43	- min_unmapped_ratio
44	- mmap_min_addr
45	- mmap_rnd_bits
46	- mmap_rnd_compat_bits
47	- nr_hugepages
48	- nr_overcommit_hugepages
49	- nr_trim_pages         (only if CONFIG_MMU=n)
50	- numa_zonelist_order
51	- oom_dump_tasks
52	- oom_kill_allocating_task
53	- overcommit_kbytes
54	- overcommit_memory
55	- overcommit_ratio
56	- page-cluster
57	- panic_on_oom
58	- percpu_pagelist_fraction
59	- stat_interval
60	- stat_refresh
61	- swappiness
62	- user_reserve_kbytes
63	- vfs_cache_pressure
64	- watermark_scale_factor
65	- zone_reclaim_mode
67	==============================================================
69	admin_reserve_kbytes
71	The amount of free memory in the system that should be reserved for users
72	with the capability cap_sys_admin.
74	admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
76	That should provide enough for the admin to log in and kill a process,
77	if necessary, under the default overcommit 'guess' mode.
79	Systems running under overcommit 'never' should increase this to account
80	for the full Virtual Memory Size of programs used to recover. Otherwise,
81	root may not be able to log in to recover the system.
83	How do you calculate a minimum useful reserve?
85	sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
87	For overcommit 'guess', we can sum resident set sizes (RSS).
88	On x86_64 this is about 8MB.
90	For overcommit 'never', we can take the max of their virtual sizes (VSZ)
91	and add the sum of their RSS.
92	On x86_64 this is about 128MB.
94	Changing this takes effect whenever an application requests memory.
96	==============================================================
98	block_dump
100	block_dump enables block I/O debugging when set to a nonzero value. More
101	information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
103	==============================================================
105	compact_memory
107	Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
108	all zones are compacted such that free memory is available in contiguous
109	blocks where possible. This can be important for example in the allocation of
110	huge pages although processes will also directly compact memory as required.
112	==============================================================
114	compact_unevictable_allowed
116	Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
117	allowed to examine the unevictable lru (mlocked pages) for pages to compact.
118	This should be used on systems where stalls for minor page faults are an
119	acceptable trade for large contiguous free memory.  Set to 0 to prevent
120	compaction from moving pages that are unevictable.  Default value is 1.
122	==============================================================
124	dirty_background_bytes
126	Contains the amount of dirty memory at which the background kernel
127	flusher threads will start writeback.
129	Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
130	one of them may be specified at a time. When one sysctl is written it is
131	immediately taken into account to evaluate the dirty memory limits and the
132	other appears as 0 when read.
134	==============================================================
136	dirty_background_ratio
138	Contains, as a percentage of total available memory that contains free pages
139	and reclaimable pages, the number of pages at which the background kernel
140	flusher threads will start writing out dirty data.
142	The total available memory is not equal to total system memory.
144	==============================================================
146	dirty_bytes
148	Contains the amount of dirty memory at which a process generating disk writes
149	will itself start writeback.
151	Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
152	specified at a time. When one sysctl is written it is immediately taken into
153	account to evaluate the dirty memory limits and the other appears as 0 when
154	read.
156	Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
157	value lower than this limit will be ignored and the old configuration will be
158	retained.
160	==============================================================
162	dirty_expire_centisecs
164	This tunable is used to define when dirty data is old enough to be eligible
165	for writeout by the kernel flusher threads.  It is expressed in 100'ths
166	of a second.  Data which has been dirty in-memory for longer than this
167	interval will be written out next time a flusher thread wakes up.
169	==============================================================
171	dirty_ratio
173	Contains, as a percentage of total available memory that contains free pages
174	and reclaimable pages, the number of pages at which a process which is
175	generating disk writes will itself start writing out dirty data.
177	The total available memory is not equal to total system memory.
179	==============================================================
181	dirty_writeback_centisecs
183	The kernel flusher threads will periodically wake up and write `old' data
184	out to disk.  This tunable expresses the interval between those wakeups, in
185	100'ths of a second.
187	Setting this to zero disables periodic writeback altogether.
189	==============================================================
191	drop_caches
193	Writing to this will cause the kernel to drop clean caches, as well as
194	reclaimable slab objects like dentries and inodes.  Once dropped, their
195	memory becomes free.
197	To free pagecache:
198		echo 1 > /proc/sys/vm/drop_caches
199	To free reclaimable slab objects (includes dentries and inodes):
200		echo 2 > /proc/sys/vm/drop_caches
201	To free slab objects and pagecache:
202		echo 3 > /proc/sys/vm/drop_caches
204	This is a non-destructive operation and will not free any dirty objects.
205	To 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
207	number of dirty objects on the system and create more candidates to be
208	dropped.
210	This file is not a means to control the growth of the various kernel caches
211	(inodes, dentries, pagecache, etc...)  These objects are automatically
212	reclaimed by the kernel when memory is needed elsewhere on the system.
214	Use of this file can cause performance problems.  Since it discards cached
215	objects, it may cost a significant amount of I/O and CPU to recreate the
216	dropped objects, especially if they were under heavy use.  Because of this,
217	use outside of a testing or debugging environment is not recommended.
219	You may see informational messages in your kernel log when this file is
220	used:
222		cat (1234): drop_caches: 3
224	These are informational only.  They do not mean that anything is wrong
225	with your system.  To disable them, echo 4 (bit 3) into drop_caches.
227	==============================================================
229	extfrag_threshold
231	This parameter affects whether the kernel will compact memory or direct
232	reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
233	debugfs shows what the fragmentation index for each order is in each zone in
234	the system. Values tending towards 0 imply allocations would fail due to lack
235	of memory, values towards 1000 imply failures are due to fragmentation and -1
236	implies that the allocation will succeed as long as watermarks are met.
238	The kernel will not compact memory in a zone if the
239	fragmentation index is <= extfrag_threshold. The default value is 500.
241	==============================================================
243	hugepages_treat_as_movable
245	This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
246	or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
247	ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
248	so this parameter has no effect if used without kernelcore=.
250	Hugepage migration is now available in some situations which depend on the
251	architecture and/or the hugepage size. If a hugepage supports migration,
252	allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
253	of the value of this parameter.
254	IOW, this parameter affects only non-migratable hugepages.
256	Assuming that hugepages are not migratable in your system, one usecase of
257	this parameter is that users can make hugepage pool more extensible by
258	enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
259	page reclaim/migration/compaction work more and you can get contiguous
260	memory more likely. Note that using ZONE_MOVABLE for non-migratable
261	hugepages can do harm to other features like memory hotremove (because
262	memory hotremove expects that memory blocks on ZONE_MOVABLE are always
263	removable,) so it's a trade-off responsible for the users.
265	==============================================================
267	hugetlb_shm_group
269	hugetlb_shm_group contains group id that is allowed to create SysV
270	shared memory segment using hugetlb page.
272	==============================================================
274	laptop_mode
276	laptop_mode is a knob that controls "laptop mode". All the things that are
277	controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
279	==============================================================
281	legacy_va_layout
283	If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
284	will use the legacy (2.4) layout for all processes.
286	==============================================================
288	lowmem_reserve_ratio
290	For some specialised workloads on highmem machines it is dangerous for
291	the kernel to allow process memory to be allocated from the "lowmem"
292	zone.  This is because that memory could then be pinned via the mlock()
293	system call, or by unavailability of swapspace.
295	And on large highmem machines this lack of reclaimable lowmem memory
296	can be fatal.
298	So the Linux page allocator has a mechanism which prevents allocations
299	which _could_ use highmem from using too much lowmem.  This means that
300	a certain amount of lowmem is defended from the possibility of being
301	captured into pinned user memory.
303	(The same argument applies to the old 16 megabyte ISA DMA region.  This
304	mechanism will also defend that region from allocations which could use
305	highmem or lowmem).
307	The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
308	in defending these lower zones.
310	If you have a machine which uses highmem or ISA DMA and your
311	applications are using mlock(), or if you are running with no swap then
312	you probably should change the lowmem_reserve_ratio setting.
314	The lowmem_reserve_ratio is an array. You can see them by reading this file.
315	-
316	% cat /proc/sys/vm/lowmem_reserve_ratio
317	256     256     32
318	-
319	Note: # of this elements is one fewer than number of zones. Because the highest
320	      zone's value is not necessary for following calculation.
322	But, these values are not used directly. The kernel calculates # of protection
323	pages for each zones from them. These are shown as array of protection pages
324	in /proc/zoneinfo like followings. (This is an example of x86-64 box).
325	Each zone has an array of protection pages like this.
327	-
328	Node 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	-
342	These protections are added to score to judge whether this zone should be used
343	for page allocation or should be reclaimed.
345	In this example, if normal pages (index=2) are required to this DMA zone and
346	watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
347	not be used because pages_free(1355) is smaller than watermark + protection[2]
348	(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
349	normal page requirement. If requirement is DMA zone(index=0), protection[0]
350	(=0) is used.
352	zone[i]'s protection[j] is calculated by following expression.
354	(i < j):
355	  zone[i]->protection[j]
356	  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
357	    / lowmem_reserve_ratio[i];
358	(i = j):
359	   (should not be protected. = 0;
360	(i > j):
361	   (not necessary, but looks 0)
363	The default values of lowmem_reserve_ratio[i] are
364	    256 (if zone[i] means DMA or DMA32 zone)
365	    32  (others).
366	As above expression, they are reciprocal number of ratio.
367	256 means 1/256. # of protection pages becomes about "0.39%" of total managed
368	pages of higher zones on the node.
370	If you would like to protect more pages, smaller values are effective.
371	The minimum value is 1 (1/1 -> 100%).
373	==============================================================
375	max_map_count:
377	This file contains the maximum number of memory map areas a process
378	may have. Memory map areas are used as a side-effect of calling
379	malloc, directly by mmap and mprotect, and also when loading shared
380	libraries.
382	While most applications need less than a thousand maps, certain
383	programs, particularly malloc debuggers, may consume lots of them,
384	e.g., up to one or two maps per allocation.
386	The default value is 65536.
388	=============================================================
390	memory_failure_early_kill:
392	Control how to kill processes when uncorrected memory error (typically
393	a 2bit error in a memory module) is detected in the background by hardware
394	that cannot be handled by the kernel. In some cases (like the page
395	still having a valid copy on disk) the kernel will handle the failure
396	transparently without affecting any applications. But if there is
397	no other uptodate copy of the data it will kill to prevent any data
398	corruptions from propagating.
400	1: Kill all processes that have the corrupted and not reloadable page mapped
401	as soon as the corruption is detected.  Note this is not supported
402	for a few types of pages, like kernel internally allocated data or
403	the swap cache, but works for the majority of user pages.
405	0: Only unmap the corrupted page from all processes and only kill a process
406	who tries to access it.
408	The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
409	handle this if they want to.
411	This is only active on architectures/platforms with advanced machine
412	check handling and depends on the hardware capabilities.
414	Applications can override this setting individually with the PR_MCE_KILL prctl
416	==============================================================
418	memory_failure_recovery
420	Enable memory failure recovery (when supported by the platform)
422	1: Attempt recovery.
424	0: Always panic on a memory failure.
426	==============================================================
428	min_free_kbytes:
430	This is used to force the Linux VM to keep a minimum number
431	of kilobytes free.  The VM uses this number to compute a
432	watermark[WMARK_MIN] value for each lowmem zone in the system.
433	Each lowmem zone gets a number of reserved free pages based
434	proportionally on its size.
436	Some minimal amount of memory is needed to satisfy PF_MEMALLOC
437	allocations; if you set this to lower than 1024KB, your system will
438	become subtly broken, and prone to deadlock under high loads.
440	Setting this too high will OOM your machine instantly.
442	=============================================================
444	min_slab_ratio:
446	This is available only on NUMA kernels.
448	A percentage of the total pages in each zone.  On Zone reclaim
449	(fallback from the local zone occurs) slabs will be reclaimed if more
450	than this percentage of pages in a zone are reclaimable slab pages.
451	This insures that the slab growth stays under control even in NUMA
452	systems that rarely perform global reclaim.
454	The default is 5 percent.
456	Note that slab reclaim is triggered in a per zone / node fashion.
457	The process of reclaiming slab memory is currently not node specific
458	and may not be fast.
460	=============================================================
462	min_unmapped_ratio:
464	This is available only on NUMA kernels.
466	This is a percentage of the total pages in each zone. Zone reclaim will
467	only occur if more than this percentage of pages are in a state that
468	zone_reclaim_mode allows to be reclaimed.
470	If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
471	against all file-backed unmapped pages including swapcache pages and tmpfs
472	files. Otherwise, only unmapped pages backed by normal files but not tmpfs
473	files and similar are considered.
475	The default is 1 percent.
477	==============================================================
479	mmap_min_addr
481	This file indicates the amount of address space  which a user process will
482	be restricted from mmapping.  Since kernel null dereference bugs could
483	accidentally operate based on the information in the first couple of pages
484	of memory userspace processes should not be allowed to write to them.  By
485	default this value is set to 0 and no protections will be enforced by the
486	security module.  Setting this value to something like 64k will allow the
487	vast majority of applications to work correctly and provide defense in depth
488	against future potential kernel bugs.
490	==============================================================
492	mmap_rnd_bits:
494	This value can be used to select the number of bits to use to
495	determine the random offset to the base address of vma regions
496	resulting from mmap allocations on architectures which support
497	tuning address space randomization.  This value will be bounded
498	by the architecture's minimum and maximum supported values.
500	This value can be changed after boot using the
501	/proc/sys/vm/mmap_rnd_bits tunable
503	==============================================================
505	mmap_rnd_compat_bits:
507	This value can be used to select the number of bits to use to
508	determine the random offset to the base address of vma regions
509	resulting from mmap allocations for applications run in
510	compatibility mode on architectures which support tuning address
511	space randomization.  This value will be bounded by the
512	architecture's minimum and maximum supported values.
514	This value can be changed after boot using the
515	/proc/sys/vm/mmap_rnd_compat_bits tunable
517	==============================================================
519	nr_hugepages
521	Change the minimum size of the hugepage pool.
523	See Documentation/vm/hugetlbpage.txt
525	==============================================================
527	nr_overcommit_hugepages
529	Change the maximum size of the hugepage pool. The maximum is
530	nr_hugepages + nr_overcommit_hugepages.
532	See Documentation/vm/hugetlbpage.txt
534	==============================================================
536	nr_trim_pages
538	This is available only on NOMMU kernels.
540	This value adjusts the excess page trimming behaviour of power-of-2 aligned
541	NOMMU mmap allocations.
543	A value of 0 disables trimming of allocations entirely, while a value of 1
544	trims excess pages aggressively. Any value >= 1 acts as the watermark where
545	trimming of allocations is initiated.
547	The default value is 1.
549	See Documentation/nommu-mmap.txt for more information.
551	==============================================================
553	numa_zonelist_order
555	This 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...)
560	In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
562	This means that a memory allocation request for GFP_KERNEL will
563	get memory from ZONE_DMA only when ZONE_NORMAL is not available.
565	In NUMA case, you can think of following 2 types of order.
566	Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
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.
571	Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
572	will be used before ZONE_NORMAL exhaustion. This increases possibility of
573	out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
575	Type(B) cannot offer the best locality but is more robust against OOM of
576	the DMA zone.
578	Type(A) is called as "Node" order. Type (B) is "Zone" order.
580	"Node order" orders the zonelists by node, then by zone within each node.
581	Specify "[Nn]ode" for node order
583	"Zone Order" orders the zonelists by zone type, then by node within each
584	zone.  Specify "[Zz]one" for zone order.
586	Specify "[Dd]efault" to request automatic configuration.
588	On 32-bit, the Normal zone needs to be preserved for allocations accessible
589	by the kernel, so "zone" order will be selected.
591	On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
592	order will be selected.
594	Default order is recommended unless this is causing problems for your
595	system/application.
597	==============================================================
599	oom_dump_tasks
601	Enables a system-wide task dump (excluding kernel threads) to be produced
602	when the kernel performs an OOM-killing and includes such information as
603	pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj
604	score, and name.  This is helpful to determine why the OOM killer was
605	invoked, to identify the rogue task that caused it, and to determine why
606	the OOM killer chose the task it did to kill.
608	If this is set to zero, this information is suppressed.  On very
609	large systems with thousands of tasks it may not be feasible to dump
610	the memory state information for each one.  Such systems should not
611	be forced to incur a performance penalty in OOM conditions when the
612	information may not be desired.
614	If this is set to non-zero, this information is shown whenever the
615	OOM killer actually kills a memory-hogging task.
617	The default value is 1 (enabled).
619	==============================================================
621	oom_kill_allocating_task
623	This enables or disables killing the OOM-triggering task in
624	out-of-memory situations.
626	If this is set to zero, the OOM killer will scan through the entire
627	tasklist and select a task based on heuristics to kill.  This normally
628	selects a rogue memory-hogging task that frees up a large amount of
629	memory when killed.
631	If this is set to non-zero, the OOM killer simply kills the task that
632	triggered the out-of-memory condition.  This avoids the expensive
633	tasklist scan.
635	If panic_on_oom is selected, it takes precedence over whatever value
636	is used in oom_kill_allocating_task.
638	The default value is 0.
640	==============================================================
642	overcommit_kbytes:
644	When overcommit_memory is set to 2, the committed address space is not
645	permitted to exceed swap plus this amount of physical RAM. See below.
647	Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
648	of them may be specified at a time. Setting one disables the other (which
649	then appears as 0 when read).
651	==============================================================
653	overcommit_memory:
655	This value contains a flag that enables memory overcommitment.
657	When this flag is 0, the kernel attempts to estimate the amount
658	of free memory left when userspace requests more memory.
660	When this flag is 1, the kernel pretends there is always enough
661	memory until it actually runs out.
663	When this flag is 2, the kernel uses a "never overcommit"
664	policy that attempts to prevent any overcommit of memory.
665	Note that user_reserve_kbytes affects this policy.
667	This feature can be very useful because there are a lot of
668	programs that malloc() huge amounts of memory "just-in-case"
669	and don't use much of it.
671	The default value is 0.
673	See Documentation/vm/overcommit-accounting and
674	mm/mmap.c::__vm_enough_memory() for more information.
676	==============================================================
678	overcommit_ratio:
680	When overcommit_memory is set to 2, the committed address
681	space is not permitted to exceed swap plus this percentage
682	of physical RAM.  See above.
684	==============================================================
686	page-cluster
688	page-cluster controls the number of pages up to which consecutive pages
689	are read in from swap in a single attempt. This is the swap counterpart
690	to page cache readahead.
691	The mentioned consecutivity is not in terms of virtual/physical addresses,
692	but consecutive on swap space - that means they were swapped out together.
694	It is a logarithmic value - setting it to zero means "1 page", setting
695	it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
696	Zero disables swap readahead completely.
698	The default value is three (eight pages at a time).  There may be some
699	small benefits in tuning this to a different value if your workload is
700	swap-intensive.
702	Lower values mean lower latencies for initial faults, but at the same time
703	extra faults and I/O delays for following faults if they would have been part of
704	that consecutive pages readahead would have brought in.
706	=============================================================
708	panic_on_oom
710	This enables or disables panic on out-of-memory feature.
712	If this is set to 0, the kernel will kill some rogue process,
713	called oom_killer.  Usually, oom_killer can kill rogue processes and
714	system will survive.
716	If this is set to 1, the kernel panics when out-of-memory happens.
717	However, if a process limits using nodes by mempolicy/cpusets,
718	and those nodes become memory exhaustion status, one process
719	may be killed by oom-killer. No panic occurs in this case.
720	Because other nodes' memory may be free. This means system total status
721	may be not fatal yet.
723	If this is set to 2, the kernel panics compulsorily even on the
724	above-mentioned. Even oom happens under memory cgroup, the whole
725	system panics.
727	The default value is 0.
728	1 and 2 are for failover of clustering. Please select either
729	according to your policy of failover.
730	panic_on_oom=2+kdump gives you very strong tool to investigate
731	why oom happens. You can get snapshot.
733	=============================================================
735	percpu_pagelist_fraction
737	This is the fraction of pages at most (high mark pcp->high) in each zone that
738	are allocated for each per cpu page list.  The min value for this is 8.  It
739	means that we don't allow more than 1/8th of pages in each zone to be
740	allocated in any single per_cpu_pagelist.  This entry only changes the value
741	of hot per cpu pagelists.  User can specify a number like 100 to allocate
742	1/100th of each zone to each per cpu page list.
744	The batch value of each per cpu pagelist is also updated as a result.  It is
745	set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
747	The initial value is zero.  Kernel does not use this value at boot time to set
748	the high water marks for each per cpu page list.  If the user writes '0' to this
749	sysctl, it will revert to this default behavior.
751	==============================================================
753	stat_interval
755	The time interval between which vm statistics are updated.  The default
756	is 1 second.
758	==============================================================
760	stat_refresh
762	Any read or write (by root only) flushes all the per-cpu vm statistics
763	into their global totals, for more accurate reports when testing
764	e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
766	As a side-effect, it also checks for negative totals (elsewhere reported
767	as 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,
769	with no ill effects: errors and warnings on these stats are suppressed.)
771	==============================================================
773	swappiness
775	This control is used to define how aggressive the kernel will swap
776	memory pages.  Higher values will increase agressiveness, lower values
777	decrease the amount of swap.  A value of 0 instructs the kernel not to
778	initiate swap until the amount of free and file-backed pages is less
779	than the high water mark in a zone.
781	The default value is 60.
783	==============================================================
785	- user_reserve_kbytes
787	When overcommit_memory is set to 2, "never overcommit" mode, reserve
788	min(3% of current process size, user_reserve_kbytes) of free memory.
789	This is intended to prevent a user from starting a single memory hogging
790	process, such that they cannot recover (kill the hog).
792	user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
794	If this is reduced to zero, then the user will be allowed to allocate
795	all free memory with a single process, minus admin_reserve_kbytes.
796	Any subsequent attempts to execute a command will result in
797	"fork: Cannot allocate memory".
799	Changing this takes effect whenever an application requests memory.
801	==============================================================
803	vfs_cache_pressure
804	------------------
806	This percentage value controls the tendency of the kernel to reclaim
807	the memory which is used for caching of directory and inode objects.
809	At the default value of vfs_cache_pressure=100 the kernel will attempt to
810	reclaim dentries and inodes at a "fair" rate with respect to pagecache and
811	swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
812	to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
813	never reclaim dentries and inodes due to memory pressure and this can easily
814	lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
815	causes the kernel to prefer to reclaim dentries and inodes.
817	Increasing vfs_cache_pressure significantly beyond 100 may have negative
818	performance impact. Reclaim code needs to take various locks to find freeable
819	directory and inode objects. With vfs_cache_pressure=1000, it will look for
820	ten times more freeable objects than there are.
822	=============================================================
824	watermark_scale_factor:
826	This factor controls the aggressiveness of kswapd. It defines the
827	amount of memory left in a node/system before kswapd is woken up and
828	how much memory needs to be free before kswapd goes back to sleep.
830	The unit is in fractions of 10,000. The default value of 10 means the
831	distances between watermarks are 0.1% of the available memory in the
832	node/system. The maximum value is 1000, or 10% of memory.
834	A high rate of threads entering direct reclaim (allocstall) or kswapd
835	going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
836	that the number of free pages kswapd maintains for latency reasons is
837	too small for the allocation bursts occurring in the system. This knob
838	can then be used to tune kswapd aggressiveness accordingly.
840	==============================================================
842	zone_reclaim_mode:
844	Zone_reclaim_mode allows someone to set more or less aggressive approaches to
845	reclaim memory when a zone runs out of memory. If it is set to zero then no
846	zone reclaim occurs. Allocations will be satisfied from other zones / nodes
847	in the system.
849	This is value ORed together of
851	1	= Zone reclaim on
852	2	= Zone reclaim writes dirty pages out
853	4	= Zone reclaim swaps pages
855	zone_reclaim_mode is disabled by default.  For file servers or workloads
856	that benefit from having their data cached, zone_reclaim_mode should be
857	left disabled as the caching effect is likely to be more important than
858	data locality.
860	zone_reclaim may be enabled if it's known that the workload is partitioned
861	such that each partition fits within a NUMA node and that accessing remote
862	memory would cause a measurable performance reduction.  The page allocator
863	will then reclaim easily reusable pages (those page cache pages that are
864	currently not used) before allocating off node pages.
866	Allowing zone reclaim to write out pages stops processes that are
867	writing large amounts of data from dirtying pages on other nodes. Zone
868	reclaim will write out dirty pages if a zone fills up and so effectively
869	throttle the process. This may decrease the performance of a single process
870	since it cannot use all of system memory to buffer the outgoing writes
871	anymore but it preserve the memory on other nodes so that the performance
872	of other processes running on other nodes will not be affected.
874	Allowing regular swap effectively restricts allocations to the local
875	node unless explicitly overridden by memory policies or cpuset
876	configurations.
878	============ End of Document =================================
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