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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 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	- zone_reclaim_mode
66	==============================================================
68	admin_reserve_kbytes
70	The amount of free memory in the system that should be reserved for users
71	with the capability cap_sys_admin.
73	admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
75	That should provide enough for the admin to log in and kill a process,
76	if necessary, under the default overcommit 'guess' mode.
78	Systems running under overcommit 'never' should increase this to account
79	for the full Virtual Memory Size of programs used to recover. Otherwise,
80	root may not be able to log in to recover the system.
82	How do you calculate a minimum useful reserve?
84	sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
86	For overcommit 'guess', we can sum resident set sizes (RSS).
87	On x86_64 this is about 8MB.
89	For overcommit 'never', we can take the max of their virtual sizes (VSZ)
90	and add the sum of their RSS.
91	On x86_64 this is about 128MB.
93	Changing this takes effect whenever an application requests memory.
95	==============================================================
97	block_dump
99	block_dump enables block I/O debugging when set to a nonzero value. More
100	information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
102	==============================================================
104	compact_memory
106	Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
107	all zones are compacted such that free memory is available in contiguous
108	blocks where possible. This can be important for example in the allocation of
109	huge pages although processes will also directly compact memory as required.
111	==============================================================
113	compact_unevictable_allowed
115	Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
116	allowed to examine the unevictable lru (mlocked pages) for pages to compact.
117	This should be used on systems where stalls for minor page faults are an
118	acceptable trade for large contiguous free memory.  Set to 0 to prevent
119	compaction from moving pages that are unevictable.  Default value is 1.
121	==============================================================
123	dirty_background_bytes
125	Contains the amount of dirty memory at which the background kernel
126	flusher threads will start writeback.
128	Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
129	one of them may be specified at a time. When one sysctl is written it is
130	immediately taken into account to evaluate the dirty memory limits and the
131	other appears as 0 when read.
133	==============================================================
135	dirty_background_ratio
137	Contains, as a percentage of total available memory that contains free pages
138	and reclaimable pages, the number of pages at which the background kernel
139	flusher threads will start writing out dirty data.
141	The total available memory is not equal to total system memory.
143	==============================================================
145	dirty_bytes
147	Contains the amount of dirty memory at which a process generating disk writes
148	will itself start writeback.
150	Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
151	specified at a time. When one sysctl is written it is immediately taken into
152	account to evaluate the dirty memory limits and the other appears as 0 when
153	read.
155	Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
156	value lower than this limit will be ignored and the old configuration will be
157	retained.
159	==============================================================
161	dirty_expire_centisecs
163	This tunable is used to define when dirty data is old enough to be eligible
164	for writeout by the kernel flusher threads.  It is expressed in 100'ths
165	of a second.  Data which has been dirty in-memory for longer than this
166	interval will be written out next time a flusher thread wakes up.
168	==============================================================
170	dirty_ratio
172	Contains, as a percentage of total available memory that contains free pages
173	and reclaimable pages, the number of pages at which a process which is
174	generating disk writes will itself start writing out dirty data.
176	The total available memory is not equal to total system memory.
178	==============================================================
180	dirty_writeback_centisecs
182	The kernel flusher threads will periodically wake up and write `old' data
183	out to disk.  This tunable expresses the interval between those wakeups, in
184	100'ths of a second.
186	Setting this to zero disables periodic writeback altogether.
188	==============================================================
190	drop_caches
192	Writing to this will cause the kernel to drop clean caches, as well as
193	reclaimable slab objects like dentries and inodes.  Once dropped, their
194	memory becomes free.
196	To free pagecache:
197		echo 1 > /proc/sys/vm/drop_caches
198	To free reclaimable slab objects (includes dentries and inodes):
199		echo 2 > /proc/sys/vm/drop_caches
200	To free slab objects and pagecache:
201		echo 3 > /proc/sys/vm/drop_caches
203	This is a non-destructive operation and will not free any dirty objects.
204	To increase the number of objects freed by this operation, the user may run
205	`sync' prior to writing to /proc/sys/vm/drop_caches.  This will minimize the
206	number of dirty objects on the system and create more candidates to be
207	dropped.
209	This file is not a means to control the growth of the various kernel caches
210	(inodes, dentries, pagecache, etc...)  These objects are automatically
211	reclaimed by the kernel when memory is needed elsewhere on the system.
213	Use of this file can cause performance problems.  Since it discards cached
214	objects, it may cost a significant amount of I/O and CPU to recreate the
215	dropped objects, especially if they were under heavy use.  Because of this,
216	use outside of a testing or debugging environment is not recommended.
218	You may see informational messages in your kernel log when this file is
219	used:
221		cat (1234): drop_caches: 3
223	These are informational only.  They do not mean that anything is wrong
224	with your system.  To disable them, echo 4 (bit 3) into drop_caches.
226	==============================================================
228	extfrag_threshold
230	This parameter affects whether the kernel will compact memory or direct
231	reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
232	debugfs shows what the fragmentation index for each order is in each zone in
233	the system. Values tending towards 0 imply allocations would fail due to lack
234	of memory, values towards 1000 imply failures are due to fragmentation and -1
235	implies that the allocation will succeed as long as watermarks are met.
237	The kernel will not compact memory in a zone if the
238	fragmentation index is <= extfrag_threshold. The default value is 500.
240	==============================================================
242	hugepages_treat_as_movable
244	This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
245	or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
246	ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
247	so this parameter has no effect if used without kernelcore=.
249	Hugepage migration is now available in some situations which depend on the
250	architecture and/or the hugepage size. If a hugepage supports migration,
251	allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
252	of the value of this parameter.
253	IOW, this parameter affects only non-migratable hugepages.
255	Assuming that hugepages are not migratable in your system, one usecase of
256	this parameter is that users can make hugepage pool more extensible by
257	enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
258	page reclaim/migration/compaction work more and you can get contiguous
259	memory more likely. Note that using ZONE_MOVABLE for non-migratable
260	hugepages can do harm to other features like memory hotremove (because
261	memory hotremove expects that memory blocks on ZONE_MOVABLE are always
262	removable,) so it's a trade-off responsible for the users.
264	==============================================================
266	hugetlb_shm_group
268	hugetlb_shm_group contains group id that is allowed to create SysV
269	shared memory segment using hugetlb page.
271	==============================================================
273	laptop_mode
275	laptop_mode is a knob that controls "laptop mode". All the things that are
276	controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
278	==============================================================
280	legacy_va_layout
282	If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
283	will use the legacy (2.4) layout for all processes.
285	==============================================================
287	lowmem_reserve_ratio
289	For some specialised workloads on highmem machines it is dangerous for
290	the kernel to allow process memory to be allocated from the "lowmem"
291	zone.  This is because that memory could then be pinned via the mlock()
292	system call, or by unavailability of swapspace.
294	And on large highmem machines this lack of reclaimable lowmem memory
295	can be fatal.
297	So the Linux page allocator has a mechanism which prevents allocations
298	which _could_ use highmem from using too much lowmem.  This means that
299	a certain amount of lowmem is defended from the possibility of being
300	captured into pinned user memory.
302	(The same argument applies to the old 16 megabyte ISA DMA region.  This
303	mechanism will also defend that region from allocations which could use
304	highmem or lowmem).
306	The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
307	in defending these lower zones.
309	If you have a machine which uses highmem or ISA DMA and your
310	applications are using mlock(), or if you are running with no swap then
311	you probably should change the lowmem_reserve_ratio setting.
313	The lowmem_reserve_ratio is an array. You can see them by reading this file.
314	-
315	% cat /proc/sys/vm/lowmem_reserve_ratio
316	256     256     32
317	-
318	Note: # of this elements is one fewer than number of zones. Because the highest
319	      zone's value is not necessary for following calculation.
321	But, these values are not used directly. The kernel calculates # of protection
322	pages for each zones from them. These are shown as array of protection pages
323	in /proc/zoneinfo like followings. (This is an example of x86-64 box).
324	Each zone has an array of protection pages like this.
326	-
327	Node 0, zone      DMA
328	  pages free     1355
329	        min      3
330	        low      3
331	        high     4
332		:
333		:
334	    numa_other   0
335	        protection: (0, 2004, 2004, 2004)
336		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
337	  pagesets
338	    cpu: 0 pcp: 0
339	        :
340	-
341	These protections are added to score to judge whether this zone should be used
342	for page allocation or should be reclaimed.
344	In this example, if normal pages (index=2) are required to this DMA zone and
345	watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
346	not be used because pages_free(1355) is smaller than watermark + protection[2]
347	(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
348	normal page requirement. If requirement is DMA zone(index=0), protection[0]
349	(=0) is used.
351	zone[i]'s protection[j] is calculated by following expression.
353	(i < j):
354	  zone[i]->protection[j]
355	  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
356	    / lowmem_reserve_ratio[i];
357	(i = j):
358	   (should not be protected. = 0;
359	(i > j):
360	   (not necessary, but looks 0)
362	The default values of lowmem_reserve_ratio[i] are
363	    256 (if zone[i] means DMA or DMA32 zone)
364	    32  (others).
365	As above expression, they are reciprocal number of ratio.
366	256 means 1/256. # of protection pages becomes about "0.39%" of total managed
367	pages of higher zones on the node.
369	If you would like to protect more pages, smaller values are effective.
370	The minimum value is 1 (1/1 -> 100%).
372	==============================================================
374	max_map_count:
376	This file contains the maximum number of memory map areas a process
377	may have. Memory map areas are used as a side-effect of calling
378	malloc, directly by mmap and mprotect, and also when loading shared
379	libraries.
381	While most applications need less than a thousand maps, certain
382	programs, particularly malloc debuggers, may consume lots of them,
383	e.g., up to one or two maps per allocation.
385	The default value is 65536.
387	=============================================================
389	memory_failure_early_kill:
391	Control how to kill processes when uncorrected memory error (typically
392	a 2bit error in a memory module) is detected in the background by hardware
393	that cannot be handled by the kernel. In some cases (like the page
394	still having a valid copy on disk) the kernel will handle the failure
395	transparently without affecting any applications. But if there is
396	no other uptodate copy of the data it will kill to prevent any data
397	corruptions from propagating.
399	1: Kill all processes that have the corrupted and not reloadable page mapped
400	as soon as the corruption is detected.  Note this is not supported
401	for a few types of pages, like kernel internally allocated data or
402	the swap cache, but works for the majority of user pages.
404	0: Only unmap the corrupted page from all processes and only kill a process
405	who tries to access it.
407	The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
408	handle this if they want to.
410	This is only active on architectures/platforms with advanced machine
411	check handling and depends on the hardware capabilities.
413	Applications can override this setting individually with the PR_MCE_KILL prctl
415	==============================================================
417	memory_failure_recovery
419	Enable memory failure recovery (when supported by the platform)
421	1: Attempt recovery.
423	0: Always panic on a memory failure.
425	==============================================================
427	min_free_kbytes:
429	This is used to force the Linux VM to keep a minimum number
430	of kilobytes free.  The VM uses this number to compute a
431	watermark[WMARK_MIN] value for each lowmem zone in the system.
432	Each lowmem zone gets a number of reserved free pages based
433	proportionally on its size.
435	Some minimal amount of memory is needed to satisfy PF_MEMALLOC
436	allocations; if you set this to lower than 1024KB, your system will
437	become subtly broken, and prone to deadlock under high loads.
439	Setting this too high will OOM your machine instantly.
441	=============================================================
443	min_slab_ratio:
445	This is available only on NUMA kernels.
447	A percentage of the total pages in each zone.  On Zone reclaim
448	(fallback from the local zone occurs) slabs will be reclaimed if more
449	than this percentage of pages in a zone are reclaimable slab pages.
450	This insures that the slab growth stays under control even in NUMA
451	systems that rarely perform global reclaim.
453	The default is 5 percent.
455	Note that slab reclaim is triggered in a per zone / node fashion.
456	The process of reclaiming slab memory is currently not node specific
457	and may not be fast.
459	=============================================================
461	min_unmapped_ratio:
463	This is available only on NUMA kernels.
465	This is a percentage of the total pages in each zone. Zone reclaim will
466	only occur if more than this percentage of pages are in a state that
467	zone_reclaim_mode allows to be reclaimed.
469	If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
470	against all file-backed unmapped pages including swapcache pages and tmpfs
471	files. Otherwise, only unmapped pages backed by normal files but not tmpfs
472	files and similar are considered.
474	The default is 1 percent.
476	==============================================================
478	mmap_min_addr
480	This file indicates the amount of address space  which a user process will
481	be restricted from mmapping.  Since kernel null dereference bugs could
482	accidentally operate based on the information in the first couple of pages
483	of memory userspace processes should not be allowed to write to them.  By
484	default this value is set to 0 and no protections will be enforced by the
485	security module.  Setting this value to something like 64k will allow the
486	vast majority of applications to work correctly and provide defense in depth
487	against future potential kernel bugs.
489	==============================================================
491	mmap_rnd_bits:
493	This value can be used to select the number of bits to use to
494	determine the random offset to the base address of vma regions
495	resulting from mmap allocations on architectures which support
496	tuning address space randomization.  This value will be bounded
497	by the architecture's minimum and maximum supported values.
499	This value can be changed after boot using the
500	/proc/sys/vm/mmap_rnd_bits tunable
502	==============================================================
504	mmap_rnd_compat_bits:
506	This value can be used to select the number of bits to use to
507	determine the random offset to the base address of vma regions
508	resulting from mmap allocations for applications run in
509	compatibility mode on architectures which support tuning address
510	space randomization.  This value will be bounded by the
511	architecture's minimum and maximum supported values.
513	This value can be changed after boot using the
514	/proc/sys/vm/mmap_rnd_compat_bits tunable
516	==============================================================
518	nr_hugepages
520	Change the minimum size of the hugepage pool.
522	See Documentation/vm/hugetlbpage.txt
524	==============================================================
526	nr_overcommit_hugepages
528	Change the maximum size of the hugepage pool. The maximum is
529	nr_hugepages + nr_overcommit_hugepages.
531	See Documentation/vm/hugetlbpage.txt
533	==============================================================
535	nr_trim_pages
537	This is available only on NOMMU kernels.
539	This value adjusts the excess page trimming behaviour of power-of-2 aligned
540	NOMMU mmap allocations.
542	A value of 0 disables trimming of allocations entirely, while a value of 1
543	trims excess pages aggressively. Any value >= 1 acts as the watermark where
544	trimming of allocations is initiated.
546	The default value is 1.
548	See Documentation/nommu-mmap.txt for more information.
550	==============================================================
552	numa_zonelist_order
554	This sysctl is only for NUMA.
555	'where the memory is allocated from' is controlled by zonelists.
556	(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
557	 you may be able to read ZONE_DMA as ZONE_DMA32...)
559	In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
561	This means that a memory allocation request for GFP_KERNEL will
562	get memory from ZONE_DMA only when ZONE_NORMAL is not available.
564	In NUMA case, you can think of following 2 types of order.
565	Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
567	(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
568	(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
570	Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
571	will be used before ZONE_NORMAL exhaustion. This increases possibility of
572	out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
574	Type(B) cannot offer the best locality but is more robust against OOM of
575	the DMA zone.
577	Type(A) is called as "Node" order. Type (B) is "Zone" order.
579	"Node order" orders the zonelists by node, then by zone within each node.
580	Specify "[Nn]ode" for node order
582	"Zone Order" orders the zonelists by zone type, then by node within each
583	zone.  Specify "[Zz]one" for zone order.
585	Specify "[Dd]efault" to request automatic configuration.
587	On 32-bit, the Normal zone needs to be preserved for allocations accessible
588	by the kernel, so "zone" order will be selected.
590	On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
591	order will be selected.
593	Default order is recommended unless this is causing problems for your
594	system/application.
596	==============================================================
598	oom_dump_tasks
600	Enables a system-wide task dump (excluding kernel threads) to be produced
601	when the kernel performs an OOM-killing and includes such information as
602	pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj
603	score, and name.  This is helpful to determine why the OOM killer was
604	invoked, to identify the rogue task that caused it, and to determine why
605	the OOM killer chose the task it did to kill.
607	If this is set to zero, this information is suppressed.  On very
608	large systems with thousands of tasks it may not be feasible to dump
609	the memory state information for each one.  Such systems should not
610	be forced to incur a performance penalty in OOM conditions when the
611	information may not be desired.
613	If this is set to non-zero, this information is shown whenever the
614	OOM killer actually kills a memory-hogging task.
616	The default value is 1 (enabled).
618	==============================================================
620	oom_kill_allocating_task
622	This enables or disables killing the OOM-triggering task in
623	out-of-memory situations.
625	If this is set to zero, the OOM killer will scan through the entire
626	tasklist and select a task based on heuristics to kill.  This normally
627	selects a rogue memory-hogging task that frees up a large amount of
628	memory when killed.
630	If this is set to non-zero, the OOM killer simply kills the task that
631	triggered the out-of-memory condition.  This avoids the expensive
632	tasklist scan.
634	If panic_on_oom is selected, it takes precedence over whatever value
635	is used in oom_kill_allocating_task.
637	The default value is 0.
639	==============================================================
641	overcommit_kbytes:
643	When overcommit_memory is set to 2, the committed address space is not
644	permitted to exceed swap plus this amount of physical RAM. See below.
646	Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
647	of them may be specified at a time. Setting one disables the other (which
648	then appears as 0 when read).
650	==============================================================
652	overcommit_memory:
654	This value contains a flag that enables memory overcommitment.
656	When this flag is 0, the kernel attempts to estimate the amount
657	of free memory left when userspace requests more memory.
659	When this flag is 1, the kernel pretends there is always enough
660	memory until it actually runs out.
662	When this flag is 2, the kernel uses a "never overcommit"
663	policy that attempts to prevent any overcommit of memory.
664	Note that user_reserve_kbytes affects this policy.
666	This feature can be very useful because there are a lot of
667	programs that malloc() huge amounts of memory "just-in-case"
668	and don't use much of it.
670	The default value is 0.
672	See Documentation/vm/overcommit-accounting and
673	mm/mmap.c::__vm_enough_memory() for more information.
675	==============================================================
677	overcommit_ratio:
679	When overcommit_memory is set to 2, the committed address
680	space is not permitted to exceed swap plus this percentage
681	of physical RAM.  See above.
683	==============================================================
685	page-cluster
687	page-cluster controls the number of pages up to which consecutive pages
688	are read in from swap in a single attempt. This is the swap counterpart
689	to page cache readahead.
690	The mentioned consecutivity is not in terms of virtual/physical addresses,
691	but consecutive on swap space - that means they were swapped out together.
693	It is a logarithmic value - setting it to zero means "1 page", setting
694	it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
695	Zero disables swap readahead completely.
697	The default value is three (eight pages at a time).  There may be some
698	small benefits in tuning this to a different value if your workload is
699	swap-intensive.
701	Lower values mean lower latencies for initial faults, but at the same time
702	extra faults and I/O delays for following faults if they would have been part of
703	that consecutive pages readahead would have brought in.
705	=============================================================
707	panic_on_oom
709	This enables or disables panic on out-of-memory feature.
711	If this is set to 0, the kernel will kill some rogue process,
712	called oom_killer.  Usually, oom_killer can kill rogue processes and
713	system will survive.
715	If this is set to 1, the kernel panics when out-of-memory happens.
716	However, if a process limits using nodes by mempolicy/cpusets,
717	and those nodes become memory exhaustion status, one process
718	may be killed by oom-killer. No panic occurs in this case.
719	Because other nodes' memory may be free. This means system total status
720	may be not fatal yet.
722	If this is set to 2, the kernel panics compulsorily even on the
723	above-mentioned. Even oom happens under memory cgroup, the whole
724	system panics.
726	The default value is 0.
727	1 and 2 are for failover of clustering. Please select either
728	according to your policy of failover.
729	panic_on_oom=2+kdump gives you very strong tool to investigate
730	why oom happens. You can get snapshot.
732	=============================================================
734	percpu_pagelist_fraction
736	This is the fraction of pages at most (high mark pcp->high) in each zone that
737	are allocated for each per cpu page list.  The min value for this is 8.  It
738	means that we don't allow more than 1/8th of pages in each zone to be
739	allocated in any single per_cpu_pagelist.  This entry only changes the value
740	of hot per cpu pagelists.  User can specify a number like 100 to allocate
741	1/100th of each zone to each per cpu page list.
743	The batch value of each per cpu pagelist is also updated as a result.  It is
744	set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
746	The initial value is zero.  Kernel does not use this value at boot time to set
747	the high water marks for each per cpu page list.  If the user writes '0' to this
748	sysctl, it will revert to this default behavior.
750	==============================================================
752	stat_interval
754	The time interval between which vm statistics are updated.  The default
755	is 1 second.
757	==============================================================
759	stat_refresh
761	Any read or write (by root only) flushes all the per-cpu vm statistics
762	into their global totals, for more accurate reports when testing
763	e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
765	As a side-effect, it also checks for negative totals (elsewhere reported
766	as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
767	(At time of writing, a few stats are known sometimes to be found negative,
768	with no ill effects: errors and warnings on these stats are suppressed.)
770	==============================================================
772	swappiness
774	This control is used to define how aggressive the kernel will swap
775	memory pages.  Higher values will increase agressiveness, lower values
776	decrease the amount of swap.  A value of 0 instructs the kernel not to
777	initiate swap until the amount of free and file-backed pages is less
778	than the high water mark in a zone.
780	The default value is 60.
782	==============================================================
784	- user_reserve_kbytes
786	When overcommit_memory is set to 2, "never overcommit" mode, reserve
787	min(3% of current process size, user_reserve_kbytes) of free memory.
788	This is intended to prevent a user from starting a single memory hogging
789	process, such that they cannot recover (kill the hog).
791	user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
793	If this is reduced to zero, then the user will be allowed to allocate
794	all free memory with a single process, minus admin_reserve_kbytes.
795	Any subsequent attempts to execute a command will result in
796	"fork: Cannot allocate memory".
798	Changing this takes effect whenever an application requests memory.
800	==============================================================
802	vfs_cache_pressure
803	------------------
805	This percentage value controls the tendency of the kernel to reclaim
806	the memory which is used for caching of directory and inode objects.
808	At the default value of vfs_cache_pressure=100 the kernel will attempt to
809	reclaim dentries and inodes at a "fair" rate with respect to pagecache and
810	swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
811	to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
812	never reclaim dentries and inodes due to memory pressure and this can easily
813	lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
814	causes the kernel to prefer to reclaim dentries and inodes.
816	Increasing vfs_cache_pressure significantly beyond 100 may have negative
817	performance impact. Reclaim code needs to take various locks to find freeable
818	directory and inode objects. With vfs_cache_pressure=1000, it will look for
819	ten times more freeable objects than there are.
821	=============================================================
823	watermark_scale_factor:
825	This factor controls the aggressiveness of kswapd. It defines the
826	amount of memory left in a node/system before kswapd is woken up and
827	how much memory needs to be free before kswapd goes back to sleep.
829	The unit is in fractions of 10,000. The default value of 10 means the
830	distances between watermarks are 0.1% of the available memory in the
831	node/system. The maximum value is 1000, or 10% of memory.
833	A high rate of threads entering direct reclaim (allocstall) or kswapd
834	going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
835	that the number of free pages kswapd maintains for latency reasons is
836	too small for the allocation bursts occurring in the system. This knob
837	can then be used to tune kswapd aggressiveness accordingly.
839	==============================================================
841	zone_reclaim_mode:
843	Zone_reclaim_mode allows someone to set more or less aggressive approaches to
844	reclaim memory when a zone runs out of memory. If it is set to zero then no
845	zone reclaim occurs. Allocations will be satisfied from other zones / nodes
846	in the system.
848	This is value ORed together of
850	1	= Zone reclaim on
851	2	= Zone reclaim writes dirty pages out
852	4	= Zone reclaim swaps pages
854	zone_reclaim_mode is disabled by default.  For file servers or workloads
855	that benefit from having their data cached, zone_reclaim_mode should be
856	left disabled as the caching effect is likely to be more important than
857	data locality.
859	zone_reclaim may be enabled if it's known that the workload is partitioned
860	such that each partition fits within a NUMA node and that accessing remote
861	memory would cause a measurable performance reduction.  The page allocator
862	will then reclaim easily reusable pages (those page cache pages that are
863	currently not used) before allocating off node pages.
865	Allowing zone reclaim to write out pages stops processes that are
866	writing large amounts of data from dirtying pages on other nodes. Zone
867	reclaim will write out dirty pages if a zone fills up and so effectively
868	throttle the process. This may decrease the performance of a single process
869	since it cannot use all of system memory to buffer the outgoing writes
870	anymore but it preserve the memory on other nodes so that the performance
871	of other processes running on other nodes will not be affected.
873	Allowing regular swap effectively restricts allocations to the local
874	node unless explicitly overridden by memory policies or cpuset
875	configurations.
877	============ End of Document =================================
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