About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / sysctl / vm.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:04 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	- dirty_background_bytes
25	- dirty_background_ratio
26	- dirty_bytes
27	- dirty_expire_centisecs
28	- dirty_ratio
29	- dirty_writeback_centisecs
30	- drop_caches
31	- extfrag_threshold
32	- hugepages_treat_as_movable
33	- hugetlb_shm_group
34	- laptop_mode
35	- legacy_va_layout
36	- lowmem_reserve_ratio
37	- max_map_count
38	- memory_failure_early_kill
39	- memory_failure_recovery
40	- min_free_kbytes
41	- min_slab_ratio
42	- min_unmapped_ratio
43	- mmap_min_addr
44	- nr_hugepages
45	- nr_overcommit_hugepages
46	- nr_trim_pages         (only if CONFIG_MMU=n)
47	- numa_zonelist_order
48	- oom_dump_tasks
49	- oom_kill_allocating_task
50	- overcommit_memory
51	- overcommit_ratio
52	- page-cluster
53	- panic_on_oom
54	- percpu_pagelist_fraction
55	- stat_interval
56	- swappiness
57	- user_reserve_kbytes
58	- vfs_cache_pressure
59	- zone_reclaim_mode
60	
61	==============================================================
62	
63	admin_reserve_kbytes
64	
65	The amount of free memory in the system that should be reserved for users
66	with the capability cap_sys_admin.
67	
68	admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
69	
70	That should provide enough for the admin to log in and kill a process,
71	if necessary, under the default overcommit 'guess' mode.
72	
73	Systems running under overcommit 'never' should increase this to account
74	for the full Virtual Memory Size of programs used to recover. Otherwise,
75	root may not be able to log in to recover the system.
76	
77	How do you calculate a minimum useful reserve?
78	
79	sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
80	
81	For overcommit 'guess', we can sum resident set sizes (RSS).
82	On x86_64 this is about 8MB.
83	
84	For overcommit 'never', we can take the max of their virtual sizes (VSZ)
85	and add the sum of their RSS.
86	On x86_64 this is about 128MB.
87	
88	Changing this takes effect whenever an application requests memory.
89	
90	==============================================================
91	
92	block_dump
93	
94	block_dump enables block I/O debugging when set to a nonzero value. More
95	information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
96	
97	==============================================================
98	
99	compact_memory
100	
101	Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
102	all zones are compacted such that free memory is available in contiguous
103	blocks where possible. This can be important for example in the allocation of
104	huge pages although processes will also directly compact memory as required.
105	
106	==============================================================
107	
108	dirty_background_bytes
109	
110	Contains the amount of dirty memory at which the background kernel
111	flusher threads will start writeback.
112	
113	Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
114	one of them may be specified at a time. When one sysctl is written it is
115	immediately taken into account to evaluate the dirty memory limits and the
116	other appears as 0 when read.
117	
118	==============================================================
119	
120	dirty_background_ratio
121	
122	Contains, as a percentage of total available memory that contains free pages
123	and reclaimable pages, the number of pages at which the background kernel
124	flusher threads will start writing out dirty data.
125	
126	The total avaiable memory is not equal to total system memory.
127	
128	==============================================================
129	
130	dirty_bytes
131	
132	Contains the amount of dirty memory at which a process generating disk writes
133	will itself start writeback.
134	
135	Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
136	specified at a time. When one sysctl is written it is immediately taken into
137	account to evaluate the dirty memory limits and the other appears as 0 when
138	read.
139	
140	Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
141	value lower than this limit will be ignored and the old configuration will be
142	retained.
143	
144	==============================================================
145	
146	dirty_expire_centisecs
147	
148	This tunable is used to define when dirty data is old enough to be eligible
149	for writeout by the kernel flusher threads.  It is expressed in 100'ths
150	of a second.  Data which has been dirty in-memory for longer than this
151	interval will be written out next time a flusher thread wakes up.
152	
153	==============================================================
154	
155	dirty_ratio
156	
157	Contains, as a percentage of total available memory that contains free pages
158	and reclaimable pages, the number of pages at which a process which is
159	generating disk writes will itself start writing out dirty data.
160	
161	The total avaiable memory is not equal to total system memory.
162	
163	==============================================================
164	
165	dirty_writeback_centisecs
166	
167	The kernel flusher threads will periodically wake up and write `old' data
168	out to disk.  This tunable expresses the interval between those wakeups, in
169	100'ths of a second.
170	
171	Setting this to zero disables periodic writeback altogether.
172	
173	==============================================================
174	
175	drop_caches
176	
177	Writing to this will cause the kernel to drop clean caches, dentries and
178	inodes from memory, causing that memory to become free.
179	
180	To free pagecache:
181		echo 1 > /proc/sys/vm/drop_caches
182	To free dentries and inodes:
183		echo 2 > /proc/sys/vm/drop_caches
184	To free pagecache, dentries and inodes:
185		echo 3 > /proc/sys/vm/drop_caches
186	
187	As this is a non-destructive operation and dirty objects are not freeable, the
188	user should run `sync' first.
189	
190	==============================================================
191	
192	extfrag_threshold
193	
194	This parameter affects whether the kernel will compact memory or direct
195	reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what
196	the fragmentation index for each order is in each zone in the system. Values
197	tending towards 0 imply allocations would fail due to lack of memory,
198	values towards 1000 imply failures are due to fragmentation and -1 implies
199	that the allocation will succeed as long as watermarks are met.
200	
201	The kernel will not compact memory in a zone if the
202	fragmentation index is <= extfrag_threshold. The default value is 500.
203	
204	==============================================================
205	
206	hugepages_treat_as_movable
207	
208	This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
209	or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
210	ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
211	so this parameter has no effect if used without kernelcore=.
212	
213	Hugepage migration is now available in some situations which depend on the
214	architecture and/or the hugepage size. If a hugepage supports migration,
215	allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
216	of the value of this parameter.
217	IOW, this parameter affects only non-migratable hugepages.
218	
219	Assuming that hugepages are not migratable in your system, one usecase of
220	this parameter is that users can make hugepage pool more extensible by
221	enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
222	page reclaim/migration/compaction work more and you can get contiguous
223	memory more likely. Note that using ZONE_MOVABLE for non-migratable
224	hugepages can do harm to other features like memory hotremove (because
225	memory hotremove expects that memory blocks on ZONE_MOVABLE are always
226	removable,) so it's a trade-off responsible for the users.
227	
228	==============================================================
229	
230	hugetlb_shm_group
231	
232	hugetlb_shm_group contains group id that is allowed to create SysV
233	shared memory segment using hugetlb page.
234	
235	==============================================================
236	
237	laptop_mode
238	
239	laptop_mode is a knob that controls "laptop mode". All the things that are
240	controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
241	
242	==============================================================
243	
244	legacy_va_layout
245	
246	If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
247	will use the legacy (2.4) layout for all processes.
248	
249	==============================================================
250	
251	lowmem_reserve_ratio
252	
253	For some specialised workloads on highmem machines it is dangerous for
254	the kernel to allow process memory to be allocated from the "lowmem"
255	zone.  This is because that memory could then be pinned via the mlock()
256	system call, or by unavailability of swapspace.
257	
258	And on large highmem machines this lack of reclaimable lowmem memory
259	can be fatal.
260	
261	So the Linux page allocator has a mechanism which prevents allocations
262	which _could_ use highmem from using too much lowmem.  This means that
263	a certain amount of lowmem is defended from the possibility of being
264	captured into pinned user memory.
265	
266	(The same argument applies to the old 16 megabyte ISA DMA region.  This
267	mechanism will also defend that region from allocations which could use
268	highmem or lowmem).
269	
270	The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
271	in defending these lower zones.
272	
273	If you have a machine which uses highmem or ISA DMA and your
274	applications are using mlock(), or if you are running with no swap then
275	you probably should change the lowmem_reserve_ratio setting.
276	
277	The lowmem_reserve_ratio is an array. You can see them by reading this file.
278	-
279	% cat /proc/sys/vm/lowmem_reserve_ratio
280	256     256     32
281	-
282	Note: # of this elements is one fewer than number of zones. Because the highest
283	      zone's value is not necessary for following calculation.
284	
285	But, these values are not used directly. The kernel calculates # of protection
286	pages for each zones from them. These are shown as array of protection pages
287	in /proc/zoneinfo like followings. (This is an example of x86-64 box).
288	Each zone has an array of protection pages like this.
289	
290	-
291	Node 0, zone      DMA
292	  pages free     1355
293	        min      3
294	        low      3
295	        high     4
296		:
297		:
298	    numa_other   0
299	        protection: (0, 2004, 2004, 2004)
300		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
301	  pagesets
302	    cpu: 0 pcp: 0
303	        :
304	-
305	These protections are added to score to judge whether this zone should be used
306	for page allocation or should be reclaimed.
307	
308	In this example, if normal pages (index=2) are required to this DMA zone and
309	watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
310	not be used because pages_free(1355) is smaller than watermark + protection[2]
311	(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
312	normal page requirement. If requirement is DMA zone(index=0), protection[0]
313	(=0) is used.
314	
315	zone[i]'s protection[j] is calculated by following expression.
316	
317	(i < j):
318	  zone[i]->protection[j]
319	  = (total sums of present_pages from zone[i+1] to zone[j] on the node)
320	    / lowmem_reserve_ratio[i];
321	(i = j):
322	   (should not be protected. = 0;
323	(i > j):
324	   (not necessary, but looks 0)
325	
326	The default values of lowmem_reserve_ratio[i] are
327	    256 (if zone[i] means DMA or DMA32 zone)
328	    32  (others).
329	As above expression, they are reciprocal number of ratio.
330	256 means 1/256. # of protection pages becomes about "0.39%" of total present
331	pages of higher zones on the node.
332	
333	If you would like to protect more pages, smaller values are effective.
334	The minimum value is 1 (1/1 -> 100%).
335	
336	==============================================================
337	
338	max_map_count:
339	
340	This file contains the maximum number of memory map areas a process
341	may have. Memory map areas are used as a side-effect of calling
342	malloc, directly by mmap and mprotect, and also when loading shared
343	libraries.
344	
345	While most applications need less than a thousand maps, certain
346	programs, particularly malloc debuggers, may consume lots of them,
347	e.g., up to one or two maps per allocation.
348	
349	The default value is 65536.
350	
351	=============================================================
352	
353	memory_failure_early_kill:
354	
355	Control how to kill processes when uncorrected memory error (typically
356	a 2bit error in a memory module) is detected in the background by hardware
357	that cannot be handled by the kernel. In some cases (like the page
358	still having a valid copy on disk) the kernel will handle the failure
359	transparently without affecting any applications. But if there is
360	no other uptodate copy of the data it will kill to prevent any data
361	corruptions from propagating.
362	
363	1: Kill all processes that have the corrupted and not reloadable page mapped
364	as soon as the corruption is detected.  Note this is not supported
365	for a few types of pages, like kernel internally allocated data or
366	the swap cache, but works for the majority of user pages.
367	
368	0: Only unmap the corrupted page from all processes and only kill a process
369	who tries to access it.
370	
371	The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
372	handle this if they want to.
373	
374	This is only active on architectures/platforms with advanced machine
375	check handling and depends on the hardware capabilities.
376	
377	Applications can override this setting individually with the PR_MCE_KILL prctl
378	
379	==============================================================
380	
381	memory_failure_recovery
382	
383	Enable memory failure recovery (when supported by the platform)
384	
385	1: Attempt recovery.
386	
387	0: Always panic on a memory failure.
388	
389	==============================================================
390	
391	min_free_kbytes:
392	
393	This is used to force the Linux VM to keep a minimum number
394	of kilobytes free.  The VM uses this number to compute a
395	watermark[WMARK_MIN] value for each lowmem zone in the system.
396	Each lowmem zone gets a number of reserved free pages based
397	proportionally on its size.
398	
399	Some minimal amount of memory is needed to satisfy PF_MEMALLOC
400	allocations; if you set this to lower than 1024KB, your system will
401	become subtly broken, and prone to deadlock under high loads.
402	
403	Setting this too high will OOM your machine instantly.
404	
405	=============================================================
406	
407	min_slab_ratio:
408	
409	This is available only on NUMA kernels.
410	
411	A percentage of the total pages in each zone.  On Zone reclaim
412	(fallback from the local zone occurs) slabs will be reclaimed if more
413	than this percentage of pages in a zone are reclaimable slab pages.
414	This insures that the slab growth stays under control even in NUMA
415	systems that rarely perform global reclaim.
416	
417	The default is 5 percent.
418	
419	Note that slab reclaim is triggered in a per zone / node fashion.
420	The process of reclaiming slab memory is currently not node specific
421	and may not be fast.
422	
423	=============================================================
424	
425	min_unmapped_ratio:
426	
427	This is available only on NUMA kernels.
428	
429	This is a percentage of the total pages in each zone. Zone reclaim will
430	only occur if more than this percentage of pages are in a state that
431	zone_reclaim_mode allows to be reclaimed.
432	
433	If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
434	against all file-backed unmapped pages including swapcache pages and tmpfs
435	files. Otherwise, only unmapped pages backed by normal files but not tmpfs
436	files and similar are considered.
437	
438	The default is 1 percent.
439	
440	==============================================================
441	
442	mmap_min_addr
443	
444	This file indicates the amount of address space  which a user process will
445	be restricted from mmapping.  Since kernel null dereference bugs could
446	accidentally operate based on the information in the first couple of pages
447	of memory userspace processes should not be allowed to write to them.  By
448	default this value is set to 0 and no protections will be enforced by the
449	security module.  Setting this value to something like 64k will allow the
450	vast majority of applications to work correctly and provide defense in depth
451	against future potential kernel bugs.
452	
453	==============================================================
454	
455	nr_hugepages
456	
457	Change the minimum size of the hugepage pool.
458	
459	See Documentation/vm/hugetlbpage.txt
460	
461	==============================================================
462	
463	nr_overcommit_hugepages
464	
465	Change the maximum size of the hugepage pool. The maximum is
466	nr_hugepages + nr_overcommit_hugepages.
467	
468	See Documentation/vm/hugetlbpage.txt
469	
470	==============================================================
471	
472	nr_trim_pages
473	
474	This is available only on NOMMU kernels.
475	
476	This value adjusts the excess page trimming behaviour of power-of-2 aligned
477	NOMMU mmap allocations.
478	
479	A value of 0 disables trimming of allocations entirely, while a value of 1
480	trims excess pages aggressively. Any value >= 1 acts as the watermark where
481	trimming of allocations is initiated.
482	
483	The default value is 1.
484	
485	See Documentation/nommu-mmap.txt for more information.
486	
487	==============================================================
488	
489	numa_zonelist_order
490	
491	This sysctl is only for NUMA.
492	'where the memory is allocated from' is controlled by zonelists.
493	(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
494	 you may be able to read ZONE_DMA as ZONE_DMA32...)
495	
496	In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
497	ZONE_NORMAL -> ZONE_DMA
498	This means that a memory allocation request for GFP_KERNEL will
499	get memory from ZONE_DMA only when ZONE_NORMAL is not available.
500	
501	In NUMA case, you can think of following 2 types of order.
502	Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
503	
504	(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
505	(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
506	
507	Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
508	will be used before ZONE_NORMAL exhaustion. This increases possibility of
509	out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
510	
511	Type(B) cannot offer the best locality but is more robust against OOM of
512	the DMA zone.
513	
514	Type(A) is called as "Node" order. Type (B) is "Zone" order.
515	
516	"Node order" orders the zonelists by node, then by zone within each node.
517	Specify "[Nn]ode" for node order
518	
519	"Zone Order" orders the zonelists by zone type, then by node within each
520	zone.  Specify "[Zz]one" for zone order.
521	
522	Specify "[Dd]efault" to request automatic configuration.  Autoconfiguration
523	will select "node" order in following case.
524	(1) if the DMA zone does not exist or
525	(2) if the DMA zone comprises greater than 50% of the available memory or
526	(3) if any node's DMA zone comprises greater than 70% of its local memory and
527	    the amount of local memory is big enough.
528	
529	Otherwise, "zone" order will be selected. Default order is recommended unless
530	this is causing problems for your system/application.
531	
532	==============================================================
533	
534	oom_dump_tasks
535	
536	Enables a system-wide task dump (excluding kernel threads) to be
537	produced when the kernel performs an OOM-killing and includes such
538	information as pid, uid, tgid, vm size, rss, nr_ptes, swapents,
539	oom_score_adj score, and name.  This is helpful to determine why the
540	OOM killer was invoked, to identify the rogue task that caused it,
541	and to determine why the OOM killer chose the task it did to kill.
542	
543	If this is set to zero, this information is suppressed.  On very
544	large systems with thousands of tasks it may not be feasible to dump
545	the memory state information for each one.  Such systems should not
546	be forced to incur a performance penalty in OOM conditions when the
547	information may not be desired.
548	
549	If this is set to non-zero, this information is shown whenever the
550	OOM killer actually kills a memory-hogging task.
551	
552	The default value is 1 (enabled).
553	
554	==============================================================
555	
556	oom_kill_allocating_task
557	
558	This enables or disables killing the OOM-triggering task in
559	out-of-memory situations.
560	
561	If this is set to zero, the OOM killer will scan through the entire
562	tasklist and select a task based on heuristics to kill.  This normally
563	selects a rogue memory-hogging task that frees up a large amount of
564	memory when killed.
565	
566	If this is set to non-zero, the OOM killer simply kills the task that
567	triggered the out-of-memory condition.  This avoids the expensive
568	tasklist scan.
569	
570	If panic_on_oom is selected, it takes precedence over whatever value
571	is used in oom_kill_allocating_task.
572	
573	The default value is 0.
574	
575	==============================================================
576	
577	overcommit_memory:
578	
579	This value contains a flag that enables memory overcommitment.
580	
581	When this flag is 0, the kernel attempts to estimate the amount
582	of free memory left when userspace requests more memory.
583	
584	When this flag is 1, the kernel pretends there is always enough
585	memory until it actually runs out.
586	
587	When this flag is 2, the kernel uses a "never overcommit"
588	policy that attempts to prevent any overcommit of memory.
589	Note that user_reserve_kbytes affects this policy.
590	
591	This feature can be very useful because there are a lot of
592	programs that malloc() huge amounts of memory "just-in-case"
593	and don't use much of it.
594	
595	The default value is 0.
596	
597	See Documentation/vm/overcommit-accounting and
598	security/commoncap.c::cap_vm_enough_memory() for more information.
599	
600	==============================================================
601	
602	overcommit_ratio:
603	
604	When overcommit_memory is set to 2, the committed address
605	space is not permitted to exceed swap plus this percentage
606	of physical RAM.  See above.
607	
608	==============================================================
609	
610	page-cluster
611	
612	page-cluster controls the number of pages up to which consecutive pages
613	are read in from swap in a single attempt. This is the swap counterpart
614	to page cache readahead.
615	The mentioned consecutivity is not in terms of virtual/physical addresses,
616	but consecutive on swap space - that means they were swapped out together.
617	
618	It is a logarithmic value - setting it to zero means "1 page", setting
619	it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
620	Zero disables swap readahead completely.
621	
622	The default value is three (eight pages at a time).  There may be some
623	small benefits in tuning this to a different value if your workload is
624	swap-intensive.
625	
626	Lower values mean lower latencies for initial faults, but at the same time
627	extra faults and I/O delays for following faults if they would have been part of
628	that consecutive pages readahead would have brought in.
629	
630	=============================================================
631	
632	panic_on_oom
633	
634	This enables or disables panic on out-of-memory feature.
635	
636	If this is set to 0, the kernel will kill some rogue process,
637	called oom_killer.  Usually, oom_killer can kill rogue processes and
638	system will survive.
639	
640	If this is set to 1, the kernel panics when out-of-memory happens.
641	However, if a process limits using nodes by mempolicy/cpusets,
642	and those nodes become memory exhaustion status, one process
643	may be killed by oom-killer. No panic occurs in this case.
644	Because other nodes' memory may be free. This means system total status
645	may be not fatal yet.
646	
647	If this is set to 2, the kernel panics compulsorily even on the
648	above-mentioned. Even oom happens under memory cgroup, the whole
649	system panics.
650	
651	The default value is 0.
652	1 and 2 are for failover of clustering. Please select either
653	according to your policy of failover.
654	panic_on_oom=2+kdump gives you very strong tool to investigate
655	why oom happens. You can get snapshot.
656	
657	=============================================================
658	
659	percpu_pagelist_fraction
660	
661	This is the fraction of pages at most (high mark pcp->high) in each zone that
662	are allocated for each per cpu page list.  The min value for this is 8.  It
663	means that we don't allow more than 1/8th of pages in each zone to be
664	allocated in any single per_cpu_pagelist.  This entry only changes the value
665	of hot per cpu pagelists.  User can specify a number like 100 to allocate
666	1/100th of each zone to each per cpu page list.
667	
668	The batch value of each per cpu pagelist is also updated as a result.  It is
669	set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
670	
671	The initial value is zero.  Kernel does not use this value at boot time to set
672	the high water marks for each per cpu page list.
673	
674	==============================================================
675	
676	stat_interval
677	
678	The time interval between which vm statistics are updated.  The default
679	is 1 second.
680	
681	==============================================================
682	
683	swappiness
684	
685	This control is used to define how aggressive the kernel will swap
686	memory pages.  Higher values will increase agressiveness, lower values
687	decrease the amount of swap.
688	
689	The default value is 60.
690	
691	==============================================================
692	
693	- user_reserve_kbytes
694	
695	When overcommit_memory is set to 2, "never overommit" mode, reserve
696	min(3% of current process size, user_reserve_kbytes) of free memory.
697	This is intended to prevent a user from starting a single memory hogging
698	process, such that they cannot recover (kill the hog).
699	
700	user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
701	
702	If this is reduced to zero, then the user will be allowed to allocate
703	all free memory with a single process, minus admin_reserve_kbytes.
704	Any subsequent attempts to execute a command will result in
705	"fork: Cannot allocate memory".
706	
707	Changing this takes effect whenever an application requests memory.
708	
709	==============================================================
710	
711	vfs_cache_pressure
712	------------------
713	
714	Controls the tendency of the kernel to reclaim the memory which is used for
715	caching of directory and inode objects.
716	
717	At the default value of vfs_cache_pressure=100 the kernel will attempt to
718	reclaim dentries and inodes at a "fair" rate with respect to pagecache and
719	swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
720	to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
721	never reclaim dentries and inodes due to memory pressure and this can easily
722	lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
723	causes the kernel to prefer to reclaim dentries and inodes.
724	
725	==============================================================
726	
727	zone_reclaim_mode:
728	
729	Zone_reclaim_mode allows someone to set more or less aggressive approaches to
730	reclaim memory when a zone runs out of memory. If it is set to zero then no
731	zone reclaim occurs. Allocations will be satisfied from other zones / nodes
732	in the system.
733	
734	This is value ORed together of
735	
736	1	= Zone reclaim on
737	2	= Zone reclaim writes dirty pages out
738	4	= Zone reclaim swaps pages
739	
740	zone_reclaim_mode is set during bootup to 1 if it is determined that pages
741	from remote zones will cause a measurable performance reduction. The
742	page allocator will then reclaim easily reusable pages (those page
743	cache pages that are currently not used) before allocating off node pages.
744	
745	It may be beneficial to switch off zone reclaim if the system is
746	used for a file server and all of memory should be used for caching files
747	from disk. In that case the caching effect is more important than
748	data locality.
749	
750	Allowing zone reclaim to write out pages stops processes that are
751	writing large amounts of data from dirtying pages on other nodes. Zone
752	reclaim will write out dirty pages if a zone fills up and so effectively
753	throttle the process. This may decrease the performance of a single process
754	since it cannot use all of system memory to buffer the outgoing writes
755	anymore but it preserve the memory on other nodes so that the performance
756	of other processes running on other nodes will not be affected.
757	
758	Allowing regular swap effectively restricts allocations to the local
759	node unless explicitly overridden by memory policies or cpuset
760	configurations.
761	
762	============ End of Document =================================
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.