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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 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>
4	
5	For general info and legal blurb, please look in README.
6	
7	==============================================================
8	
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.
11	
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.
15	
16	Default values and initialization routines for most of these
17	files can be found in mm/swap.c.
18	
19	Currently, these files are in /proc/sys/vm:
20	
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
66	
67	==============================================================
68	
69	admin_reserve_kbytes
70	
71	The amount of free memory in the system that should be reserved for users
72	with the capability cap_sys_admin.
73	
74	admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
75	
76	That should provide enough for the admin to log in and kill a process,
77	if necessary, under the default overcommit 'guess' mode.
78	
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.
82	
83	How do you calculate a minimum useful reserve?
84	
85	sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
86	
87	For overcommit 'guess', we can sum resident set sizes (RSS).
88	On x86_64 this is about 8MB.
89	
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.
93	
94	Changing this takes effect whenever an application requests memory.
95	
96	==============================================================
97	
98	block_dump
99	
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.
102	
103	==============================================================
104	
105	compact_memory
106	
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.
111	
112	==============================================================
113	
114	compact_unevictable_allowed
115	
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.
121	
122	==============================================================
123	
124	dirty_background_bytes
125	
126	Contains the amount of dirty memory at which the background kernel
127	flusher threads will start writeback.
128	
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.
133	
134	==============================================================
135	
136	dirty_background_ratio
137	
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.
141	
142	The total available memory is not equal to total system memory.
143	
144	==============================================================
145	
146	dirty_bytes
147	
148	Contains the amount of dirty memory at which a process generating disk writes
149	will itself start writeback.
150	
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.
155	
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.
159	
160	==============================================================
161	
162	dirty_expire_centisecs
163	
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.
168	
169	==============================================================
170	
171	dirty_ratio
172	
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.
176	
177	The total available memory is not equal to total system memory.
178	
179	==============================================================
180	
181	dirty_writeback_centisecs
182	
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.
186	
187	Setting this to zero disables periodic writeback altogether.
188	
189	==============================================================
190	
191	drop_caches
192	
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.
196	
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
203	
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.
209	
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.
213	
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.
218	
219	You may see informational messages in your kernel log when this file is
220	used:
221	
222		cat (1234): drop_caches: 3
223	
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.
226	
227	==============================================================
228	
229	extfrag_threshold
230	
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.
237	
238	The kernel will not compact memory in a zone if the
239	fragmentation index is <= extfrag_threshold. The default value is 500.
240	
241	==============================================================
242	
243	highmem_is_dirtyable
244	
245	Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
246	
247	This parameter controls whether the high memory is considered for dirty
248	writers throttling.  This is not the case by default which means that
249	only the amount of memory directly visible/usable by the kernel can
250	be dirtied. As a result, on systems with a large amount of memory and
251	lowmem basically depleted writers might be throttled too early and
252	streaming writes can get very slow.
253	
254	Changing the value to non zero would allow more memory to be dirtied
255	and thus allow writers to write more data which can be flushed to the
256	storage more effectively. Note this also comes with a risk of pre-mature
257	OOM killer because some writers (e.g. direct block device writes) can
258	only use the low memory and they can fill it up with dirty data without
259	any throttling.
260	
261	==============================================================
262	
263	hugepages_treat_as_movable
264	
265	This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
266	or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
267	ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
268	so this parameter has no effect if used without kernelcore=.
269	
270	Hugepage migration is now available in some situations which depend on the
271	architecture and/or the hugepage size. If a hugepage supports migration,
272	allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
273	of the value of this parameter.
274	IOW, this parameter affects only non-migratable hugepages.
275	
276	Assuming that hugepages are not migratable in your system, one usecase of
277	this parameter is that users can make hugepage pool more extensible by
278	enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
279	page reclaim/migration/compaction work more and you can get contiguous
280	memory more likely. Note that using ZONE_MOVABLE for non-migratable
281	hugepages can do harm to other features like memory hotremove (because
282	memory hotremove expects that memory blocks on ZONE_MOVABLE are always
283	removable,) so it's a trade-off responsible for the users.
284	
285	==============================================================
286	
287	hugetlb_shm_group
288	
289	hugetlb_shm_group contains group id that is allowed to create SysV
290	shared memory segment using hugetlb page.
291	
292	==============================================================
293	
294	laptop_mode
295	
296	laptop_mode is a knob that controls "laptop mode". All the things that are
297	controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
298	
299	==============================================================
300	
301	legacy_va_layout
302	
303	If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
304	will use the legacy (2.4) layout for all processes.
305	
306	==============================================================
307	
308	lowmem_reserve_ratio
309	
310	For some specialised workloads on highmem machines it is dangerous for
311	the kernel to allow process memory to be allocated from the "lowmem"
312	zone.  This is because that memory could then be pinned via the mlock()
313	system call, or by unavailability of swapspace.
314	
315	And on large highmem machines this lack of reclaimable lowmem memory
316	can be fatal.
317	
318	So the Linux page allocator has a mechanism which prevents allocations
319	which _could_ use highmem from using too much lowmem.  This means that
320	a certain amount of lowmem is defended from the possibility of being
321	captured into pinned user memory.
322	
323	(The same argument applies to the old 16 megabyte ISA DMA region.  This
324	mechanism will also defend that region from allocations which could use
325	highmem or lowmem).
326	
327	The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
328	in defending these lower zones.
329	
330	If you have a machine which uses highmem or ISA DMA and your
331	applications are using mlock(), or if you are running with no swap then
332	you probably should change the lowmem_reserve_ratio setting.
333	
334	The lowmem_reserve_ratio is an array. You can see them by reading this file.
335	-
336	% cat /proc/sys/vm/lowmem_reserve_ratio
337	256     256     32
338	-
339	Note: # of this elements is one fewer than number of zones. Because the highest
340	      zone's value is not necessary for following calculation.
341	
342	But, these values are not used directly. The kernel calculates # of protection
343	pages for each zones from them. These are shown as array of protection pages
344	in /proc/zoneinfo like followings. (This is an example of x86-64 box).
345	Each zone has an array of protection pages like this.
346	
347	-
348	Node 0, zone      DMA
349	  pages free     1355
350	        min      3
351	        low      3
352	        high     4
353		:
354		:
355	    numa_other   0
356	        protection: (0, 2004, 2004, 2004)
357		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
358	  pagesets
359	    cpu: 0 pcp: 0
360	        :
361	-
362	These protections are added to score to judge whether this zone should be used
363	for page allocation or should be reclaimed.
364	
365	In this example, if normal pages (index=2) are required to this DMA zone and
366	watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
367	not be used because pages_free(1355) is smaller than watermark + protection[2]
368	(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
369	normal page requirement. If requirement is DMA zone(index=0), protection[0]
370	(=0) is used.
371	
372	zone[i]'s protection[j] is calculated by following expression.
373	
374	(i < j):
375	  zone[i]->protection[j]
376	  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
377	    / lowmem_reserve_ratio[i];
378	(i = j):
379	   (should not be protected. = 0;
380	(i > j):
381	   (not necessary, but looks 0)
382	
383	The default values of lowmem_reserve_ratio[i] are
384	    256 (if zone[i] means DMA or DMA32 zone)
385	    32  (others).
386	As above expression, they are reciprocal number of ratio.
387	256 means 1/256. # of protection pages becomes about "0.39%" of total managed
388	pages of higher zones on the node.
389	
390	If you would like to protect more pages, smaller values are effective.
391	The minimum value is 1 (1/1 -> 100%).
392	
393	==============================================================
394	
395	max_map_count:
396	
397	This file contains the maximum number of memory map areas a process
398	may have. Memory map areas are used as a side-effect of calling
399	malloc, directly by mmap, mprotect, and madvise, and also when loading
400	shared libraries.
401	
402	While most applications need less than a thousand maps, certain
403	programs, particularly malloc debuggers, may consume lots of them,
404	e.g., up to one or two maps per allocation.
405	
406	The default value is 65536.
407	
408	=============================================================
409	
410	memory_failure_early_kill:
411	
412	Control how to kill processes when uncorrected memory error (typically
413	a 2bit error in a memory module) is detected in the background by hardware
414	that cannot be handled by the kernel. In some cases (like the page
415	still having a valid copy on disk) the kernel will handle the failure
416	transparently without affecting any applications. But if there is
417	no other uptodate copy of the data it will kill to prevent any data
418	corruptions from propagating.
419	
420	1: Kill all processes that have the corrupted and not reloadable page mapped
421	as soon as the corruption is detected.  Note this is not supported
422	for a few types of pages, like kernel internally allocated data or
423	the swap cache, but works for the majority of user pages.
424	
425	0: Only unmap the corrupted page from all processes and only kill a process
426	who tries to access it.
427	
428	The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
429	handle this if they want to.
430	
431	This is only active on architectures/platforms with advanced machine
432	check handling and depends on the hardware capabilities.
433	
434	Applications can override this setting individually with the PR_MCE_KILL prctl
435	
436	==============================================================
437	
438	memory_failure_recovery
439	
440	Enable memory failure recovery (when supported by the platform)
441	
442	1: Attempt recovery.
443	
444	0: Always panic on a memory failure.
445	
446	==============================================================
447	
448	min_free_kbytes:
449	
450	This is used to force the Linux VM to keep a minimum number
451	of kilobytes free.  The VM uses this number to compute a
452	watermark[WMARK_MIN] value for each lowmem zone in the system.
453	Each lowmem zone gets a number of reserved free pages based
454	proportionally on its size.
455	
456	Some minimal amount of memory is needed to satisfy PF_MEMALLOC
457	allocations; if you set this to lower than 1024KB, your system will
458	become subtly broken, and prone to deadlock under high loads.
459	
460	Setting this too high will OOM your machine instantly.
461	
462	=============================================================
463	
464	min_slab_ratio:
465	
466	This is available only on NUMA kernels.
467	
468	A percentage of the total pages in each zone.  On Zone reclaim
469	(fallback from the local zone occurs) slabs will be reclaimed if more
470	than this percentage of pages in a zone are reclaimable slab pages.
471	This insures that the slab growth stays under control even in NUMA
472	systems that rarely perform global reclaim.
473	
474	The default is 5 percent.
475	
476	Note that slab reclaim is triggered in a per zone / node fashion.
477	The process of reclaiming slab memory is currently not node specific
478	and may not be fast.
479	
480	=============================================================
481	
482	min_unmapped_ratio:
483	
484	This is available only on NUMA kernels.
485	
486	This is a percentage of the total pages in each zone. Zone reclaim will
487	only occur if more than this percentage of pages are in a state that
488	zone_reclaim_mode allows to be reclaimed.
489	
490	If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
491	against all file-backed unmapped pages including swapcache pages and tmpfs
492	files. Otherwise, only unmapped pages backed by normal files but not tmpfs
493	files and similar are considered.
494	
495	The default is 1 percent.
496	
497	==============================================================
498	
499	mmap_min_addr
500	
501	This file indicates the amount of address space  which a user process will
502	be restricted from mmapping.  Since kernel null dereference bugs could
503	accidentally operate based on the information in the first couple of pages
504	of memory userspace processes should not be allowed to write to them.  By
505	default this value is set to 0 and no protections will be enforced by the
506	security module.  Setting this value to something like 64k will allow the
507	vast majority of applications to work correctly and provide defense in depth
508	against future potential kernel bugs.
509	
510	==============================================================
511	
512	mmap_rnd_bits:
513	
514	This value can be used to select the number of bits to use to
515	determine the random offset to the base address of vma regions
516	resulting from mmap allocations on architectures which support
517	tuning address space randomization.  This value will be bounded
518	by the architecture's minimum and maximum supported values.
519	
520	This value can be changed after boot using the
521	/proc/sys/vm/mmap_rnd_bits tunable
522	
523	==============================================================
524	
525	mmap_rnd_compat_bits:
526	
527	This value can be used to select the number of bits to use to
528	determine the random offset to the base address of vma regions
529	resulting from mmap allocations for applications run in
530	compatibility mode on architectures which support tuning address
531	space randomization.  This value will be bounded by the
532	architecture's minimum and maximum supported values.
533	
534	This value can be changed after boot using the
535	/proc/sys/vm/mmap_rnd_compat_bits tunable
536	
537	==============================================================
538	
539	nr_hugepages
540	
541	Change the minimum size of the hugepage pool.
542	
543	See Documentation/vm/hugetlbpage.txt
544	
545	==============================================================
546	
547	nr_overcommit_hugepages
548	
549	Change the maximum size of the hugepage pool. The maximum is
550	nr_hugepages + nr_overcommit_hugepages.
551	
552	See Documentation/vm/hugetlbpage.txt
553	
554	==============================================================
555	
556	nr_trim_pages
557	
558	This is available only on NOMMU kernels.
559	
560	This value adjusts the excess page trimming behaviour of power-of-2 aligned
561	NOMMU mmap allocations.
562	
563	A value of 0 disables trimming of allocations entirely, while a value of 1
564	trims excess pages aggressively. Any value >= 1 acts as the watermark where
565	trimming of allocations is initiated.
566	
567	The default value is 1.
568	
569	See Documentation/nommu-mmap.txt for more information.
570	
571	==============================================================
572	
573	numa_zonelist_order
574	
575	This sysctl is only for NUMA.
576	'where the memory is allocated from' is controlled by zonelists.
577	(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
578	 you may be able to read ZONE_DMA as ZONE_DMA32...)
579	
580	In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
581	ZONE_NORMAL -> ZONE_DMA
582	This means that a memory allocation request for GFP_KERNEL will
583	get memory from ZONE_DMA only when ZONE_NORMAL is not available.
584	
585	In NUMA case, you can think of following 2 types of order.
586	Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
587	
588	(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
589	(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
590	
591	Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
592	will be used before ZONE_NORMAL exhaustion. This increases possibility of
593	out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
594	
595	Type(B) cannot offer the best locality but is more robust against OOM of
596	the DMA zone.
597	
598	Type(A) is called as "Node" order. Type (B) is "Zone" order.
599	
600	"Node order" orders the zonelists by node, then by zone within each node.
601	Specify "[Nn]ode" for node order
602	
603	"Zone Order" orders the zonelists by zone type, then by node within each
604	zone.  Specify "[Zz]one" for zone order.
605	
606	Specify "[Dd]efault" to request automatic configuration.
607	
608	On 32-bit, the Normal zone needs to be preserved for allocations accessible
609	by the kernel, so "zone" order will be selected.
610	
611	On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
612	order will be selected.
613	
614	Default order is recommended unless this is causing problems for your
615	system/application.
616	
617	==============================================================
618	
619	oom_dump_tasks
620	
621	Enables a system-wide task dump (excluding kernel threads) to be produced
622	when the kernel performs an OOM-killing and includes such information as
623	pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj
624	score, and name.  This is helpful to determine why the OOM killer was
625	invoked, to identify the rogue task that caused it, and to determine why
626	the OOM killer chose the task it did to kill.
627	
628	If this is set to zero, this information is suppressed.  On very
629	large systems with thousands of tasks it may not be feasible to dump
630	the memory state information for each one.  Such systems should not
631	be forced to incur a performance penalty in OOM conditions when the
632	information may not be desired.
633	
634	If this is set to non-zero, this information is shown whenever the
635	OOM killer actually kills a memory-hogging task.
636	
637	The default value is 1 (enabled).
638	
639	==============================================================
640	
641	oom_kill_allocating_task
642	
643	This enables or disables killing the OOM-triggering task in
644	out-of-memory situations.
645	
646	If this is set to zero, the OOM killer will scan through the entire
647	tasklist and select a task based on heuristics to kill.  This normally
648	selects a rogue memory-hogging task that frees up a large amount of
649	memory when killed.
650	
651	If this is set to non-zero, the OOM killer simply kills the task that
652	triggered the out-of-memory condition.  This avoids the expensive
653	tasklist scan.
654	
655	If panic_on_oom is selected, it takes precedence over whatever value
656	is used in oom_kill_allocating_task.
657	
658	The default value is 0.
659	
660	==============================================================
661	
662	overcommit_kbytes:
663	
664	When overcommit_memory is set to 2, the committed address space is not
665	permitted to exceed swap plus this amount of physical RAM. See below.
666	
667	Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
668	of them may be specified at a time. Setting one disables the other (which
669	then appears as 0 when read).
670	
671	==============================================================
672	
673	overcommit_memory:
674	
675	This value contains a flag that enables memory overcommitment.
676	
677	When this flag is 0, the kernel attempts to estimate the amount
678	of free memory left when userspace requests more memory.
679	
680	When this flag is 1, the kernel pretends there is always enough
681	memory until it actually runs out.
682	
683	When this flag is 2, the kernel uses a "never overcommit"
684	policy that attempts to prevent any overcommit of memory.
685	Note that user_reserve_kbytes affects this policy.
686	
687	This feature can be very useful because there are a lot of
688	programs that malloc() huge amounts of memory "just-in-case"
689	and don't use much of it.
690	
691	The default value is 0.
692	
693	See Documentation/vm/overcommit-accounting and
694	mm/mmap.c::__vm_enough_memory() for more information.
695	
696	==============================================================
697	
698	overcommit_ratio:
699	
700	When overcommit_memory is set to 2, the committed address
701	space is not permitted to exceed swap plus this percentage
702	of physical RAM.  See above.
703	
704	==============================================================
705	
706	page-cluster
707	
708	page-cluster controls the number of pages up to which consecutive pages
709	are read in from swap in a single attempt. This is the swap counterpart
710	to page cache readahead.
711	The mentioned consecutivity is not in terms of virtual/physical addresses,
712	but consecutive on swap space - that means they were swapped out together.
713	
714	It is a logarithmic value - setting it to zero means "1 page", setting
715	it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
716	Zero disables swap readahead completely.
717	
718	The default value is three (eight pages at a time).  There may be some
719	small benefits in tuning this to a different value if your workload is
720	swap-intensive.
721	
722	Lower values mean lower latencies for initial faults, but at the same time
723	extra faults and I/O delays for following faults if they would have been part of
724	that consecutive pages readahead would have brought in.
725	
726	=============================================================
727	
728	panic_on_oom
729	
730	This enables or disables panic on out-of-memory feature.
731	
732	If this is set to 0, the kernel will kill some rogue process,
733	called oom_killer.  Usually, oom_killer can kill rogue processes and
734	system will survive.
735	
736	If this is set to 1, the kernel panics when out-of-memory happens.
737	However, if a process limits using nodes by mempolicy/cpusets,
738	and those nodes become memory exhaustion status, one process
739	may be killed by oom-killer. No panic occurs in this case.
740	Because other nodes' memory may be free. This means system total status
741	may be not fatal yet.
742	
743	If this is set to 2, the kernel panics compulsorily even on the
744	above-mentioned. Even oom happens under memory cgroup, the whole
745	system panics.
746	
747	The default value is 0.
748	1 and 2 are for failover of clustering. Please select either
749	according to your policy of failover.
750	panic_on_oom=2+kdump gives you very strong tool to investigate
751	why oom happens. You can get snapshot.
752	
753	=============================================================
754	
755	percpu_pagelist_fraction
756	
757	This is the fraction of pages at most (high mark pcp->high) in each zone that
758	are allocated for each per cpu page list.  The min value for this is 8.  It
759	means that we don't allow more than 1/8th of pages in each zone to be
760	allocated in any single per_cpu_pagelist.  This entry only changes the value
761	of hot per cpu pagelists.  User can specify a number like 100 to allocate
762	1/100th of each zone to each per cpu page list.
763	
764	The batch value of each per cpu pagelist is also updated as a result.  It is
765	set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
766	
767	The initial value is zero.  Kernel does not use this value at boot time to set
768	the high water marks for each per cpu page list.  If the user writes '0' to this
769	sysctl, it will revert to this default behavior.
770	
771	==============================================================
772	
773	stat_interval
774	
775	The time interval between which vm statistics are updated.  The default
776	is 1 second.
777	
778	==============================================================
779	
780	stat_refresh
781	
782	Any read or write (by root only) flushes all the per-cpu vm statistics
783	into their global totals, for more accurate reports when testing
784	e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
785	
786	As a side-effect, it also checks for negative totals (elsewhere reported
787	as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
788	(At time of writing, a few stats are known sometimes to be found negative,
789	with no ill effects: errors and warnings on these stats are suppressed.)
790	
791	==============================================================
792	
793	swappiness
794	
795	This control is used to define how aggressive the kernel will swap
796	memory pages.  Higher values will increase agressiveness, lower values
797	decrease the amount of swap.  A value of 0 instructs the kernel not to
798	initiate swap until the amount of free and file-backed pages is less
799	than the high water mark in a zone.
800	
801	The default value is 60.
802	
803	==============================================================
804	
805	- user_reserve_kbytes
806	
807	When overcommit_memory is set to 2, "never overcommit" mode, reserve
808	min(3% of current process size, user_reserve_kbytes) of free memory.
809	This is intended to prevent a user from starting a single memory hogging
810	process, such that they cannot recover (kill the hog).
811	
812	user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
813	
814	If this is reduced to zero, then the user will be allowed to allocate
815	all free memory with a single process, minus admin_reserve_kbytes.
816	Any subsequent attempts to execute a command will result in
817	"fork: Cannot allocate memory".
818	
819	Changing this takes effect whenever an application requests memory.
820	
821	==============================================================
822	
823	vfs_cache_pressure
824	------------------
825	
826	This percentage value controls the tendency of the kernel to reclaim
827	the memory which is used for caching of directory and inode objects.
828	
829	At the default value of vfs_cache_pressure=100 the kernel will attempt to
830	reclaim dentries and inodes at a "fair" rate with respect to pagecache and
831	swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
832	to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
833	never reclaim dentries and inodes due to memory pressure and this can easily
834	lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
835	causes the kernel to prefer to reclaim dentries and inodes.
836	
837	Increasing vfs_cache_pressure significantly beyond 100 may have negative
838	performance impact. Reclaim code needs to take various locks to find freeable
839	directory and inode objects. With vfs_cache_pressure=1000, it will look for
840	ten times more freeable objects than there are.
841	
842	=============================================================
843	
844	watermark_scale_factor:
845	
846	This factor controls the aggressiveness of kswapd. It defines the
847	amount of memory left in a node/system before kswapd is woken up and
848	how much memory needs to be free before kswapd goes back to sleep.
849	
850	The unit is in fractions of 10,000. The default value of 10 means the
851	distances between watermarks are 0.1% of the available memory in the
852	node/system. The maximum value is 1000, or 10% of memory.
853	
854	A high rate of threads entering direct reclaim (allocstall) or kswapd
855	going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
856	that the number of free pages kswapd maintains for latency reasons is
857	too small for the allocation bursts occurring in the system. This knob
858	can then be used to tune kswapd aggressiveness accordingly.
859	
860	==============================================================
861	
862	zone_reclaim_mode:
863	
864	Zone_reclaim_mode allows someone to set more or less aggressive approaches to
865	reclaim memory when a zone runs out of memory. If it is set to zero then no
866	zone reclaim occurs. Allocations will be satisfied from other zones / nodes
867	in the system.
868	
869	This is value ORed together of
870	
871	1	= Zone reclaim on
872	2	= Zone reclaim writes dirty pages out
873	4	= Zone reclaim swaps pages
874	
875	zone_reclaim_mode is disabled by default.  For file servers or workloads
876	that benefit from having their data cached, zone_reclaim_mode should be
877	left disabled as the caching effect is likely to be more important than
878	data locality.
879	
880	zone_reclaim may be enabled if it's known that the workload is partitioned
881	such that each partition fits within a NUMA node and that accessing remote
882	memory would cause a measurable performance reduction.  The page allocator
883	will then reclaim easily reusable pages (those page cache pages that are
884	currently not used) before allocating off node pages.
885	
886	Allowing zone reclaim to write out pages stops processes that are
887	writing large amounts of data from dirtying pages on other nodes. Zone
888	reclaim will write out dirty pages if a zone fills up and so effectively
889	throttle the process. This may decrease the performance of a single process
890	since it cannot use all of system memory to buffer the outgoing writes
891	anymore but it preserve the memory on other nodes so that the performance
892	of other processes running on other nodes will not be affected.
893	
894	Allowing regular swap effectively restricts allocations to the local
895	node unless explicitly overridden by memory policies or cpuset
896	configurations.
897	
898	============ End of Document =================================
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