Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.
1 Documentation for /proc/sys/vm/* kernel version 2.6.29 2 (c) 1998, 1999, Rik van Riel <email@example.com> 3 (c) 2008 Peter W. Morreale <firstname.lastname@example.org> 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_kbytes 51 - overcommit_memory 52 - overcommit_ratio 53 - page-cluster 54 - panic_on_oom 55 - percpu_pagelist_fraction 56 - stat_interval 57 - swappiness 58 - user_reserve_kbytes 59 - vfs_cache_pressure 60 - zone_reclaim_mode 61 62 ============================================================== 63 64 admin_reserve_kbytes 65 66 The amount of free memory in the system that should be reserved for users 67 with the capability cap_sys_admin. 68 69 admin_reserve_kbytes defaults to min(3% of free pages, 8MB) 70 71 That should provide enough for the admin to log in and kill a process, 72 if necessary, under the default overcommit 'guess' mode. 73 74 Systems running under overcommit 'never' should increase this to account 75 for the full Virtual Memory Size of programs used to recover. Otherwise, 76 root may not be able to log in to recover the system. 77 78 How do you calculate a minimum useful reserve? 79 80 sshd or login + bash (or some other shell) + top (or ps, kill, etc.) 81 82 For overcommit 'guess', we can sum resident set sizes (RSS). 83 On x86_64 this is about 8MB. 84 85 For overcommit 'never', we can take the max of their virtual sizes (VSZ) 86 and add the sum of their RSS. 87 On x86_64 this is about 128MB. 88 89 Changing this takes effect whenever an application requests memory. 90 91 ============================================================== 92 93 block_dump 94 95 block_dump enables block I/O debugging when set to a nonzero value. More 96 information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. 97 98 ============================================================== 99 100 compact_memory 101 102 Available only when CONFIG_COMPACTION is set. When 1 is written to the file, 103 all zones are compacted such that free memory is available in contiguous 104 blocks where possible. This can be important for example in the allocation of 105 huge pages although processes will also directly compact memory as required. 106 107 ============================================================== 108 109 dirty_background_bytes 110 111 Contains the amount of dirty memory at which the background kernel 112 flusher threads will start writeback. 113 114 Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only 115 one of them may be specified at a time. When one sysctl is written it is 116 immediately taken into account to evaluate the dirty memory limits and the 117 other appears as 0 when read. 118 119 ============================================================== 120 121 dirty_background_ratio 122 123 Contains, as a percentage of total available memory that contains free pages 124 and reclaimable pages, the number of pages at which the background kernel 125 flusher threads will start writing out dirty data. 126 127 The total avaiable memory is not equal to total system memory. 128 129 ============================================================== 130 131 dirty_bytes 132 133 Contains the amount of dirty memory at which a process generating disk writes 134 will itself start writeback. 135 136 Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be 137 specified at a time. When one sysctl is written it is immediately taken into 138 account to evaluate the dirty memory limits and the other appears as 0 when 139 read. 140 141 Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any 142 value lower than this limit will be ignored and the old configuration will be 143 retained. 144 145 ============================================================== 146 147 dirty_expire_centisecs 148 149 This tunable is used to define when dirty data is old enough to be eligible 150 for writeout by the kernel flusher threads. It is expressed in 100'ths 151 of a second. Data which has been dirty in-memory for longer than this 152 interval will be written out next time a flusher thread wakes up. 153 154 ============================================================== 155 156 dirty_ratio 157 158 Contains, as a percentage of total available memory that contains free pages 159 and reclaimable pages, the number of pages at which a process which is 160 generating disk writes will itself start writing out dirty data. 161 162 The total avaiable memory is not equal to total system memory. 163 164 ============================================================== 165 166 dirty_writeback_centisecs 167 168 The kernel flusher threads will periodically wake up and write `old' data 169 out to disk. This tunable expresses the interval between those wakeups, in 170 100'ths of a second. 171 172 Setting this to zero disables periodic writeback altogether. 173 174 ============================================================== 175 176 drop_caches 177 178 Writing to this will cause the kernel to drop clean caches, as well as 179 reclaimable slab objects like dentries and inodes. Once dropped, their 180 memory becomes free. 181 182 To free pagecache: 183 echo 1 > /proc/sys/vm/drop_caches 184 To free reclaimable slab objects (includes dentries and inodes): 185 echo 2 > /proc/sys/vm/drop_caches 186 To free slab objects and pagecache: 187 echo 3 > /proc/sys/vm/drop_caches 188 189 This is a non-destructive operation and will not free any dirty objects. 190 To increase the number of objects freed by this operation, the user may run 191 `sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the 192 number of dirty objects on the system and create more candidates to be 193 dropped. 194 195 This file is not a means to control the growth of the various kernel caches 196 (inodes, dentries, pagecache, etc...) These objects are automatically 197 reclaimed by the kernel when memory is needed elsewhere on the system. 198 199 Use of this file can cause performance problems. Since it discards cached 200 objects, it may cost a significant amount of I/O and CPU to recreate the 201 dropped objects, especially if they were under heavy use. Because of this, 202 use outside of a testing or debugging environment is not recommended. 203 204 You may see informational messages in your kernel log when this file is 205 used: 206 207 cat (1234): drop_caches: 3 208 209 These are informational only. They do not mean that anything is wrong 210 with your system. To disable them, echo 4 (bit 3) into drop_caches. 211 212 ============================================================== 213 214 extfrag_threshold 215 216 This parameter affects whether the kernel will compact memory or direct 217 reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what 218 the fragmentation index for each order is in each zone in the system. Values 219 tending towards 0 imply allocations would fail due to lack of memory, 220 values towards 1000 imply failures are due to fragmentation and -1 implies 221 that the allocation will succeed as long as watermarks are met. 222 223 The kernel will not compact memory in a zone if the 224 fragmentation index is <= extfrag_threshold. The default value is 500. 225 226 ============================================================== 227 228 hugepages_treat_as_movable 229 230 This parameter controls whether we can allocate hugepages from ZONE_MOVABLE 231 or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE. 232 ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified, 233 so this parameter has no effect if used without kernelcore=. 234 235 Hugepage migration is now available in some situations which depend on the 236 architecture and/or the hugepage size. If a hugepage supports migration, 237 allocation from ZONE_MOVABLE is always enabled for the hugepage regardless 238 of the value of this parameter. 239 IOW, this parameter affects only non-migratable hugepages. 240 241 Assuming that hugepages are not migratable in your system, one usecase of 242 this parameter is that users can make hugepage pool more extensible by 243 enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE 244 page reclaim/migration/compaction work more and you can get contiguous 245 memory more likely. Note that using ZONE_MOVABLE for non-migratable 246 hugepages can do harm to other features like memory hotremove (because 247 memory hotremove expects that memory blocks on ZONE_MOVABLE are always 248 removable,) so it's a trade-off responsible for the users. 249 250 ============================================================== 251 252 hugetlb_shm_group 253 254 hugetlb_shm_group contains group id that is allowed to create SysV 255 shared memory segment using hugetlb page. 256 257 ============================================================== 258 259 laptop_mode 260 261 laptop_mode is a knob that controls "laptop mode". All the things that are 262 controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. 263 264 ============================================================== 265 266 legacy_va_layout 267 268 If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel 269 will use the legacy (2.4) layout for all processes. 270 271 ============================================================== 272 273 lowmem_reserve_ratio 274 275 For some specialised workloads on highmem machines it is dangerous for 276 the kernel to allow process memory to be allocated from the "lowmem" 277 zone. This is because that memory could then be pinned via the mlock() 278 system call, or by unavailability of swapspace. 279 280 And on large highmem machines this lack of reclaimable lowmem memory 281 can be fatal. 282 283 So the Linux page allocator has a mechanism which prevents allocations 284 which _could_ use highmem from using too much lowmem. This means that 285 a certain amount of lowmem is defended from the possibility of being 286 captured into pinned user memory. 287 288 (The same argument applies to the old 16 megabyte ISA DMA region. This 289 mechanism will also defend that region from allocations which could use 290 highmem or lowmem). 291 292 The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is 293 in defending these lower zones. 294 295 If you have a machine which uses highmem or ISA DMA and your 296 applications are using mlock(), or if you are running with no swap then 297 you probably should change the lowmem_reserve_ratio setting. 298 299 The lowmem_reserve_ratio is an array. You can see them by reading this file. 300 - 301 % cat /proc/sys/vm/lowmem_reserve_ratio 302 256 256 32 303 - 304 Note: # of this elements is one fewer than number of zones. Because the highest 305 zone's value is not necessary for following calculation. 306 307 But, these values are not used directly. The kernel calculates # of protection 308 pages for each zones from them. These are shown as array of protection pages 309 in /proc/zoneinfo like followings. (This is an example of x86-64 box). 310 Each zone has an array of protection pages like this. 311 312 - 313 Node 0, zone DMA 314 pages free 1355 315 min 3 316 low 3 317 high 4 318 : 319 : 320 numa_other 0 321 protection: (0, 2004, 2004, 2004) 322 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 323 pagesets 324 cpu: 0 pcp: 0 325 : 326 - 327 These protections are added to score to judge whether this zone should be used 328 for page allocation or should be reclaimed. 329 330 In this example, if normal pages (index=2) are required to this DMA zone and 331 watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should 332 not be used because pages_free(1355) is smaller than watermark + protection 333 (4 + 2004 = 2008). If this protection value is 0, this zone would be used for 334 normal page requirement. If requirement is DMA zone(index=0), protection 335 (=0) is used. 336 337 zone[i]'s protection[j] is calculated by following expression. 338 339 (i < j): 340 zone[i]->protection[j] 341 = (total sums of present_pages from zone[i+1] to zone[j] on the node) 342 / lowmem_reserve_ratio[i]; 343 (i = j): 344 (should not be protected. = 0; 345 (i > j): 346 (not necessary, but looks 0) 347 348 The default values of lowmem_reserve_ratio[i] are 349 256 (if zone[i] means DMA or DMA32 zone) 350 32 (others). 351 As above expression, they are reciprocal number of ratio. 352 256 means 1/256. # of protection pages becomes about "0.39%" of total present 353 pages of higher zones on the node. 354 355 If you would like to protect more pages, smaller values are effective. 356 The minimum value is 1 (1/1 -> 100%). 357 358 ============================================================== 359 360 max_map_count: 361 362 This file contains the maximum number of memory map areas a process 363 may have. Memory map areas are used as a side-effect of calling 364 malloc, directly by mmap and mprotect, and also when loading shared 365 libraries. 366 367 While most applications need less than a thousand maps, certain 368 programs, particularly malloc debuggers, may consume lots of them, 369 e.g., up to one or two maps per allocation. 370 371 The default value is 65536. 372 373 ============================================================= 374 375 memory_failure_early_kill: 376 377 Control how to kill processes when uncorrected memory error (typically 378 a 2bit error in a memory module) is detected in the background by hardware 379 that cannot be handled by the kernel. In some cases (like the page 380 still having a valid copy on disk) the kernel will handle the failure 381 transparently without affecting any applications. But if there is 382 no other uptodate copy of the data it will kill to prevent any data 383 corruptions from propagating. 384 385 1: Kill all processes that have the corrupted and not reloadable page mapped 386 as soon as the corruption is detected. Note this is not supported 387 for a few types of pages, like kernel internally allocated data or 388 the swap cache, but works for the majority of user pages. 389 390 0: Only unmap the corrupted page from all processes and only kill a process 391 who tries to access it. 392 393 The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can 394 handle this if they want to. 395 396 This is only active on architectures/platforms with advanced machine 397 check handling and depends on the hardware capabilities. 398 399 Applications can override this setting individually with the PR_MCE_KILL prctl 400 401 ============================================================== 402 403 memory_failure_recovery 404 405 Enable memory failure recovery (when supported by the platform) 406 407 1: Attempt recovery. 408 409 0: Always panic on a memory failure. 410 411 ============================================================== 412 413 min_free_kbytes: 414 415 This is used to force the Linux VM to keep a minimum number 416 of kilobytes free. The VM uses this number to compute a 417 watermark[WMARK_MIN] value for each lowmem zone in the system. 418 Each lowmem zone gets a number of reserved free pages based 419 proportionally on its size. 420 421 Some minimal amount of memory is needed to satisfy PF_MEMALLOC 422 allocations; if you set this to lower than 1024KB, your system will 423 become subtly broken, and prone to deadlock under high loads. 424 425 Setting this too high will OOM your machine instantly. 426 427 ============================================================= 428 429 min_slab_ratio: 430 431 This is available only on NUMA kernels. 432 433 A percentage of the total pages in each zone. On Zone reclaim 434 (fallback from the local zone occurs) slabs will be reclaimed if more 435 than this percentage of pages in a zone are reclaimable slab pages. 436 This insures that the slab growth stays under control even in NUMA 437 systems that rarely perform global reclaim. 438 439 The default is 5 percent. 440 441 Note that slab reclaim is triggered in a per zone / node fashion. 442 The process of reclaiming slab memory is currently not node specific 443 and may not be fast. 444 445 ============================================================= 446 447 min_unmapped_ratio: 448 449 This is available only on NUMA kernels. 450 451 This is a percentage of the total pages in each zone. Zone reclaim will 452 only occur if more than this percentage of pages are in a state that 453 zone_reclaim_mode allows to be reclaimed. 454 455 If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared 456 against all file-backed unmapped pages including swapcache pages and tmpfs 457 files. Otherwise, only unmapped pages backed by normal files but not tmpfs 458 files and similar are considered. 459 460 The default is 1 percent. 461 462 ============================================================== 463 464 mmap_min_addr 465 466 This file indicates the amount of address space which a user process will 467 be restricted from mmapping. Since kernel null dereference bugs could 468 accidentally operate based on the information in the first couple of pages 469 of memory userspace processes should not be allowed to write to them. By 470 default this value is set to 0 and no protections will be enforced by the 471 security module. Setting this value to something like 64k will allow the 472 vast majority of applications to work correctly and provide defense in depth 473 against future potential kernel bugs. 474 475 ============================================================== 476 477 nr_hugepages 478 479 Change the minimum size of the hugepage pool. 480 481 See Documentation/vm/hugetlbpage.txt 482 483 ============================================================== 484 485 nr_overcommit_hugepages 486 487 Change the maximum size of the hugepage pool. The maximum is 488 nr_hugepages + nr_overcommit_hugepages. 489 490 See Documentation/vm/hugetlbpage.txt 491 492 ============================================================== 493 494 nr_trim_pages 495 496 This is available only on NOMMU kernels. 497 498 This value adjusts the excess page trimming behaviour of power-of-2 aligned 499 NOMMU mmap allocations. 500 501 A value of 0 disables trimming of allocations entirely, while a value of 1 502 trims excess pages aggressively. Any value >= 1 acts as the watermark where 503 trimming of allocations is initiated. 504 505 The default value is 1. 506 507 See Documentation/nommu-mmap.txt for more information. 508 509 ============================================================== 510 511 numa_zonelist_order 512 513 This sysctl is only for NUMA. 514 'where the memory is allocated from' is controlled by zonelists. 515 (This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. 516 you may be able to read ZONE_DMA as ZONE_DMA32...) 517 518 In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. 519 ZONE_NORMAL -> ZONE_DMA 520 This means that a memory allocation request for GFP_KERNEL will 521 get memory from ZONE_DMA only when ZONE_NORMAL is not available. 522 523 In NUMA case, you can think of following 2 types of order. 524 Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL 525 526 (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL 527 (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. 528 529 Type(A) offers the best locality for processes on Node(0), but ZONE_DMA 530 will be used before ZONE_NORMAL exhaustion. This increases possibility of 531 out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. 532 533 Type(B) cannot offer the best locality but is more robust against OOM of 534 the DMA zone. 535 536 Type(A) is called as "Node" order. Type (B) is "Zone" order. 537 538 "Node order" orders the zonelists by node, then by zone within each node. 539 Specify "[Nn]ode" for node order 540 541 "Zone Order" orders the zonelists by zone type, then by node within each 542 zone. Specify "[Zz]one" for zone order. 543 544 Specify "[Dd]efault" to request automatic configuration. Autoconfiguration 545 will select "node" order in following case. 546 (1) if the DMA zone does not exist or 547 (2) if the DMA zone comprises greater than 50% of the available memory or 548 (3) if any node's DMA zone comprises greater than 70% of its local memory and 549 the amount of local memory is big enough. 550 551 Otherwise, "zone" order will be selected. Default order is recommended unless 552 this is causing problems for your system/application. 553 554 ============================================================== 555 556 oom_dump_tasks 557 558 Enables a system-wide task dump (excluding kernel threads) to be 559 produced when the kernel performs an OOM-killing and includes such 560 information as pid, uid, tgid, vm size, rss, nr_ptes, swapents, 561 oom_score_adj score, and name. This is helpful to determine why the 562 OOM killer was invoked, to identify the rogue task that caused it, 563 and to determine why the OOM killer chose the task it did to kill. 564 565 If this is set to zero, this information is suppressed. On very 566 large systems with thousands of tasks it may not be feasible to dump 567 the memory state information for each one. Such systems should not 568 be forced to incur a performance penalty in OOM conditions when the 569 information may not be desired. 570 571 If this is set to non-zero, this information is shown whenever the 572 OOM killer actually kills a memory-hogging task. 573 574 The default value is 1 (enabled). 575 576 ============================================================== 577 578 oom_kill_allocating_task 579 580 This enables or disables killing the OOM-triggering task in 581 out-of-memory situations. 582 583 If this is set to zero, the OOM killer will scan through the entire 584 tasklist and select a task based on heuristics to kill. This normally 585 selects a rogue memory-hogging task that frees up a large amount of 586 memory when killed. 587 588 If this is set to non-zero, the OOM killer simply kills the task that 589 triggered the out-of-memory condition. This avoids the expensive 590 tasklist scan. 591 592 If panic_on_oom is selected, it takes precedence over whatever value 593 is used in oom_kill_allocating_task. 594 595 The default value is 0. 596 597 ============================================================== 598 599 overcommit_kbytes: 600 601 When overcommit_memory is set to 2, the committed address space is not 602 permitted to exceed swap plus this amount of physical RAM. See below. 603 604 Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one 605 of them may be specified at a time. Setting one disables the other (which 606 then appears as 0 when read). 607 608 ============================================================== 609 610 overcommit_memory: 611 612 This value contains a flag that enables memory overcommitment. 613 614 When this flag is 0, the kernel attempts to estimate the amount 615 of free memory left when userspace requests more memory. 616 617 When this flag is 1, the kernel pretends there is always enough 618 memory until it actually runs out. 619 620 When this flag is 2, the kernel uses a "never overcommit" 621 policy that attempts to prevent any overcommit of memory. 622 Note that user_reserve_kbytes affects this policy. 623 624 This feature can be very useful because there are a lot of 625 programs that malloc() huge amounts of memory "just-in-case" 626 and don't use much of it. 627 628 The default value is 0. 629 630 See Documentation/vm/overcommit-accounting and 631 security/commoncap.c::cap_vm_enough_memory() for more information. 632 633 ============================================================== 634 635 overcommit_ratio: 636 637 When overcommit_memory is set to 2, the committed address 638 space is not permitted to exceed swap plus this percentage 639 of physical RAM. See above. 640 641 ============================================================== 642 643 page-cluster 644 645 page-cluster controls the number of pages up to which consecutive pages 646 are read in from swap in a single attempt. This is the swap counterpart 647 to page cache readahead. 648 The mentioned consecutivity is not in terms of virtual/physical addresses, 649 but consecutive on swap space - that means they were swapped out together. 650 651 It is a logarithmic value - setting it to zero means "1 page", setting 652 it to 1 means "2 pages", setting it to 2 means "4 pages", etc. 653 Zero disables swap readahead completely. 654 655 The default value is three (eight pages at a time). There may be some 656 small benefits in tuning this to a different value if your workload is 657 swap-intensive. 658 659 Lower values mean lower latencies for initial faults, but at the same time 660 extra faults and I/O delays for following faults if they would have been part of 661 that consecutive pages readahead would have brought in. 662 663 ============================================================= 664 665 panic_on_oom 666 667 This enables or disables panic on out-of-memory feature. 668 669 If this is set to 0, the kernel will kill some rogue process, 670 called oom_killer. Usually, oom_killer can kill rogue processes and 671 system will survive. 672 673 If this is set to 1, the kernel panics when out-of-memory happens. 674 However, if a process limits using nodes by mempolicy/cpusets, 675 and those nodes become memory exhaustion status, one process 676 may be killed by oom-killer. No panic occurs in this case. 677 Because other nodes' memory may be free. This means system total status 678 may be not fatal yet. 679 680 If this is set to 2, the kernel panics compulsorily even on the 681 above-mentioned. Even oom happens under memory cgroup, the whole 682 system panics. 683 684 The default value is 0. 685 1 and 2 are for failover of clustering. Please select either 686 according to your policy of failover. 687 panic_on_oom=2+kdump gives you very strong tool to investigate 688 why oom happens. You can get snapshot. 689 690 ============================================================= 691 692 percpu_pagelist_fraction 693 694 This is the fraction of pages at most (high mark pcp->high) in each zone that 695 are allocated for each per cpu page list. The min value for this is 8. It 696 means that we don't allow more than 1/8th of pages in each zone to be 697 allocated in any single per_cpu_pagelist. This entry only changes the value 698 of hot per cpu pagelists. User can specify a number like 100 to allocate 699 1/100th of each zone to each per cpu page list. 700 701 The batch value of each per cpu pagelist is also updated as a result. It is 702 set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) 703 704 The initial value is zero. Kernel does not use this value at boot time to set 705 the high water marks for each per cpu page list. If the user writes '0' to this 706 sysctl, it will revert to this default behavior. 707 708 ============================================================== 709 710 stat_interval 711 712 The time interval between which vm statistics are updated. The default 713 is 1 second. 714 715 ============================================================== 716 717 swappiness 718 719 This control is used to define how aggressive the kernel will swap 720 memory pages. Higher values will increase agressiveness, lower values 721 decrease the amount of swap. A value of 0 instructs the kernel not to 722 initiate swap until the amount of free and file-backed pages is less 723 than the high water mark in a zone. 724 725 The default value is 60. 726 727 ============================================================== 728 729 - user_reserve_kbytes 730 731 When overcommit_memory is set to 2, "never overommit" mode, reserve 732 min(3% of current process size, user_reserve_kbytes) of free memory. 733 This is intended to prevent a user from starting a single memory hogging 734 process, such that they cannot recover (kill the hog). 735 736 user_reserve_kbytes defaults to min(3% of the current process size, 128MB). 737 738 If this is reduced to zero, then the user will be allowed to allocate 739 all free memory with a single process, minus admin_reserve_kbytes. 740 Any subsequent attempts to execute a command will result in 741 "fork: Cannot allocate memory". 742 743 Changing this takes effect whenever an application requests memory. 744 745 ============================================================== 746 747 vfs_cache_pressure 748 ------------------ 749 750 This percentage value controls the tendency of the kernel to reclaim 751 the memory which is used for caching of directory and inode objects. 752 753 At the default value of vfs_cache_pressure=100 the kernel will attempt to 754 reclaim dentries and inodes at a "fair" rate with respect to pagecache and 755 swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer 756 to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will 757 never reclaim dentries and inodes due to memory pressure and this can easily 758 lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 759 causes the kernel to prefer to reclaim dentries and inodes. 760 761 Increasing vfs_cache_pressure significantly beyond 100 may have negative 762 performance impact. Reclaim code needs to take various locks to find freeable 763 directory and inode objects. With vfs_cache_pressure=1000, it will look for 764 ten times more freeable objects than there are. 765 766 ============================================================== 767 768 zone_reclaim_mode: 769 770 Zone_reclaim_mode allows someone to set more or less aggressive approaches to 771 reclaim memory when a zone runs out of memory. If it is set to zero then no 772 zone reclaim occurs. Allocations will be satisfied from other zones / nodes 773 in the system. 774 775 This is value ORed together of 776 777 1 = Zone reclaim on 778 2 = Zone reclaim writes dirty pages out 779 4 = Zone reclaim swaps pages 780 781 zone_reclaim_mode is disabled by default. For file servers or workloads 782 that benefit from having their data cached, zone_reclaim_mode should be 783 left disabled as the caching effect is likely to be more important than 784 data locality. 785 786 zone_reclaim may be enabled if it's known that the workload is partitioned 787 such that each partition fits within a NUMA node and that accessing remote 788 memory would cause a measurable performance reduction. The page allocator 789 will then reclaim easily reusable pages (those page cache pages that are 790 currently not used) before allocating off node pages. 791 792 Allowing zone reclaim to write out pages stops processes that are 793 writing large amounts of data from dirtying pages on other nodes. Zone 794 reclaim will write out dirty pages if a zone fills up and so effectively 795 throttle the process. This may decrease the performance of a single process 796 since it cannot use all of system memory to buffer the outgoing writes 797 anymore but it preserve the memory on other nodes so that the performance 798 of other processes running on other nodes will not be affected. 799 800 Allowing regular swap effectively restricts allocations to the local 801 node unless explicitly overridden by memory policies or cpuset 802 configurations. 803 804 ============ End of Document =================================