Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.
1 Documentation for /proc/sys/vm/* kernel version 2.6.29 2 (c) 1998, 1999, Rik van Riel <firstname.lastname@example.org> 3 (c) 2008 Peter W. Morreale <email@example.com> 4 5 For general info and legal blurb, please look in README. 6 7 ============================================================== 8 9 This file contains the documentation for the sysctl files in 10 /proc/sys/vm and is valid for Linux kernel version 2.6.29. 11 12 The files in this directory can be used to tune the operation 13 of the virtual memory (VM) subsystem of the Linux kernel and 14 the writeout of dirty data to disk. 15 16 Default values and initialization routines for most of these 17 files can be found in mm/swap.c. 18 19 Currently, these files are in /proc/sys/vm: 20 21 - admin_reserve_kbytes 22 - block_dump 23 - compact_memory 24 - compact_unevictable_allowed 25 - dirty_background_bytes 26 - dirty_background_ratio 27 - dirty_bytes 28 - dirty_expire_centisecs 29 - dirty_ratio 30 - dirty_writeback_centisecs 31 - drop_caches 32 - extfrag_threshold 33 - hugepages_treat_as_movable 34 - hugetlb_shm_group 35 - laptop_mode 36 - legacy_va_layout 37 - lowmem_reserve_ratio 38 - max_map_count 39 - memory_failure_early_kill 40 - memory_failure_recovery 41 - min_free_kbytes 42 - min_slab_ratio 43 - min_unmapped_ratio 44 - mmap_min_addr 45 - nr_hugepages 46 - nr_overcommit_hugepages 47 - nr_trim_pages (only if CONFIG_MMU=n) 48 - numa_zonelist_order 49 - oom_dump_tasks 50 - oom_kill_allocating_task 51 - overcommit_kbytes 52 - overcommit_memory 53 - overcommit_ratio 54 - page-cluster 55 - panic_on_oom 56 - percpu_pagelist_fraction 57 - stat_interval 58 - swappiness 59 - user_reserve_kbytes 60 - vfs_cache_pressure 61 - zone_reclaim_mode 62 63 ============================================================== 64 65 admin_reserve_kbytes 66 67 The amount of free memory in the system that should be reserved for users 68 with the capability cap_sys_admin. 69 70 admin_reserve_kbytes defaults to min(3% of free pages, 8MB) 71 72 That should provide enough for the admin to log in and kill a process, 73 if necessary, under the default overcommit 'guess' mode. 74 75 Systems running under overcommit 'never' should increase this to account 76 for the full Virtual Memory Size of programs used to recover. Otherwise, 77 root may not be able to log in to recover the system. 78 79 How do you calculate a minimum useful reserve? 80 81 sshd or login + bash (or some other shell) + top (or ps, kill, etc.) 82 83 For overcommit 'guess', we can sum resident set sizes (RSS). 84 On x86_64 this is about 8MB. 85 86 For overcommit 'never', we can take the max of their virtual sizes (VSZ) 87 and add the sum of their RSS. 88 On x86_64 this is about 128MB. 89 90 Changing this takes effect whenever an application requests memory. 91 92 ============================================================== 93 94 block_dump 95 96 block_dump enables block I/O debugging when set to a nonzero value. More 97 information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. 98 99 ============================================================== 100 101 compact_memory 102 103 Available only when CONFIG_COMPACTION is set. When 1 is written to the file, 104 all zones are compacted such that free memory is available in contiguous 105 blocks where possible. This can be important for example in the allocation of 106 huge pages although processes will also directly compact memory as required. 107 108 ============================================================== 109 110 compact_unevictable_allowed 111 112 Available only when CONFIG_COMPACTION is set. When set to 1, compaction is 113 allowed to examine the unevictable lru (mlocked pages) for pages to compact. 114 This should be used on systems where stalls for minor page faults are an 115 acceptable trade for large contiguous free memory. Set to 0 to prevent 116 compaction from moving pages that are unevictable. Default value is 1. 117 118 ============================================================== 119 120 dirty_background_bytes 121 122 Contains the amount of dirty memory at which the background kernel 123 flusher threads will start writeback. 124 125 Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only 126 one of them may be specified at a time. When one sysctl is written it is 127 immediately taken into account to evaluate the dirty memory limits and the 128 other appears as 0 when read. 129 130 ============================================================== 131 132 dirty_background_ratio 133 134 Contains, as a percentage of total available memory that contains free pages 135 and reclaimable pages, the number of pages at which the background kernel 136 flusher threads will start writing out dirty data. 137 138 The total avaiable memory is not equal to total system memory. 139 140 ============================================================== 141 142 dirty_bytes 143 144 Contains the amount of dirty memory at which a process generating disk writes 145 will itself start writeback. 146 147 Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be 148 specified at a time. When one sysctl is written it is immediately taken into 149 account to evaluate the dirty memory limits and the other appears as 0 when 150 read. 151 152 Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any 153 value lower than this limit will be ignored and the old configuration will be 154 retained. 155 156 ============================================================== 157 158 dirty_expire_centisecs 159 160 This tunable is used to define when dirty data is old enough to be eligible 161 for writeout by the kernel flusher threads. It is expressed in 100'ths 162 of a second. Data which has been dirty in-memory for longer than this 163 interval will be written out next time a flusher thread wakes up. 164 165 ============================================================== 166 167 dirty_ratio 168 169 Contains, as a percentage of total available memory that contains free pages 170 and reclaimable pages, the number of pages at which a process which is 171 generating disk writes will itself start writing out dirty data. 172 173 The total avaiable memory is not equal to total system memory. 174 175 ============================================================== 176 177 dirty_writeback_centisecs 178 179 The kernel flusher threads will periodically wake up and write `old' data 180 out to disk. This tunable expresses the interval between those wakeups, in 181 100'ths of a second. 182 183 Setting this to zero disables periodic writeback altogether. 184 185 ============================================================== 186 187 drop_caches 188 189 Writing to this will cause the kernel to drop clean caches, as well as 190 reclaimable slab objects like dentries and inodes. Once dropped, their 191 memory becomes free. 192 193 To free pagecache: 194 echo 1 > /proc/sys/vm/drop_caches 195 To free reclaimable slab objects (includes dentries and inodes): 196 echo 2 > /proc/sys/vm/drop_caches 197 To free slab objects and pagecache: 198 echo 3 > /proc/sys/vm/drop_caches 199 200 This is a non-destructive operation and will not free any dirty objects. 201 To increase the number of objects freed by this operation, the user may run 202 `sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the 203 number of dirty objects on the system and create more candidates to be 204 dropped. 205 206 This file is not a means to control the growth of the various kernel caches 207 (inodes, dentries, pagecache, etc...) These objects are automatically 208 reclaimed by the kernel when memory is needed elsewhere on the system. 209 210 Use of this file can cause performance problems. Since it discards cached 211 objects, it may cost a significant amount of I/O and CPU to recreate the 212 dropped objects, especially if they were under heavy use. Because of this, 213 use outside of a testing or debugging environment is not recommended. 214 215 You may see informational messages in your kernel log when this file is 216 used: 217 218 cat (1234): drop_caches: 3 219 220 These are informational only. They do not mean that anything is wrong 221 with your system. To disable them, echo 4 (bit 3) into drop_caches. 222 223 ============================================================== 224 225 extfrag_threshold 226 227 This parameter affects whether the kernel will compact memory or direct 228 reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what 229 the fragmentation index for each order is in each zone in the system. Values 230 tending towards 0 imply allocations would fail due to lack of memory, 231 values towards 1000 imply failures are due to fragmentation and -1 implies 232 that the allocation will succeed as long as watermarks are met. 233 234 The kernel will not compact memory in a zone if the 235 fragmentation index is <= extfrag_threshold. The default value is 500. 236 237 ============================================================== 238 239 hugepages_treat_as_movable 240 241 This parameter controls whether we can allocate hugepages from ZONE_MOVABLE 242 or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE. 243 ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified, 244 so this parameter has no effect if used without kernelcore=. 245 246 Hugepage migration is now available in some situations which depend on the 247 architecture and/or the hugepage size. If a hugepage supports migration, 248 allocation from ZONE_MOVABLE is always enabled for the hugepage regardless 249 of the value of this parameter. 250 IOW, this parameter affects only non-migratable hugepages. 251 252 Assuming that hugepages are not migratable in your system, one usecase of 253 this parameter is that users can make hugepage pool more extensible by 254 enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE 255 page reclaim/migration/compaction work more and you can get contiguous 256 memory more likely. Note that using ZONE_MOVABLE for non-migratable 257 hugepages can do harm to other features like memory hotremove (because 258 memory hotremove expects that memory blocks on ZONE_MOVABLE are always 259 removable,) so it's a trade-off responsible for the users. 260 261 ============================================================== 262 263 hugetlb_shm_group 264 265 hugetlb_shm_group contains group id that is allowed to create SysV 266 shared memory segment using hugetlb page. 267 268 ============================================================== 269 270 laptop_mode 271 272 laptop_mode is a knob that controls "laptop mode". All the things that are 273 controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. 274 275 ============================================================== 276 277 legacy_va_layout 278 279 If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel 280 will use the legacy (2.4) layout for all processes. 281 282 ============================================================== 283 284 lowmem_reserve_ratio 285 286 For some specialised workloads on highmem machines it is dangerous for 287 the kernel to allow process memory to be allocated from the "lowmem" 288 zone. This is because that memory could then be pinned via the mlock() 289 system call, or by unavailability of swapspace. 290 291 And on large highmem machines this lack of reclaimable lowmem memory 292 can be fatal. 293 294 So the Linux page allocator has a mechanism which prevents allocations 295 which _could_ use highmem from using too much lowmem. This means that 296 a certain amount of lowmem is defended from the possibility of being 297 captured into pinned user memory. 298 299 (The same argument applies to the old 16 megabyte ISA DMA region. This 300 mechanism will also defend that region from allocations which could use 301 highmem or lowmem). 302 303 The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is 304 in defending these lower zones. 305 306 If you have a machine which uses highmem or ISA DMA and your 307 applications are using mlock(), or if you are running with no swap then 308 you probably should change the lowmem_reserve_ratio setting. 309 310 The lowmem_reserve_ratio is an array. You can see them by reading this file. 311 - 312 % cat /proc/sys/vm/lowmem_reserve_ratio 313 256 256 32 314 - 315 Note: # of this elements is one fewer than number of zones. Because the highest 316 zone's value is not necessary for following calculation. 317 318 But, these values are not used directly. The kernel calculates # of protection 319 pages for each zones from them. These are shown as array of protection pages 320 in /proc/zoneinfo like followings. (This is an example of x86-64 box). 321 Each zone has an array of protection pages like this. 322 323 - 324 Node 0, zone DMA 325 pages free 1355 326 min 3 327 low 3 328 high 4 329 : 330 : 331 numa_other 0 332 protection: (0, 2004, 2004, 2004) 333 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 334 pagesets 335 cpu: 0 pcp: 0 336 : 337 - 338 These protections are added to score to judge whether this zone should be used 339 for page allocation or should be reclaimed. 340 341 In this example, if normal pages (index=2) are required to this DMA zone and 342 watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should 343 not be used because pages_free(1355) is smaller than watermark + protection 344 (4 + 2004 = 2008). If this protection value is 0, this zone would be used for 345 normal page requirement. If requirement is DMA zone(index=0), protection 346 (=0) is used. 347 348 zone[i]'s protection[j] is calculated by following expression. 349 350 (i < j): 351 zone[i]->protection[j] 352 = (total sums of present_pages from zone[i+1] to zone[j] on the node) 353 / lowmem_reserve_ratio[i]; 354 (i = j): 355 (should not be protected. = 0; 356 (i > j): 357 (not necessary, but looks 0) 358 359 The default values of lowmem_reserve_ratio[i] are 360 256 (if zone[i] means DMA or DMA32 zone) 361 32 (others). 362 As above expression, they are reciprocal number of ratio. 363 256 means 1/256. # of protection pages becomes about "0.39%" of total present 364 pages of higher zones on the node. 365 366 If you would like to protect more pages, smaller values are effective. 367 The minimum value is 1 (1/1 -> 100%). 368 369 ============================================================== 370 371 max_map_count: 372 373 This file contains the maximum number of memory map areas a process 374 may have. Memory map areas are used as a side-effect of calling 375 malloc, directly by mmap and mprotect, and also when loading shared 376 libraries. 377 378 While most applications need less than a thousand maps, certain 379 programs, particularly malloc debuggers, may consume lots of them, 380 e.g., up to one or two maps per allocation. 381 382 The default value is 65536. 383 384 ============================================================= 385 386 memory_failure_early_kill: 387 388 Control how to kill processes when uncorrected memory error (typically 389 a 2bit error in a memory module) is detected in the background by hardware 390 that cannot be handled by the kernel. In some cases (like the page 391 still having a valid copy on disk) the kernel will handle the failure 392 transparently without affecting any applications. But if there is 393 no other uptodate copy of the data it will kill to prevent any data 394 corruptions from propagating. 395 396 1: Kill all processes that have the corrupted and not reloadable page mapped 397 as soon as the corruption is detected. Note this is not supported 398 for a few types of pages, like kernel internally allocated data or 399 the swap cache, but works for the majority of user pages. 400 401 0: Only unmap the corrupted page from all processes and only kill a process 402 who tries to access it. 403 404 The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can 405 handle this if they want to. 406 407 This is only active on architectures/platforms with advanced machine 408 check handling and depends on the hardware capabilities. 409 410 Applications can override this setting individually with the PR_MCE_KILL prctl 411 412 ============================================================== 413 414 memory_failure_recovery 415 416 Enable memory failure recovery (when supported by the platform) 417 418 1: Attempt recovery. 419 420 0: Always panic on a memory failure. 421 422 ============================================================== 423 424 min_free_kbytes: 425 426 This is used to force the Linux VM to keep a minimum number 427 of kilobytes free. The VM uses this number to compute a 428 watermark[WMARK_MIN] value for each lowmem zone in the system. 429 Each lowmem zone gets a number of reserved free pages based 430 proportionally on its size. 431 432 Some minimal amount of memory is needed to satisfy PF_MEMALLOC 433 allocations; if you set this to lower than 1024KB, your system will 434 become subtly broken, and prone to deadlock under high loads. 435 436 Setting this too high will OOM your machine instantly. 437 438 ============================================================= 439 440 min_slab_ratio: 441 442 This is available only on NUMA kernels. 443 444 A percentage of the total pages in each zone. On Zone reclaim 445 (fallback from the local zone occurs) slabs will be reclaimed if more 446 than this percentage of pages in a zone are reclaimable slab pages. 447 This insures that the slab growth stays under control even in NUMA 448 systems that rarely perform global reclaim. 449 450 The default is 5 percent. 451 452 Note that slab reclaim is triggered in a per zone / node fashion. 453 The process of reclaiming slab memory is currently not node specific 454 and may not be fast. 455 456 ============================================================= 457 458 min_unmapped_ratio: 459 460 This is available only on NUMA kernels. 461 462 This is a percentage of the total pages in each zone. Zone reclaim will 463 only occur if more than this percentage of pages are in a state that 464 zone_reclaim_mode allows to be reclaimed. 465 466 If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared 467 against all file-backed unmapped pages including swapcache pages and tmpfs 468 files. Otherwise, only unmapped pages backed by normal files but not tmpfs 469 files and similar are considered. 470 471 The default is 1 percent. 472 473 ============================================================== 474 475 mmap_min_addr 476 477 This file indicates the amount of address space which a user process will 478 be restricted from mmapping. Since kernel null dereference bugs could 479 accidentally operate based on the information in the first couple of pages 480 of memory userspace processes should not be allowed to write to them. By 481 default this value is set to 0 and no protections will be enforced by the 482 security module. Setting this value to something like 64k will allow the 483 vast majority of applications to work correctly and provide defense in depth 484 against future potential kernel bugs. 485 486 ============================================================== 487 488 nr_hugepages 489 490 Change the minimum size of the hugepage pool. 491 492 See Documentation/vm/hugetlbpage.txt 493 494 ============================================================== 495 496 nr_overcommit_hugepages 497 498 Change the maximum size of the hugepage pool. The maximum is 499 nr_hugepages + nr_overcommit_hugepages. 500 501 See Documentation/vm/hugetlbpage.txt 502 503 ============================================================== 504 505 nr_trim_pages 506 507 This is available only on NOMMU kernels. 508 509 This value adjusts the excess page trimming behaviour of power-of-2 aligned 510 NOMMU mmap allocations. 511 512 A value of 0 disables trimming of allocations entirely, while a value of 1 513 trims excess pages aggressively. Any value >= 1 acts as the watermark where 514 trimming of allocations is initiated. 515 516 The default value is 1. 517 518 See Documentation/nommu-mmap.txt for more information. 519 520 ============================================================== 521 522 numa_zonelist_order 523 524 This sysctl is only for NUMA. 525 'where the memory is allocated from' is controlled by zonelists. 526 (This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. 527 you may be able to read ZONE_DMA as ZONE_DMA32...) 528 529 In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. 530 ZONE_NORMAL -> ZONE_DMA 531 This means that a memory allocation request for GFP_KERNEL will 532 get memory from ZONE_DMA only when ZONE_NORMAL is not available. 533 534 In NUMA case, you can think of following 2 types of order. 535 Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL 536 537 (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL 538 (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. 539 540 Type(A) offers the best locality for processes on Node(0), but ZONE_DMA 541 will be used before ZONE_NORMAL exhaustion. This increases possibility of 542 out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. 543 544 Type(B) cannot offer the best locality but is more robust against OOM of 545 the DMA zone. 546 547 Type(A) is called as "Node" order. Type (B) is "Zone" order. 548 549 "Node order" orders the zonelists by node, then by zone within each node. 550 Specify "[Nn]ode" for node order 551 552 "Zone Order" orders the zonelists by zone type, then by node within each 553 zone. Specify "[Zz]one" for zone order. 554 555 Specify "[Dd]efault" to request automatic configuration. Autoconfiguration 556 will select "node" order in following case. 557 (1) if the DMA zone does not exist or 558 (2) if the DMA zone comprises greater than 50% of the available memory or 559 (3) if any node's DMA zone comprises greater than 70% of its local memory and 560 the amount of local memory is big enough. 561 562 Otherwise, "zone" order will be selected. Default order is recommended unless 563 this is causing problems for your system/application. 564 565 ============================================================== 566 567 oom_dump_tasks 568 569 Enables a system-wide task dump (excluding kernel threads) to be produced 570 when the kernel performs an OOM-killing and includes such information as 571 pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj 572 score, and name. This is helpful to determine why the OOM killer was 573 invoked, to identify the rogue task that caused it, and to determine why 574 the OOM killer chose the task it did to kill. 575 576 If this is set to zero, this information is suppressed. On very 577 large systems with thousands of tasks it may not be feasible to dump 578 the memory state information for each one. Such systems should not 579 be forced to incur a performance penalty in OOM conditions when the 580 information may not be desired. 581 582 If this is set to non-zero, this information is shown whenever the 583 OOM killer actually kills a memory-hogging task. 584 585 The default value is 1 (enabled). 586 587 ============================================================== 588 589 oom_kill_allocating_task 590 591 This enables or disables killing the OOM-triggering task in 592 out-of-memory situations. 593 594 If this is set to zero, the OOM killer will scan through the entire 595 tasklist and select a task based on heuristics to kill. This normally 596 selects a rogue memory-hogging task that frees up a large amount of 597 memory when killed. 598 599 If this is set to non-zero, the OOM killer simply kills the task that 600 triggered the out-of-memory condition. This avoids the expensive 601 tasklist scan. 602 603 If panic_on_oom is selected, it takes precedence over whatever value 604 is used in oom_kill_allocating_task. 605 606 The default value is 0. 607 608 ============================================================== 609 610 overcommit_kbytes: 611 612 When overcommit_memory is set to 2, the committed address space is not 613 permitted to exceed swap plus this amount of physical RAM. See below. 614 615 Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one 616 of them may be specified at a time. Setting one disables the other (which 617 then appears as 0 when read). 618 619 ============================================================== 620 621 overcommit_memory: 622 623 This value contains a flag that enables memory overcommitment. 624 625 When this flag is 0, the kernel attempts to estimate the amount 626 of free memory left when userspace requests more memory. 627 628 When this flag is 1, the kernel pretends there is always enough 629 memory until it actually runs out. 630 631 When this flag is 2, the kernel uses a "never overcommit" 632 policy that attempts to prevent any overcommit of memory. 633 Note that user_reserve_kbytes affects this policy. 634 635 This feature can be very useful because there are a lot of 636 programs that malloc() huge amounts of memory "just-in-case" 637 and don't use much of it. 638 639 The default value is 0. 640 641 See Documentation/vm/overcommit-accounting and 642 security/commoncap.c::cap_vm_enough_memory() for more information. 643 644 ============================================================== 645 646 overcommit_ratio: 647 648 When overcommit_memory is set to 2, the committed address 649 space is not permitted to exceed swap plus this percentage 650 of physical RAM. See above. 651 652 ============================================================== 653 654 page-cluster 655 656 page-cluster controls the number of pages up to which consecutive pages 657 are read in from swap in a single attempt. This is the swap counterpart 658 to page cache readahead. 659 The mentioned consecutivity is not in terms of virtual/physical addresses, 660 but consecutive on swap space - that means they were swapped out together. 661 662 It is a logarithmic value - setting it to zero means "1 page", setting 663 it to 1 means "2 pages", setting it to 2 means "4 pages", etc. 664 Zero disables swap readahead completely. 665 666 The default value is three (eight pages at a time). There may be some 667 small benefits in tuning this to a different value if your workload is 668 swap-intensive. 669 670 Lower values mean lower latencies for initial faults, but at the same time 671 extra faults and I/O delays for following faults if they would have been part of 672 that consecutive pages readahead would have brought in. 673 674 ============================================================= 675 676 panic_on_oom 677 678 This enables or disables panic on out-of-memory feature. 679 680 If this is set to 0, the kernel will kill some rogue process, 681 called oom_killer. Usually, oom_killer can kill rogue processes and 682 system will survive. 683 684 If this is set to 1, the kernel panics when out-of-memory happens. 685 However, if a process limits using nodes by mempolicy/cpusets, 686 and those nodes become memory exhaustion status, one process 687 may be killed by oom-killer. No panic occurs in this case. 688 Because other nodes' memory may be free. This means system total status 689 may be not fatal yet. 690 691 If this is set to 2, the kernel panics compulsorily even on the 692 above-mentioned. Even oom happens under memory cgroup, the whole 693 system panics. 694 695 The default value is 0. 696 1 and 2 are for failover of clustering. Please select either 697 according to your policy of failover. 698 panic_on_oom=2+kdump gives you very strong tool to investigate 699 why oom happens. You can get snapshot. 700 701 ============================================================= 702 703 percpu_pagelist_fraction 704 705 This is the fraction of pages at most (high mark pcp->high) in each zone that 706 are allocated for each per cpu page list. The min value for this is 8. It 707 means that we don't allow more than 1/8th of pages in each zone to be 708 allocated in any single per_cpu_pagelist. This entry only changes the value 709 of hot per cpu pagelists. User can specify a number like 100 to allocate 710 1/100th of each zone to each per cpu page list. 711 712 The batch value of each per cpu pagelist is also updated as a result. It is 713 set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) 714 715 The initial value is zero. Kernel does not use this value at boot time to set 716 the high water marks for each per cpu page list. If the user writes '0' to this 717 sysctl, it will revert to this default behavior. 718 719 ============================================================== 720 721 stat_interval 722 723 The time interval between which vm statistics are updated. The default 724 is 1 second. 725 726 ============================================================== 727 728 swappiness 729 730 This control is used to define how aggressive the kernel will swap 731 memory pages. Higher values will increase agressiveness, lower values 732 decrease the amount of swap. A value of 0 instructs the kernel not to 733 initiate swap until the amount of free and file-backed pages is less 734 than the high water mark in a zone. 735 736 The default value is 60. 737 738 ============================================================== 739 740 - user_reserve_kbytes 741 742 When overcommit_memory is set to 2, "never overcommit" mode, reserve 743 min(3% of current process size, user_reserve_kbytes) of free memory. 744 This is intended to prevent a user from starting a single memory hogging 745 process, such that they cannot recover (kill the hog). 746 747 user_reserve_kbytes defaults to min(3% of the current process size, 128MB). 748 749 If this is reduced to zero, then the user will be allowed to allocate 750 all free memory with a single process, minus admin_reserve_kbytes. 751 Any subsequent attempts to execute a command will result in 752 "fork: Cannot allocate memory". 753 754 Changing this takes effect whenever an application requests memory. 755 756 ============================================================== 757 758 vfs_cache_pressure 759 ------------------ 760 761 This percentage value controls the tendency of the kernel to reclaim 762 the memory which is used for caching of directory and inode objects. 763 764 At the default value of vfs_cache_pressure=100 the kernel will attempt to 765 reclaim dentries and inodes at a "fair" rate with respect to pagecache and 766 swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer 767 to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will 768 never reclaim dentries and inodes due to memory pressure and this can easily 769 lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 770 causes the kernel to prefer to reclaim dentries and inodes. 771 772 Increasing vfs_cache_pressure significantly beyond 100 may have negative 773 performance impact. Reclaim code needs to take various locks to find freeable 774 directory and inode objects. With vfs_cache_pressure=1000, it will look for 775 ten times more freeable objects than there are. 776 777 ============================================================== 778 779 zone_reclaim_mode: 780 781 Zone_reclaim_mode allows someone to set more or less aggressive approaches to 782 reclaim memory when a zone runs out of memory. If it is set to zero then no 783 zone reclaim occurs. Allocations will be satisfied from other zones / nodes 784 in the system. 785 786 This is value ORed together of 787 788 1 = Zone reclaim on 789 2 = Zone reclaim writes dirty pages out 790 4 = Zone reclaim swaps pages 791 792 zone_reclaim_mode is disabled by default. For file servers or workloads 793 that benefit from having their data cached, zone_reclaim_mode should be 794 left disabled as the caching effect is likely to be more important than 795 data locality. 796 797 zone_reclaim may be enabled if it's known that the workload is partitioned 798 such that each partition fits within a NUMA node and that accessing remote 799 memory would cause a measurable performance reduction. The page allocator 800 will then reclaim easily reusable pages (those page cache pages that are 801 currently not used) before allocating off node pages. 802 803 Allowing zone reclaim to write out pages stops processes that are 804 writing large amounts of data from dirtying pages on other nodes. Zone 805 reclaim will write out dirty pages if a zone fills up and so effectively 806 throttle the process. This may decrease the performance of a single process 807 since it cannot use all of system memory to buffer the outgoing writes 808 anymore but it preserve the memory on other nodes so that the performance 809 of other processes running on other nodes will not be affected. 810 811 Allowing regular swap effectively restricts allocations to the local 812 node unless explicitly overridden by memory policies or cpuset 813 configurations. 814 815 ============ End of Document =================================