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db0fb184 | 1 | Documentation for /proc/sys/vm/* kernel version 2.6.29 |
1da177e4 | 2 | (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> |
db0fb184 | 3 | (c) 2008 Peter W. Morreale <pmorreale@novell.com> |
1da177e4 LT |
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 | |
db0fb184 | 10 | /proc/sys/vm and is valid for Linux kernel version 2.6.29. |
1da177e4 LT |
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: | |
db0fb184 | 20 | |
4eeab4f5 | 21 | - admin_reserve_kbytes |
db0fb184 | 22 | - block_dump |
76ab0f53 | 23 | - compact_memory |
5bbe3547 | 24 | - compact_unevictable_allowed |
db0fb184 | 25 | - dirty_background_bytes |
1da177e4 | 26 | - dirty_background_ratio |
db0fb184 | 27 | - dirty_bytes |
1da177e4 | 28 | - dirty_expire_centisecs |
db0fb184 | 29 | - dirty_ratio |
1da177e4 | 30 | - dirty_writeback_centisecs |
db0fb184 | 31 | - drop_caches |
5e771905 | 32 | - extfrag_threshold |
db0fb184 PM |
33 | - hugepages_treat_as_movable |
34 | - hugetlb_shm_group | |
35 | - laptop_mode | |
36 | - legacy_va_layout | |
37 | - lowmem_reserve_ratio | |
1da177e4 | 38 | - max_map_count |
6a46079c AK |
39 | - memory_failure_early_kill |
40 | - memory_failure_recovery | |
1da177e4 | 41 | - min_free_kbytes |
0ff38490 | 42 | - min_slab_ratio |
db0fb184 PM |
43 | - min_unmapped_ratio |
44 | - mmap_min_addr | |
d07e2259 DC |
45 | - mmap_rnd_bits |
46 | - mmap_rnd_compat_bits | |
d5dbac87 NA |
47 | - nr_hugepages |
48 | - nr_overcommit_hugepages | |
db0fb184 PM |
49 | - nr_trim_pages (only if CONFIG_MMU=n) |
50 | - numa_zonelist_order | |
51 | - oom_dump_tasks | |
52 | - oom_kill_allocating_task | |
49f0ce5f | 53 | - overcommit_kbytes |
db0fb184 PM |
54 | - overcommit_memory |
55 | - overcommit_ratio | |
56 | - page-cluster | |
57 | - panic_on_oom | |
58 | - percpu_pagelist_fraction | |
59 | - stat_interval | |
52b6f46b | 60 | - stat_refresh |
4518085e | 61 | - numa_stat |
db0fb184 | 62 | - swappiness |
c9b1d098 | 63 | - user_reserve_kbytes |
db0fb184 | 64 | - vfs_cache_pressure |
e6507a00 | 65 | - watermark_scale_factor |
db0fb184 PM |
66 | - zone_reclaim_mode |
67 | ||
1da177e4 LT |
68 | ============================================================== |
69 | ||
4eeab4f5 AS |
70 | admin_reserve_kbytes |
71 | ||
72 | The amount of free memory in the system that should be reserved for users | |
73 | with the capability cap_sys_admin. | |
74 | ||
75 | admin_reserve_kbytes defaults to min(3% of free pages, 8MB) | |
76 | ||
77 | That should provide enough for the admin to log in and kill a process, | |
78 | if necessary, under the default overcommit 'guess' mode. | |
79 | ||
80 | Systems running under overcommit 'never' should increase this to account | |
81 | for the full Virtual Memory Size of programs used to recover. Otherwise, | |
82 | root may not be able to log in to recover the system. | |
83 | ||
84 | How do you calculate a minimum useful reserve? | |
85 | ||
86 | sshd or login + bash (or some other shell) + top (or ps, kill, etc.) | |
87 | ||
88 | For overcommit 'guess', we can sum resident set sizes (RSS). | |
89 | On x86_64 this is about 8MB. | |
90 | ||
91 | For overcommit 'never', we can take the max of their virtual sizes (VSZ) | |
92 | and add the sum of their RSS. | |
93 | On x86_64 this is about 128MB. | |
94 | ||
95 | Changing this takes effect whenever an application requests memory. | |
96 | ||
97 | ============================================================== | |
98 | ||
db0fb184 | 99 | block_dump |
1da177e4 | 100 | |
db0fb184 PM |
101 | block_dump enables block I/O debugging when set to a nonzero value. More |
102 | information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. | |
1da177e4 LT |
103 | |
104 | ============================================================== | |
105 | ||
76ab0f53 MG |
106 | compact_memory |
107 | ||
108 | Available only when CONFIG_COMPACTION is set. When 1 is written to the file, | |
109 | all zones are compacted such that free memory is available in contiguous | |
110 | blocks where possible. This can be important for example in the allocation of | |
111 | huge pages although processes will also directly compact memory as required. | |
112 | ||
113 | ============================================================== | |
114 | ||
5bbe3547 EM |
115 | compact_unevictable_allowed |
116 | ||
117 | Available only when CONFIG_COMPACTION is set. When set to 1, compaction is | |
118 | allowed to examine the unevictable lru (mlocked pages) for pages to compact. | |
119 | This should be used on systems where stalls for minor page faults are an | |
120 | acceptable trade for large contiguous free memory. Set to 0 to prevent | |
121 | compaction from moving pages that are unevictable. Default value is 1. | |
122 | ||
123 | ============================================================== | |
124 | ||
db0fb184 | 125 | dirty_background_bytes |
1da177e4 | 126 | |
6601fac8 AB |
127 | Contains the amount of dirty memory at which the background kernel |
128 | flusher threads will start writeback. | |
1da177e4 | 129 | |
abffc020 AR |
130 | Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only |
131 | one of them may be specified at a time. When one sysctl is written it is | |
132 | immediately taken into account to evaluate the dirty memory limits and the | |
133 | other appears as 0 when read. | |
1da177e4 | 134 | |
db0fb184 | 135 | ============================================================== |
1da177e4 | 136 | |
db0fb184 | 137 | dirty_background_ratio |
1da177e4 | 138 | |
715ea41e ZL |
139 | Contains, as a percentage of total available memory that contains free pages |
140 | and reclaimable pages, the number of pages at which the background kernel | |
141 | flusher threads will start writing out dirty data. | |
142 | ||
d83e2a4e | 143 | The total available memory is not equal to total system memory. |
1da177e4 | 144 | |
db0fb184 | 145 | ============================================================== |
1da177e4 | 146 | |
db0fb184 PM |
147 | dirty_bytes |
148 | ||
149 | Contains the amount of dirty memory at which a process generating disk writes | |
150 | will itself start writeback. | |
151 | ||
abffc020 AR |
152 | Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be |
153 | specified at a time. When one sysctl is written it is immediately taken into | |
154 | account to evaluate the dirty memory limits and the other appears as 0 when | |
155 | read. | |
1da177e4 | 156 | |
9e4a5bda AR |
157 | Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any |
158 | value lower than this limit will be ignored and the old configuration will be | |
159 | retained. | |
160 | ||
1da177e4 LT |
161 | ============================================================== |
162 | ||
db0fb184 | 163 | dirty_expire_centisecs |
1da177e4 | 164 | |
db0fb184 | 165 | This tunable is used to define when dirty data is old enough to be eligible |
6601fac8 AB |
166 | for writeout by the kernel flusher threads. It is expressed in 100'ths |
167 | of a second. Data which has been dirty in-memory for longer than this | |
168 | interval will be written out next time a flusher thread wakes up. | |
db0fb184 PM |
169 | |
170 | ============================================================== | |
171 | ||
172 | dirty_ratio | |
173 | ||
715ea41e ZL |
174 | Contains, as a percentage of total available memory that contains free pages |
175 | and reclaimable pages, the number of pages at which a process which is | |
176 | generating disk writes will itself start writing out dirty data. | |
177 | ||
d83e2a4e | 178 | The total available memory is not equal to total system memory. |
1da177e4 LT |
179 | |
180 | ============================================================== | |
181 | ||
db0fb184 | 182 | dirty_writeback_centisecs |
1da177e4 | 183 | |
6601fac8 | 184 | The kernel flusher threads will periodically wake up and write `old' data |
db0fb184 PM |
185 | out to disk. This tunable expresses the interval between those wakeups, in |
186 | 100'ths of a second. | |
1da177e4 | 187 | |
db0fb184 | 188 | Setting this to zero disables periodic writeback altogether. |
1da177e4 LT |
189 | |
190 | ============================================================== | |
191 | ||
db0fb184 | 192 | drop_caches |
1da177e4 | 193 | |
5509a5d2 DH |
194 | Writing to this will cause the kernel to drop clean caches, as well as |
195 | reclaimable slab objects like dentries and inodes. Once dropped, their | |
196 | memory becomes free. | |
1da177e4 | 197 | |
db0fb184 PM |
198 | To free pagecache: |
199 | echo 1 > /proc/sys/vm/drop_caches | |
5509a5d2 | 200 | To free reclaimable slab objects (includes dentries and inodes): |
db0fb184 | 201 | echo 2 > /proc/sys/vm/drop_caches |
5509a5d2 | 202 | To free slab objects and pagecache: |
db0fb184 | 203 | echo 3 > /proc/sys/vm/drop_caches |
1da177e4 | 204 | |
5509a5d2 DH |
205 | This is a non-destructive operation and will not free any dirty objects. |
206 | To increase the number of objects freed by this operation, the user may run | |
207 | `sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the | |
208 | number of dirty objects on the system and create more candidates to be | |
209 | dropped. | |
210 | ||
211 | This file is not a means to control the growth of the various kernel caches | |
212 | (inodes, dentries, pagecache, etc...) These objects are automatically | |
213 | reclaimed by the kernel when memory is needed elsewhere on the system. | |
214 | ||
215 | Use of this file can cause performance problems. Since it discards cached | |
216 | objects, it may cost a significant amount of I/O and CPU to recreate the | |
217 | dropped objects, especially if they were under heavy use. Because of this, | |
218 | use outside of a testing or debugging environment is not recommended. | |
219 | ||
220 | You may see informational messages in your kernel log when this file is | |
221 | used: | |
222 | ||
223 | cat (1234): drop_caches: 3 | |
224 | ||
225 | These are informational only. They do not mean that anything is wrong | |
226 | with your system. To disable them, echo 4 (bit 3) into drop_caches. | |
1da177e4 LT |
227 | |
228 | ============================================================== | |
229 | ||
5e771905 MG |
230 | extfrag_threshold |
231 | ||
232 | This parameter affects whether the kernel will compact memory or direct | |
a10726bb RV |
233 | reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in |
234 | debugfs shows what the fragmentation index for each order is in each zone in | |
235 | the system. Values tending towards 0 imply allocations would fail due to lack | |
236 | of memory, values towards 1000 imply failures are due to fragmentation and -1 | |
237 | implies that the allocation will succeed as long as watermarks are met. | |
5e771905 MG |
238 | |
239 | The kernel will not compact memory in a zone if the | |
240 | fragmentation index is <= extfrag_threshold. The default value is 500. | |
241 | ||
242 | ============================================================== | |
243 | ||
d09b6468 MH |
244 | highmem_is_dirtyable |
245 | ||
246 | Available only for systems with CONFIG_HIGHMEM enabled (32b systems). | |
247 | ||
248 | This parameter controls whether the high memory is considered for dirty | |
249 | writers throttling. This is not the case by default which means that | |
250 | only the amount of memory directly visible/usable by the kernel can | |
251 | be dirtied. As a result, on systems with a large amount of memory and | |
252 | lowmem basically depleted writers might be throttled too early and | |
253 | streaming writes can get very slow. | |
254 | ||
255 | Changing the value to non zero would allow more memory to be dirtied | |
256 | and thus allow writers to write more data which can be flushed to the | |
257 | storage more effectively. Note this also comes with a risk of pre-mature | |
258 | OOM killer because some writers (e.g. direct block device writes) can | |
259 | only use the low memory and they can fill it up with dirty data without | |
260 | any throttling. | |
261 | ||
262 | ============================================================== | |
263 | ||
db0fb184 | 264 | hugepages_treat_as_movable |
1da177e4 | 265 | |
86cdb465 NH |
266 | This parameter controls whether we can allocate hugepages from ZONE_MOVABLE |
267 | or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE. | |
268 | ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified, | |
269 | so this parameter has no effect if used without kernelcore=. | |
270 | ||
271 | Hugepage migration is now available in some situations which depend on the | |
272 | architecture and/or the hugepage size. If a hugepage supports migration, | |
273 | allocation from ZONE_MOVABLE is always enabled for the hugepage regardless | |
274 | of the value of this parameter. | |
275 | IOW, this parameter affects only non-migratable hugepages. | |
276 | ||
277 | Assuming that hugepages are not migratable in your system, one usecase of | |
278 | this parameter is that users can make hugepage pool more extensible by | |
279 | enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE | |
280 | page reclaim/migration/compaction work more and you can get contiguous | |
281 | memory more likely. Note that using ZONE_MOVABLE for non-migratable | |
282 | hugepages can do harm to other features like memory hotremove (because | |
283 | memory hotremove expects that memory blocks on ZONE_MOVABLE are always | |
284 | removable,) so it's a trade-off responsible for the users. | |
24950898 | 285 | |
8ad4b1fb RS |
286 | ============================================================== |
287 | ||
db0fb184 | 288 | hugetlb_shm_group |
8ad4b1fb | 289 | |
db0fb184 PM |
290 | hugetlb_shm_group contains group id that is allowed to create SysV |
291 | shared memory segment using hugetlb page. | |
8ad4b1fb | 292 | |
db0fb184 | 293 | ============================================================== |
8ad4b1fb | 294 | |
db0fb184 | 295 | laptop_mode |
1743660b | 296 | |
db0fb184 PM |
297 | laptop_mode is a knob that controls "laptop mode". All the things that are |
298 | controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. | |
1743660b | 299 | |
db0fb184 | 300 | ============================================================== |
1743660b | 301 | |
db0fb184 | 302 | legacy_va_layout |
1b2ffb78 | 303 | |
2174efb6 | 304 | If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel |
db0fb184 | 305 | will use the legacy (2.4) layout for all processes. |
1b2ffb78 | 306 | |
db0fb184 | 307 | ============================================================== |
1b2ffb78 | 308 | |
db0fb184 PM |
309 | lowmem_reserve_ratio |
310 | ||
311 | For some specialised workloads on highmem machines it is dangerous for | |
312 | the kernel to allow process memory to be allocated from the "lowmem" | |
313 | zone. This is because that memory could then be pinned via the mlock() | |
314 | system call, or by unavailability of swapspace. | |
315 | ||
316 | And on large highmem machines this lack of reclaimable lowmem memory | |
317 | can be fatal. | |
318 | ||
319 | So the Linux page allocator has a mechanism which prevents allocations | |
320 | which _could_ use highmem from using too much lowmem. This means that | |
321 | a certain amount of lowmem is defended from the possibility of being | |
322 | captured into pinned user memory. | |
323 | ||
324 | (The same argument applies to the old 16 megabyte ISA DMA region. This | |
325 | mechanism will also defend that region from allocations which could use | |
326 | highmem or lowmem). | |
327 | ||
328 | The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is | |
329 | in defending these lower zones. | |
330 | ||
331 | If you have a machine which uses highmem or ISA DMA and your | |
332 | applications are using mlock(), or if you are running with no swap then | |
333 | you probably should change the lowmem_reserve_ratio setting. | |
334 | ||
335 | The lowmem_reserve_ratio is an array. You can see them by reading this file. | |
336 | - | |
337 | % cat /proc/sys/vm/lowmem_reserve_ratio | |
338 | 256 256 32 | |
339 | - | |
340 | Note: # of this elements is one fewer than number of zones. Because the highest | |
341 | zone's value is not necessary for following calculation. | |
342 | ||
343 | But, these values are not used directly. The kernel calculates # of protection | |
344 | pages for each zones from them. These are shown as array of protection pages | |
345 | in /proc/zoneinfo like followings. (This is an example of x86-64 box). | |
346 | Each zone has an array of protection pages like this. | |
347 | ||
348 | - | |
349 | Node 0, zone DMA | |
350 | pages free 1355 | |
351 | min 3 | |
352 | low 3 | |
353 | high 4 | |
354 | : | |
355 | : | |
356 | numa_other 0 | |
357 | protection: (0, 2004, 2004, 2004) | |
358 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
359 | pagesets | |
360 | cpu: 0 pcp: 0 | |
361 | : | |
362 | - | |
363 | These protections are added to score to judge whether this zone should be used | |
364 | for page allocation or should be reclaimed. | |
365 | ||
366 | In this example, if normal pages (index=2) are required to this DMA zone and | |
41858966 MG |
367 | watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should |
368 | not be used because pages_free(1355) is smaller than watermark + protection[2] | |
db0fb184 PM |
369 | (4 + 2004 = 2008). If this protection value is 0, this zone would be used for |
370 | normal page requirement. If requirement is DMA zone(index=0), protection[0] | |
371 | (=0) is used. | |
372 | ||
373 | zone[i]'s protection[j] is calculated by following expression. | |
374 | ||
375 | (i < j): | |
376 | zone[i]->protection[j] | |
013110a7 | 377 | = (total sums of managed_pages from zone[i+1] to zone[j] on the node) |
db0fb184 PM |
378 | / lowmem_reserve_ratio[i]; |
379 | (i = j): | |
380 | (should not be protected. = 0; | |
381 | (i > j): | |
382 | (not necessary, but looks 0) | |
383 | ||
384 | The default values of lowmem_reserve_ratio[i] are | |
385 | 256 (if zone[i] means DMA or DMA32 zone) | |
386 | 32 (others). | |
387 | As above expression, they are reciprocal number of ratio. | |
013110a7 | 388 | 256 means 1/256. # of protection pages becomes about "0.39%" of total managed |
db0fb184 PM |
389 | pages of higher zones on the node. |
390 | ||
391 | If you would like to protect more pages, smaller values are effective. | |
392 | The minimum value is 1 (1/1 -> 100%). | |
1b2ffb78 | 393 | |
db0fb184 | 394 | ============================================================== |
1b2ffb78 | 395 | |
db0fb184 | 396 | max_map_count: |
1743660b | 397 | |
db0fb184 PM |
398 | This file contains the maximum number of memory map areas a process |
399 | may have. Memory map areas are used as a side-effect of calling | |
def5efe0 DR |
400 | malloc, directly by mmap, mprotect, and madvise, and also when loading |
401 | shared libraries. | |
1743660b | 402 | |
db0fb184 PM |
403 | While most applications need less than a thousand maps, certain |
404 | programs, particularly malloc debuggers, may consume lots of them, | |
405 | e.g., up to one or two maps per allocation. | |
fadd8fbd | 406 | |
db0fb184 | 407 | The default value is 65536. |
9614634f | 408 | |
6a46079c AK |
409 | ============================================================= |
410 | ||
411 | memory_failure_early_kill: | |
412 | ||
413 | Control how to kill processes when uncorrected memory error (typically | |
414 | a 2bit error in a memory module) is detected in the background by hardware | |
415 | that cannot be handled by the kernel. In some cases (like the page | |
416 | still having a valid copy on disk) the kernel will handle the failure | |
417 | transparently without affecting any applications. But if there is | |
418 | no other uptodate copy of the data it will kill to prevent any data | |
419 | corruptions from propagating. | |
420 | ||
421 | 1: Kill all processes that have the corrupted and not reloadable page mapped | |
422 | as soon as the corruption is detected. Note this is not supported | |
423 | for a few types of pages, like kernel internally allocated data or | |
424 | the swap cache, but works for the majority of user pages. | |
425 | ||
426 | 0: Only unmap the corrupted page from all processes and only kill a process | |
427 | who tries to access it. | |
428 | ||
429 | The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can | |
430 | handle this if they want to. | |
431 | ||
432 | This is only active on architectures/platforms with advanced machine | |
433 | check handling and depends on the hardware capabilities. | |
434 | ||
435 | Applications can override this setting individually with the PR_MCE_KILL prctl | |
436 | ||
437 | ============================================================== | |
438 | ||
439 | memory_failure_recovery | |
440 | ||
441 | Enable memory failure recovery (when supported by the platform) | |
442 | ||
443 | 1: Attempt recovery. | |
444 | ||
445 | 0: Always panic on a memory failure. | |
446 | ||
db0fb184 | 447 | ============================================================== |
9614634f | 448 | |
db0fb184 | 449 | min_free_kbytes: |
9614634f | 450 | |
db0fb184 | 451 | This is used to force the Linux VM to keep a minimum number |
41858966 MG |
452 | of kilobytes free. The VM uses this number to compute a |
453 | watermark[WMARK_MIN] value for each lowmem zone in the system. | |
454 | Each lowmem zone gets a number of reserved free pages based | |
455 | proportionally on its size. | |
db0fb184 PM |
456 | |
457 | Some minimal amount of memory is needed to satisfy PF_MEMALLOC | |
458 | allocations; if you set this to lower than 1024KB, your system will | |
459 | become subtly broken, and prone to deadlock under high loads. | |
460 | ||
461 | Setting this too high will OOM your machine instantly. | |
9614634f CL |
462 | |
463 | ============================================================= | |
464 | ||
0ff38490 CL |
465 | min_slab_ratio: |
466 | ||
467 | This is available only on NUMA kernels. | |
468 | ||
469 | A percentage of the total pages in each zone. On Zone reclaim | |
470 | (fallback from the local zone occurs) slabs will be reclaimed if more | |
471 | than this percentage of pages in a zone are reclaimable slab pages. | |
472 | This insures that the slab growth stays under control even in NUMA | |
473 | systems that rarely perform global reclaim. | |
474 | ||
475 | The default is 5 percent. | |
476 | ||
477 | Note that slab reclaim is triggered in a per zone / node fashion. | |
478 | The process of reclaiming slab memory is currently not node specific | |
479 | and may not be fast. | |
480 | ||
481 | ============================================================= | |
482 | ||
db0fb184 | 483 | min_unmapped_ratio: |
fadd8fbd | 484 | |
db0fb184 | 485 | This is available only on NUMA kernels. |
fadd8fbd | 486 | |
90afa5de MG |
487 | This is a percentage of the total pages in each zone. Zone reclaim will |
488 | only occur if more than this percentage of pages are in a state that | |
489 | zone_reclaim_mode allows to be reclaimed. | |
490 | ||
491 | If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared | |
492 | against all file-backed unmapped pages including swapcache pages and tmpfs | |
493 | files. Otherwise, only unmapped pages backed by normal files but not tmpfs | |
494 | files and similar are considered. | |
2b744c01 | 495 | |
db0fb184 | 496 | The default is 1 percent. |
fadd8fbd | 497 | |
db0fb184 | 498 | ============================================================== |
2b744c01 | 499 | |
db0fb184 | 500 | mmap_min_addr |
ed032189 | 501 | |
db0fb184 | 502 | This file indicates the amount of address space which a user process will |
af901ca1 | 503 | be restricted from mmapping. Since kernel null dereference bugs could |
db0fb184 PM |
504 | accidentally operate based on the information in the first couple of pages |
505 | of memory userspace processes should not be allowed to write to them. By | |
506 | default this value is set to 0 and no protections will be enforced by the | |
507 | security module. Setting this value to something like 64k will allow the | |
508 | vast majority of applications to work correctly and provide defense in depth | |
509 | against future potential kernel bugs. | |
fe071d7e | 510 | |
db0fb184 | 511 | ============================================================== |
fef1bdd6 | 512 | |
d07e2259 DC |
513 | mmap_rnd_bits: |
514 | ||
515 | This value can be used to select the number of bits to use to | |
516 | determine the random offset to the base address of vma regions | |
517 | resulting from mmap allocations on architectures which support | |
518 | tuning address space randomization. This value will be bounded | |
519 | by the architecture's minimum and maximum supported values. | |
520 | ||
521 | This value can be changed after boot using the | |
522 | /proc/sys/vm/mmap_rnd_bits tunable | |
523 | ||
524 | ============================================================== | |
525 | ||
526 | mmap_rnd_compat_bits: | |
527 | ||
528 | This value can be used to select the number of bits to use to | |
529 | determine the random offset to the base address of vma regions | |
530 | resulting from mmap allocations for applications run in | |
531 | compatibility mode on architectures which support tuning address | |
532 | space randomization. This value will be bounded by the | |
533 | architecture's minimum and maximum supported values. | |
534 | ||
535 | This value can be changed after boot using the | |
536 | /proc/sys/vm/mmap_rnd_compat_bits tunable | |
537 | ||
538 | ============================================================== | |
539 | ||
db0fb184 | 540 | nr_hugepages |
fef1bdd6 | 541 | |
db0fb184 | 542 | Change the minimum size of the hugepage pool. |
fef1bdd6 | 543 | |
db0fb184 | 544 | See Documentation/vm/hugetlbpage.txt |
fef1bdd6 | 545 | |
db0fb184 | 546 | ============================================================== |
fef1bdd6 | 547 | |
db0fb184 | 548 | nr_overcommit_hugepages |
fef1bdd6 | 549 | |
db0fb184 PM |
550 | Change the maximum size of the hugepage pool. The maximum is |
551 | nr_hugepages + nr_overcommit_hugepages. | |
fe071d7e | 552 | |
db0fb184 | 553 | See Documentation/vm/hugetlbpage.txt |
fe071d7e | 554 | |
db0fb184 | 555 | ============================================================== |
fe071d7e | 556 | |
db0fb184 | 557 | nr_trim_pages |
ed032189 | 558 | |
db0fb184 PM |
559 | This is available only on NOMMU kernels. |
560 | ||
561 | This value adjusts the excess page trimming behaviour of power-of-2 aligned | |
562 | NOMMU mmap allocations. | |
563 | ||
564 | A value of 0 disables trimming of allocations entirely, while a value of 1 | |
565 | trims excess pages aggressively. Any value >= 1 acts as the watermark where | |
566 | trimming of allocations is initiated. | |
567 | ||
568 | The default value is 1. | |
569 | ||
570 | See Documentation/nommu-mmap.txt for more information. | |
ed032189 | 571 | |
f0c0b2b8 KH |
572 | ============================================================== |
573 | ||
574 | numa_zonelist_order | |
575 | ||
c9bff3ee MH |
576 | This sysctl is only for NUMA and it is deprecated. Anything but |
577 | Node order will fail! | |
578 | ||
f0c0b2b8 KH |
579 | 'where the memory is allocated from' is controlled by zonelists. |
580 | (This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. | |
581 | you may be able to read ZONE_DMA as ZONE_DMA32...) | |
582 | ||
583 | In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. | |
584 | ZONE_NORMAL -> ZONE_DMA | |
585 | This means that a memory allocation request for GFP_KERNEL will | |
586 | get memory from ZONE_DMA only when ZONE_NORMAL is not available. | |
587 | ||
588 | In NUMA case, you can think of following 2 types of order. | |
589 | Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL | |
590 | ||
591 | (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL | |
592 | (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. | |
593 | ||
594 | Type(A) offers the best locality for processes on Node(0), but ZONE_DMA | |
595 | will be used before ZONE_NORMAL exhaustion. This increases possibility of | |
596 | out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. | |
597 | ||
598 | Type(B) cannot offer the best locality but is more robust against OOM of | |
599 | the DMA zone. | |
600 | ||
601 | Type(A) is called as "Node" order. Type (B) is "Zone" order. | |
602 | ||
603 | "Node order" orders the zonelists by node, then by zone within each node. | |
5a3016a6 | 604 | Specify "[Nn]ode" for node order |
f0c0b2b8 KH |
605 | |
606 | "Zone Order" orders the zonelists by zone type, then by node within each | |
5a3016a6 | 607 | zone. Specify "[Zz]one" for zone order. |
f0c0b2b8 | 608 | |
7c88a292 XQ |
609 | Specify "[Dd]efault" to request automatic configuration. |
610 | ||
611 | On 32-bit, the Normal zone needs to be preserved for allocations accessible | |
612 | by the kernel, so "zone" order will be selected. | |
613 | ||
614 | On 64-bit, devices that require DMA32/DMA are relatively rare, so "node" | |
615 | order will be selected. | |
616 | ||
617 | Default order is recommended unless this is causing problems for your | |
618 | system/application. | |
d5dbac87 NA |
619 | |
620 | ============================================================== | |
621 | ||
db0fb184 | 622 | oom_dump_tasks |
d5dbac87 | 623 | |
dc6c9a35 KS |
624 | Enables a system-wide task dump (excluding kernel threads) to be produced |
625 | when the kernel performs an OOM-killing and includes such information as | |
af5b0f6a KS |
626 | pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj |
627 | score, and name. This is helpful to determine why the OOM killer was | |
628 | invoked, to identify the rogue task that caused it, and to determine why | |
629 | the OOM killer chose the task it did to kill. | |
d5dbac87 | 630 | |
db0fb184 PM |
631 | If this is set to zero, this information is suppressed. On very |
632 | large systems with thousands of tasks it may not be feasible to dump | |
633 | the memory state information for each one. Such systems should not | |
634 | be forced to incur a performance penalty in OOM conditions when the | |
635 | information may not be desired. | |
636 | ||
637 | If this is set to non-zero, this information is shown whenever the | |
638 | OOM killer actually kills a memory-hogging task. | |
639 | ||
ad915c43 | 640 | The default value is 1 (enabled). |
d5dbac87 NA |
641 | |
642 | ============================================================== | |
643 | ||
db0fb184 | 644 | oom_kill_allocating_task |
d5dbac87 | 645 | |
db0fb184 PM |
646 | This enables or disables killing the OOM-triggering task in |
647 | out-of-memory situations. | |
d5dbac87 | 648 | |
db0fb184 PM |
649 | If this is set to zero, the OOM killer will scan through the entire |
650 | tasklist and select a task based on heuristics to kill. This normally | |
651 | selects a rogue memory-hogging task that frees up a large amount of | |
652 | memory when killed. | |
653 | ||
654 | If this is set to non-zero, the OOM killer simply kills the task that | |
655 | triggered the out-of-memory condition. This avoids the expensive | |
656 | tasklist scan. | |
657 | ||
658 | If panic_on_oom is selected, it takes precedence over whatever value | |
659 | is used in oom_kill_allocating_task. | |
660 | ||
661 | The default value is 0. | |
dd8632a1 PM |
662 | |
663 | ============================================================== | |
664 | ||
49f0ce5f JM |
665 | overcommit_kbytes: |
666 | ||
667 | When overcommit_memory is set to 2, the committed address space is not | |
668 | permitted to exceed swap plus this amount of physical RAM. See below. | |
669 | ||
670 | Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one | |
671 | of them may be specified at a time. Setting one disables the other (which | |
672 | then appears as 0 when read). | |
673 | ||
674 | ============================================================== | |
675 | ||
db0fb184 | 676 | overcommit_memory: |
dd8632a1 | 677 | |
db0fb184 | 678 | This value contains a flag that enables memory overcommitment. |
dd8632a1 | 679 | |
db0fb184 PM |
680 | When this flag is 0, the kernel attempts to estimate the amount |
681 | of free memory left when userspace requests more memory. | |
dd8632a1 | 682 | |
db0fb184 PM |
683 | When this flag is 1, the kernel pretends there is always enough |
684 | memory until it actually runs out. | |
dd8632a1 | 685 | |
db0fb184 PM |
686 | When this flag is 2, the kernel uses a "never overcommit" |
687 | policy that attempts to prevent any overcommit of memory. | |
c9b1d098 | 688 | Note that user_reserve_kbytes affects this policy. |
dd8632a1 | 689 | |
db0fb184 PM |
690 | This feature can be very useful because there are a lot of |
691 | programs that malloc() huge amounts of memory "just-in-case" | |
692 | and don't use much of it. | |
693 | ||
694 | The default value is 0. | |
695 | ||
696 | See Documentation/vm/overcommit-accounting and | |
c56050c7 | 697 | mm/mmap.c::__vm_enough_memory() for more information. |
db0fb184 PM |
698 | |
699 | ============================================================== | |
700 | ||
701 | overcommit_ratio: | |
702 | ||
703 | When overcommit_memory is set to 2, the committed address | |
704 | space is not permitted to exceed swap plus this percentage | |
705 | of physical RAM. See above. | |
706 | ||
707 | ============================================================== | |
708 | ||
709 | page-cluster | |
710 | ||
df858fa8 CE |
711 | page-cluster controls the number of pages up to which consecutive pages |
712 | are read in from swap in a single attempt. This is the swap counterpart | |
713 | to page cache readahead. | |
714 | The mentioned consecutivity is not in terms of virtual/physical addresses, | |
715 | but consecutive on swap space - that means they were swapped out together. | |
db0fb184 PM |
716 | |
717 | It is a logarithmic value - setting it to zero means "1 page", setting | |
718 | it to 1 means "2 pages", setting it to 2 means "4 pages", etc. | |
df858fa8 | 719 | Zero disables swap readahead completely. |
db0fb184 PM |
720 | |
721 | The default value is three (eight pages at a time). There may be some | |
722 | small benefits in tuning this to a different value if your workload is | |
723 | swap-intensive. | |
724 | ||
df858fa8 CE |
725 | Lower values mean lower latencies for initial faults, but at the same time |
726 | extra faults and I/O delays for following faults if they would have been part of | |
727 | that consecutive pages readahead would have brought in. | |
728 | ||
db0fb184 PM |
729 | ============================================================= |
730 | ||
731 | panic_on_oom | |
732 | ||
733 | This enables or disables panic on out-of-memory feature. | |
734 | ||
735 | If this is set to 0, the kernel will kill some rogue process, | |
736 | called oom_killer. Usually, oom_killer can kill rogue processes and | |
737 | system will survive. | |
738 | ||
739 | If this is set to 1, the kernel panics when out-of-memory happens. | |
740 | However, if a process limits using nodes by mempolicy/cpusets, | |
741 | and those nodes become memory exhaustion status, one process | |
742 | may be killed by oom-killer. No panic occurs in this case. | |
743 | Because other nodes' memory may be free. This means system total status | |
744 | may be not fatal yet. | |
745 | ||
746 | If this is set to 2, the kernel panics compulsorily even on the | |
daaf1e68 KH |
747 | above-mentioned. Even oom happens under memory cgroup, the whole |
748 | system panics. | |
db0fb184 PM |
749 | |
750 | The default value is 0. | |
751 | 1 and 2 are for failover of clustering. Please select either | |
752 | according to your policy of failover. | |
daaf1e68 KH |
753 | panic_on_oom=2+kdump gives you very strong tool to investigate |
754 | why oom happens. You can get snapshot. | |
db0fb184 PM |
755 | |
756 | ============================================================= | |
757 | ||
758 | percpu_pagelist_fraction | |
759 | ||
760 | This is the fraction of pages at most (high mark pcp->high) in each zone that | |
761 | are allocated for each per cpu page list. The min value for this is 8. It | |
762 | means that we don't allow more than 1/8th of pages in each zone to be | |
763 | allocated in any single per_cpu_pagelist. This entry only changes the value | |
764 | of hot per cpu pagelists. User can specify a number like 100 to allocate | |
765 | 1/100th of each zone to each per cpu page list. | |
766 | ||
767 | The batch value of each per cpu pagelist is also updated as a result. It is | |
768 | set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) | |
769 | ||
770 | The initial value is zero. Kernel does not use this value at boot time to set | |
7cd2b0a3 DR |
771 | the high water marks for each per cpu page list. If the user writes '0' to this |
772 | sysctl, it will revert to this default behavior. | |
db0fb184 PM |
773 | |
774 | ============================================================== | |
775 | ||
776 | stat_interval | |
777 | ||
778 | The time interval between which vm statistics are updated. The default | |
779 | is 1 second. | |
780 | ||
781 | ============================================================== | |
782 | ||
52b6f46b HD |
783 | stat_refresh |
784 | ||
785 | Any read or write (by root only) flushes all the per-cpu vm statistics | |
786 | into their global totals, for more accurate reports when testing | |
787 | e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo | |
788 | ||
789 | As a side-effect, it also checks for negative totals (elsewhere reported | |
790 | as 0) and "fails" with EINVAL if any are found, with a warning in dmesg. | |
791 | (At time of writing, a few stats are known sometimes to be found negative, | |
792 | with no ill effects: errors and warnings on these stats are suppressed.) | |
793 | ||
794 | ============================================================== | |
795 | ||
4518085e KW |
796 | numa_stat |
797 | ||
798 | This interface allows runtime configuration of numa statistics. | |
799 | ||
800 | When page allocation performance becomes a bottleneck and you can tolerate | |
801 | some possible tool breakage and decreased numa counter precision, you can | |
802 | do: | |
803 | echo 0 > /proc/sys/vm/numa_stat | |
804 | ||
805 | When page allocation performance is not a bottleneck and you want all | |
806 | tooling to work, you can do: | |
807 | echo 1 > /proc/sys/vm/numa_stat | |
808 | ||
809 | ============================================================== | |
810 | ||
db0fb184 PM |
811 | swappiness |
812 | ||
813 | This control is used to define how aggressive the kernel will swap | |
2743232c | 814 | memory pages. Higher values will increase aggressiveness, lower values |
8582cb96 AT |
815 | decrease the amount of swap. A value of 0 instructs the kernel not to |
816 | initiate swap until the amount of free and file-backed pages is less | |
817 | than the high water mark in a zone. | |
db0fb184 PM |
818 | |
819 | The default value is 60. | |
820 | ||
821 | ============================================================== | |
822 | ||
c9b1d098 AS |
823 | - user_reserve_kbytes |
824 | ||
633708a4 | 825 | When overcommit_memory is set to 2, "never overcommit" mode, reserve |
c9b1d098 AS |
826 | min(3% of current process size, user_reserve_kbytes) of free memory. |
827 | This is intended to prevent a user from starting a single memory hogging | |
828 | process, such that they cannot recover (kill the hog). | |
829 | ||
830 | user_reserve_kbytes defaults to min(3% of the current process size, 128MB). | |
831 | ||
832 | If this is reduced to zero, then the user will be allowed to allocate | |
833 | all free memory with a single process, minus admin_reserve_kbytes. | |
834 | Any subsequent attempts to execute a command will result in | |
835 | "fork: Cannot allocate memory". | |
836 | ||
837 | Changing this takes effect whenever an application requests memory. | |
838 | ||
839 | ============================================================== | |
840 | ||
db0fb184 PM |
841 | vfs_cache_pressure |
842 | ------------------ | |
843 | ||
4a0da71b DV |
844 | This percentage value controls the tendency of the kernel to reclaim |
845 | the memory which is used for caching of directory and inode objects. | |
db0fb184 PM |
846 | |
847 | At the default value of vfs_cache_pressure=100 the kernel will attempt to | |
848 | reclaim dentries and inodes at a "fair" rate with respect to pagecache and | |
849 | swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer | |
55c37a84 JK |
850 | to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will |
851 | never reclaim dentries and inodes due to memory pressure and this can easily | |
852 | lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 | |
db0fb184 PM |
853 | causes the kernel to prefer to reclaim dentries and inodes. |
854 | ||
4a0da71b DV |
855 | Increasing vfs_cache_pressure significantly beyond 100 may have negative |
856 | performance impact. Reclaim code needs to take various locks to find freeable | |
857 | directory and inode objects. With vfs_cache_pressure=1000, it will look for | |
858 | ten times more freeable objects than there are. | |
859 | ||
795ae7a0 JW |
860 | ============================================================= |
861 | ||
862 | watermark_scale_factor: | |
863 | ||
864 | This factor controls the aggressiveness of kswapd. It defines the | |
865 | amount of memory left in a node/system before kswapd is woken up and | |
866 | how much memory needs to be free before kswapd goes back to sleep. | |
867 | ||
868 | The unit is in fractions of 10,000. The default value of 10 means the | |
869 | distances between watermarks are 0.1% of the available memory in the | |
870 | node/system. The maximum value is 1000, or 10% of memory. | |
871 | ||
872 | A high rate of threads entering direct reclaim (allocstall) or kswapd | |
873 | going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate | |
874 | that the number of free pages kswapd maintains for latency reasons is | |
875 | too small for the allocation bursts occurring in the system. This knob | |
876 | can then be used to tune kswapd aggressiveness accordingly. | |
877 | ||
db0fb184 PM |
878 | ============================================================== |
879 | ||
880 | zone_reclaim_mode: | |
881 | ||
882 | Zone_reclaim_mode allows someone to set more or less aggressive approaches to | |
883 | reclaim memory when a zone runs out of memory. If it is set to zero then no | |
884 | zone reclaim occurs. Allocations will be satisfied from other zones / nodes | |
885 | in the system. | |
886 | ||
887 | This is value ORed together of | |
888 | ||
889 | 1 = Zone reclaim on | |
890 | 2 = Zone reclaim writes dirty pages out | |
891 | 4 = Zone reclaim swaps pages | |
892 | ||
4f9b16a6 MG |
893 | zone_reclaim_mode is disabled by default. For file servers or workloads |
894 | that benefit from having their data cached, zone_reclaim_mode should be | |
895 | left disabled as the caching effect is likely to be more important than | |
db0fb184 PM |
896 | data locality. |
897 | ||
4f9b16a6 MG |
898 | zone_reclaim may be enabled if it's known that the workload is partitioned |
899 | such that each partition fits within a NUMA node and that accessing remote | |
900 | memory would cause a measurable performance reduction. The page allocator | |
901 | will then reclaim easily reusable pages (those page cache pages that are | |
902 | currently not used) before allocating off node pages. | |
903 | ||
db0fb184 PM |
904 | Allowing zone reclaim to write out pages stops processes that are |
905 | writing large amounts of data from dirtying pages on other nodes. Zone | |
906 | reclaim will write out dirty pages if a zone fills up and so effectively | |
907 | throttle the process. This may decrease the performance of a single process | |
908 | since it cannot use all of system memory to buffer the outgoing writes | |
909 | anymore but it preserve the memory on other nodes so that the performance | |
910 | of other processes running on other nodes will not be affected. | |
911 | ||
912 | Allowing regular swap effectively restricts allocations to the local | |
913 | node unless explicitly overridden by memory policies or cpuset | |
914 | configurations. | |
915 | ||
916 | ============ End of Document ================================= |