Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.
1 Documentation for /proc/sys/vm/* kernel version 2.6.29 2 (c) 1998, 1999, Rik van Riel <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 - mmap_rnd_bits 46 - mmap_rnd_compat_bits 47 - nr_hugepages 48 - nr_overcommit_hugepages 49 - nr_trim_pages (only if CONFIG_MMU=n) 50 - numa_zonelist_order 51 - oom_dump_tasks 52 - oom_kill_allocating_task 53 - overcommit_kbytes 54 - overcommit_memory 55 - overcommit_ratio 56 - page-cluster 57 - panic_on_oom 58 - percpu_pagelist_fraction 59 - stat_interval 60 - stat_refresh 61 - swappiness 62 - user_reserve_kbytes 63 - vfs_cache_pressure 64 - watermark_scale_factor 65 - zone_reclaim_mode 66 67 ============================================================== 68 69 admin_reserve_kbytes 70 71 The amount of free memory in the system that should be reserved for users 72 with the capability cap_sys_admin. 73 74 admin_reserve_kbytes defaults to min(3% of free pages, 8MB) 75 76 That should provide enough for the admin to log in and kill a process, 77 if necessary, under the default overcommit 'guess' mode. 78 79 Systems running under overcommit 'never' should increase this to account 80 for the full Virtual Memory Size of programs used to recover. Otherwise, 81 root may not be able to log in to recover the system. 82 83 How do you calculate a minimum useful reserve? 84 85 sshd or login + bash (or some other shell) + top (or ps, kill, etc.) 86 87 For overcommit 'guess', we can sum resident set sizes (RSS). 88 On x86_64 this is about 8MB. 89 90 For overcommit 'never', we can take the max of their virtual sizes (VSZ) 91 and add the sum of their RSS. 92 On x86_64 this is about 128MB. 93 94 Changing this takes effect whenever an application requests memory. 95 96 ============================================================== 97 98 block_dump 99 100 block_dump enables block I/O debugging when set to a nonzero value. More 101 information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. 102 103 ============================================================== 104 105 compact_memory 106 107 Available only when CONFIG_COMPACTION is set. When 1 is written to the file, 108 all zones are compacted such that free memory is available in contiguous 109 blocks where possible. This can be important for example in the allocation of 110 huge pages although processes will also directly compact memory as required. 111 112 ============================================================== 113 114 compact_unevictable_allowed 115 116 Available only when CONFIG_COMPACTION is set. When set to 1, compaction is 117 allowed to examine the unevictable lru (mlocked pages) for pages to compact. 118 This should be used on systems where stalls for minor page faults are an 119 acceptable trade for large contiguous free memory. Set to 0 to prevent 120 compaction from moving pages that are unevictable. Default value is 1. 121 122 ============================================================== 123 124 dirty_background_bytes 125 126 Contains the amount of dirty memory at which the background kernel 127 flusher threads will start writeback. 128 129 Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only 130 one of them may be specified at a time. When one sysctl is written it is 131 immediately taken into account to evaluate the dirty memory limits and the 132 other appears as 0 when read. 133 134 ============================================================== 135 136 dirty_background_ratio 137 138 Contains, as a percentage of total available memory that contains free pages 139 and reclaimable pages, the number of pages at which the background kernel 140 flusher threads will start writing out dirty data. 141 142 The total available memory is not equal to total system memory. 143 144 ============================================================== 145 146 dirty_bytes 147 148 Contains the amount of dirty memory at which a process generating disk writes 149 will itself start writeback. 150 151 Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be 152 specified at a time. When one sysctl is written it is immediately taken into 153 account to evaluate the dirty memory limits and the other appears as 0 when 154 read. 155 156 Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any 157 value lower than this limit will be ignored and the old configuration will be 158 retained. 159 160 ============================================================== 161 162 dirty_expire_centisecs 163 164 This tunable is used to define when dirty data is old enough to be eligible 165 for writeout by the kernel flusher threads. It is expressed in 100'ths 166 of a second. Data which has been dirty in-memory for longer than this 167 interval will be written out next time a flusher thread wakes up. 168 169 ============================================================== 170 171 dirty_ratio 172 173 Contains, as a percentage of total available memory that contains free pages 174 and reclaimable pages, the number of pages at which a process which is 175 generating disk writes will itself start writing out dirty data. 176 177 The total available memory is not equal to total system memory. 178 179 ============================================================== 180 181 dirty_writeback_centisecs 182 183 The kernel flusher threads will periodically wake up and write `old' data 184 out to disk. This tunable expresses the interval between those wakeups, in 185 100'ths of a second. 186 187 Setting this to zero disables periodic writeback altogether. 188 189 ============================================================== 190 191 drop_caches 192 193 Writing to this will cause the kernel to drop clean caches, as well as 194 reclaimable slab objects like dentries and inodes. Once dropped, their 195 memory becomes free. 196 197 To free pagecache: 198 echo 1 > /proc/sys/vm/drop_caches 199 To free reclaimable slab objects (includes dentries and inodes): 200 echo 2 > /proc/sys/vm/drop_caches 201 To free slab objects and pagecache: 202 echo 3 > /proc/sys/vm/drop_caches 203 204 This is a non-destructive operation and will not free any dirty objects. 205 To increase the number of objects freed by this operation, the user may run 206 `sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the 207 number of dirty objects on the system and create more candidates to be 208 dropped. 209 210 This file is not a means to control the growth of the various kernel caches 211 (inodes, dentries, pagecache, etc...) These objects are automatically 212 reclaimed by the kernel when memory is needed elsewhere on the system. 213 214 Use of this file can cause performance problems. Since it discards cached 215 objects, it may cost a significant amount of I/O and CPU to recreate the 216 dropped objects, especially if they were under heavy use. Because of this, 217 use outside of a testing or debugging environment is not recommended. 218 219 You may see informational messages in your kernel log when this file is 220 used: 221 222 cat (1234): drop_caches: 3 223 224 These are informational only. They do not mean that anything is wrong 225 with your system. To disable them, echo 4 (bit 3) into drop_caches. 226 227 ============================================================== 228 229 extfrag_threshold 230 231 This parameter affects whether the kernel will compact memory or direct 232 reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in 233 debugfs shows what the fragmentation index for each order is in each zone in 234 the system. Values tending towards 0 imply allocations would fail due to lack 235 of memory, values towards 1000 imply failures are due to fragmentation and -1 236 implies that the allocation will succeed as long as watermarks are met. 237 238 The kernel will not compact memory in a zone if the 239 fragmentation index is <= extfrag_threshold. The default value is 500. 240 241 ============================================================== 242 243 highmem_is_dirtyable 244 245 Available only for systems with CONFIG_HIGHMEM enabled (32b systems). 246 247 This parameter controls whether the high memory is considered for dirty 248 writers throttling. This is not the case by default which means that 249 only the amount of memory directly visible/usable by the kernel can 250 be dirtied. As a result, on systems with a large amount of memory and 251 lowmem basically depleted writers might be throttled too early and 252 streaming writes can get very slow. 253 254 Changing the value to non zero would allow more memory to be dirtied 255 and thus allow writers to write more data which can be flushed to the 256 storage more effectively. Note this also comes with a risk of pre-mature 257 OOM killer because some writers (e.g. direct block device writes) can 258 only use the low memory and they can fill it up with dirty data without 259 any throttling. 260 261 ============================================================== 262 263 hugepages_treat_as_movable 264 265 This parameter controls whether we can allocate hugepages from ZONE_MOVABLE 266 or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE. 267 ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified, 268 so this parameter has no effect if used without kernelcore=. 269 270 Hugepage migration is now available in some situations which depend on the 271 architecture and/or the hugepage size. If a hugepage supports migration, 272 allocation from ZONE_MOVABLE is always enabled for the hugepage regardless 273 of the value of this parameter. 274 IOW, this parameter affects only non-migratable hugepages. 275 276 Assuming that hugepages are not migratable in your system, one usecase of 277 this parameter is that users can make hugepage pool more extensible by 278 enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE 279 page reclaim/migration/compaction work more and you can get contiguous 280 memory more likely. Note that using ZONE_MOVABLE for non-migratable 281 hugepages can do harm to other features like memory hotremove (because 282 memory hotremove expects that memory blocks on ZONE_MOVABLE are always 283 removable,) so it's a trade-off responsible for the users. 284 285 ============================================================== 286 287 hugetlb_shm_group 288 289 hugetlb_shm_group contains group id that is allowed to create SysV 290 shared memory segment using hugetlb page. 291 292 ============================================================== 293 294 laptop_mode 295 296 laptop_mode is a knob that controls "laptop mode". All the things that are 297 controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. 298 299 ============================================================== 300 301 legacy_va_layout 302 303 If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel 304 will use the legacy (2.4) layout for all processes. 305 306 ============================================================== 307 308 lowmem_reserve_ratio 309 310 For some specialised workloads on highmem machines it is dangerous for 311 the kernel to allow process memory to be allocated from the "lowmem" 312 zone. This is because that memory could then be pinned via the mlock() 313 system call, or by unavailability of swapspace. 314 315 And on large highmem machines this lack of reclaimable lowmem memory 316 can be fatal. 317 318 So the Linux page allocator has a mechanism which prevents allocations 319 which _could_ use highmem from using too much lowmem. This means that 320 a certain amount of lowmem is defended from the possibility of being 321 captured into pinned user memory. 322 323 (The same argument applies to the old 16 megabyte ISA DMA region. This 324 mechanism will also defend that region from allocations which could use 325 highmem or lowmem). 326 327 The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is 328 in defending these lower zones. 329 330 If you have a machine which uses highmem or ISA DMA and your 331 applications are using mlock(), or if you are running with no swap then 332 you probably should change the lowmem_reserve_ratio setting. 333 334 The lowmem_reserve_ratio is an array. You can see them by reading this file. 335 - 336 % cat /proc/sys/vm/lowmem_reserve_ratio 337 256 256 32 338 - 339 Note: # of this elements is one fewer than number of zones. Because the highest 340 zone's value is not necessary for following calculation. 341 342 But, these values are not used directly. The kernel calculates # of protection 343 pages for each zones from them. These are shown as array of protection pages 344 in /proc/zoneinfo like followings. (This is an example of x86-64 box). 345 Each zone has an array of protection pages like this. 346 347 - 348 Node 0, zone DMA 349 pages free 1355 350 min 3 351 low 3 352 high 4 353 : 354 : 355 numa_other 0 356 protection: (0, 2004, 2004, 2004) 357 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 358 pagesets 359 cpu: 0 pcp: 0 360 : 361 - 362 These protections are added to score to judge whether this zone should be used 363 for page allocation or should be reclaimed. 364 365 In this example, if normal pages (index=2) are required to this DMA zone and 366 watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should 367 not be used because pages_free(1355) is smaller than watermark + protection 368 (4 + 2004 = 2008). If this protection value is 0, this zone would be used for 369 normal page requirement. If requirement is DMA zone(index=0), protection 370 (=0) is used. 371 372 zone[i]'s protection[j] is calculated by following expression. 373 374 (i < j): 375 zone[i]->protection[j] 376 = (total sums of managed_pages from zone[i+1] to zone[j] on the node) 377 / lowmem_reserve_ratio[i]; 378 (i = j): 379 (should not be protected. = 0; 380 (i > j): 381 (not necessary, but looks 0) 382 383 The default values of lowmem_reserve_ratio[i] are 384 256 (if zone[i] means DMA or DMA32 zone) 385 32 (others). 386 As above expression, they are reciprocal number of ratio. 387 256 means 1/256. # of protection pages becomes about "0.39%" of total managed 388 pages of higher zones on the node. 389 390 If you would like to protect more pages, smaller values are effective. 391 The minimum value is 1 (1/1 -> 100%). 392 393 ============================================================== 394 395 max_map_count: 396 397 This file contains the maximum number of memory map areas a process 398 may have. Memory map areas are used as a side-effect of calling 399 malloc, directly by mmap, mprotect, and madvise, and also when loading 400 shared libraries. 401 402 While most applications need less than a thousand maps, certain 403 programs, particularly malloc debuggers, may consume lots of them, 404 e.g., up to one or two maps per allocation. 405 406 The default value is 65536. 407 408 ============================================================= 409 410 memory_failure_early_kill: 411 412 Control how to kill processes when uncorrected memory error (typically 413 a 2bit error in a memory module) is detected in the background by hardware 414 that cannot be handled by the kernel. In some cases (like the page 415 still having a valid copy on disk) the kernel will handle the failure 416 transparently without affecting any applications. But if there is 417 no other uptodate copy of the data it will kill to prevent any data 418 corruptions from propagating. 419 420 1: Kill all processes that have the corrupted and not reloadable page mapped 421 as soon as the corruption is detected. Note this is not supported 422 for a few types of pages, like kernel internally allocated data or 423 the swap cache, but works for the majority of user pages. 424 425 0: Only unmap the corrupted page from all processes and only kill a process 426 who tries to access it. 427 428 The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can 429 handle this if they want to. 430 431 This is only active on architectures/platforms with advanced machine 432 check handling and depends on the hardware capabilities. 433 434 Applications can override this setting individually with the PR_MCE_KILL prctl 435 436 ============================================================== 437 438 memory_failure_recovery 439 440 Enable memory failure recovery (when supported by the platform) 441 442 1: Attempt recovery. 443 444 0: Always panic on a memory failure. 445 446 ============================================================== 447 448 min_free_kbytes: 449 450 This is used to force the Linux VM to keep a minimum number 451 of kilobytes free. The VM uses this number to compute a 452 watermark[WMARK_MIN] value for each lowmem zone in the system. 453 Each lowmem zone gets a number of reserved free pages based 454 proportionally on its size. 455 456 Some minimal amount of memory is needed to satisfy PF_MEMALLOC 457 allocations; if you set this to lower than 1024KB, your system will 458 become subtly broken, and prone to deadlock under high loads. 459 460 Setting this too high will OOM your machine instantly. 461 462 ============================================================= 463 464 min_slab_ratio: 465 466 This is available only on NUMA kernels. 467 468 A percentage of the total pages in each zone. On Zone reclaim 469 (fallback from the local zone occurs) slabs will be reclaimed if more 470 than this percentage of pages in a zone are reclaimable slab pages. 471 This insures that the slab growth stays under control even in NUMA 472 systems that rarely perform global reclaim. 473 474 The default is 5 percent. 475 476 Note that slab reclaim is triggered in a per zone / node fashion. 477 The process of reclaiming slab memory is currently not node specific 478 and may not be fast. 479 480 ============================================================= 481 482 min_unmapped_ratio: 483 484 This is available only on NUMA kernels. 485 486 This is a percentage of the total pages in each zone. Zone reclaim will 487 only occur if more than this percentage of pages are in a state that 488 zone_reclaim_mode allows to be reclaimed. 489 490 If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared 491 against all file-backed unmapped pages including swapcache pages and tmpfs 492 files. Otherwise, only unmapped pages backed by normal files but not tmpfs 493 files and similar are considered. 494 495 The default is 1 percent. 496 497 ============================================================== 498 499 mmap_min_addr 500 501 This file indicates the amount of address space which a user process will 502 be restricted from mmapping. Since kernel null dereference bugs could 503 accidentally operate based on the information in the first couple of pages 504 of memory userspace processes should not be allowed to write to them. By 505 default this value is set to 0 and no protections will be enforced by the 506 security module. Setting this value to something like 64k will allow the 507 vast majority of applications to work correctly and provide defense in depth 508 against future potential kernel bugs. 509 510 ============================================================== 511 512 mmap_rnd_bits: 513 514 This value can be used to select the number of bits to use to 515 determine the random offset to the base address of vma regions 516 resulting from mmap allocations on architectures which support 517 tuning address space randomization. This value will be bounded 518 by the architecture's minimum and maximum supported values. 519 520 This value can be changed after boot using the 521 /proc/sys/vm/mmap_rnd_bits tunable 522 523 ============================================================== 524 525 mmap_rnd_compat_bits: 526 527 This value can be used to select the number of bits to use to 528 determine the random offset to the base address of vma regions 529 resulting from mmap allocations for applications run in 530 compatibility mode on architectures which support tuning address 531 space randomization. This value will be bounded by the 532 architecture's minimum and maximum supported values. 533 534 This value can be changed after boot using the 535 /proc/sys/vm/mmap_rnd_compat_bits tunable 536 537 ============================================================== 538 539 nr_hugepages 540 541 Change the minimum size of the hugepage pool. 542 543 See Documentation/vm/hugetlbpage.txt 544 545 ============================================================== 546 547 nr_overcommit_hugepages 548 549 Change the maximum size of the hugepage pool. The maximum is 550 nr_hugepages + nr_overcommit_hugepages. 551 552 See Documentation/vm/hugetlbpage.txt 553 554 ============================================================== 555 556 nr_trim_pages 557 558 This is available only on NOMMU kernels. 559 560 This value adjusts the excess page trimming behaviour of power-of-2 aligned 561 NOMMU mmap allocations. 562 563 A value of 0 disables trimming of allocations entirely, while a value of 1 564 trims excess pages aggressively. Any value >= 1 acts as the watermark where 565 trimming of allocations is initiated. 566 567 The default value is 1. 568 569 See Documentation/nommu-mmap.txt for more information. 570 571 ============================================================== 572 573 numa_zonelist_order 574 575 This sysctl is only for NUMA. 576 'where the memory is allocated from' is controlled by zonelists. 577 (This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. 578 you may be able to read ZONE_DMA as ZONE_DMA32...) 579 580 In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. 581 ZONE_NORMAL -> ZONE_DMA 582 This means that a memory allocation request for GFP_KERNEL will 583 get memory from ZONE_DMA only when ZONE_NORMAL is not available. 584 585 In NUMA case, you can think of following 2 types of order. 586 Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL 587 588 (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL 589 (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. 590 591 Type(A) offers the best locality for processes on Node(0), but ZONE_DMA 592 will be used before ZONE_NORMAL exhaustion. This increases possibility of 593 out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. 594 595 Type(B) cannot offer the best locality but is more robust against OOM of 596 the DMA zone. 597 598 Type(A) is called as "Node" order. Type (B) is "Zone" order. 599 600 "Node order" orders the zonelists by node, then by zone within each node. 601 Specify "[Nn]ode" for node order 602 603 "Zone Order" orders the zonelists by zone type, then by node within each 604 zone. Specify "[Zz]one" for zone order. 605 606 Specify "[Dd]efault" to request automatic configuration. 607 608 On 32-bit, the Normal zone needs to be preserved for allocations accessible 609 by the kernel, so "zone" order will be selected. 610 611 On 64-bit, devices that require DMA32/DMA are relatively rare, so "node" 612 order will be selected. 613 614 Default order is recommended unless this is causing problems for your 615 system/application. 616 617 ============================================================== 618 619 oom_dump_tasks 620 621 Enables a system-wide task dump (excluding kernel threads) to be produced 622 when the kernel performs an OOM-killing and includes such information as 623 pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj 624 score, and name. This is helpful to determine why the OOM killer was 625 invoked, to identify the rogue task that caused it, and to determine why 626 the OOM killer chose the task it did to kill. 627 628 If this is set to zero, this information is suppressed. On very 629 large systems with thousands of tasks it may not be feasible to dump 630 the memory state information for each one. Such systems should not 631 be forced to incur a performance penalty in OOM conditions when the 632 information may not be desired. 633 634 If this is set to non-zero, this information is shown whenever the 635 OOM killer actually kills a memory-hogging task. 636 637 The default value is 1 (enabled). 638 639 ============================================================== 640 641 oom_kill_allocating_task 642 643 This enables or disables killing the OOM-triggering task in 644 out-of-memory situations. 645 646 If this is set to zero, the OOM killer will scan through the entire 647 tasklist and select a task based on heuristics to kill. This normally 648 selects a rogue memory-hogging task that frees up a large amount of 649 memory when killed. 650 651 If this is set to non-zero, the OOM killer simply kills the task that 652 triggered the out-of-memory condition. This avoids the expensive 653 tasklist scan. 654 655 If panic_on_oom is selected, it takes precedence over whatever value 656 is used in oom_kill_allocating_task. 657 658 The default value is 0. 659 660 ============================================================== 661 662 overcommit_kbytes: 663 664 When overcommit_memory is set to 2, the committed address space is not 665 permitted to exceed swap plus this amount of physical RAM. See below. 666 667 Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one 668 of them may be specified at a time. Setting one disables the other (which 669 then appears as 0 when read). 670 671 ============================================================== 672 673 overcommit_memory: 674 675 This value contains a flag that enables memory overcommitment. 676 677 When this flag is 0, the kernel attempts to estimate the amount 678 of free memory left when userspace requests more memory. 679 680 When this flag is 1, the kernel pretends there is always enough 681 memory until it actually runs out. 682 683 When this flag is 2, the kernel uses a "never overcommit" 684 policy that attempts to prevent any overcommit of memory. 685 Note that user_reserve_kbytes affects this policy. 686 687 This feature can be very useful because there are a lot of 688 programs that malloc() huge amounts of memory "just-in-case" 689 and don't use much of it. 690 691 The default value is 0. 692 693 See Documentation/vm/overcommit-accounting and 694 mm/mmap.c::__vm_enough_memory() for more information. 695 696 ============================================================== 697 698 overcommit_ratio: 699 700 When overcommit_memory is set to 2, the committed address 701 space is not permitted to exceed swap plus this percentage 702 of physical RAM. See above. 703 704 ============================================================== 705 706 page-cluster 707 708 page-cluster controls the number of pages up to which consecutive pages 709 are read in from swap in a single attempt. This is the swap counterpart 710 to page cache readahead. 711 The mentioned consecutivity is not in terms of virtual/physical addresses, 712 but consecutive on swap space - that means they were swapped out together. 713 714 It is a logarithmic value - setting it to zero means "1 page", setting 715 it to 1 means "2 pages", setting it to 2 means "4 pages", etc. 716 Zero disables swap readahead completely. 717 718 The default value is three (eight pages at a time). There may be some 719 small benefits in tuning this to a different value if your workload is 720 swap-intensive. 721 722 Lower values mean lower latencies for initial faults, but at the same time 723 extra faults and I/O delays for following faults if they would have been part of 724 that consecutive pages readahead would have brought in. 725 726 ============================================================= 727 728 panic_on_oom 729 730 This enables or disables panic on out-of-memory feature. 731 732 If this is set to 0, the kernel will kill some rogue process, 733 called oom_killer. Usually, oom_killer can kill rogue processes and 734 system will survive. 735 736 If this is set to 1, the kernel panics when out-of-memory happens. 737 However, if a process limits using nodes by mempolicy/cpusets, 738 and those nodes become memory exhaustion status, one process 739 may be killed by oom-killer. No panic occurs in this case. 740 Because other nodes' memory may be free. This means system total status 741 may be not fatal yet. 742 743 If this is set to 2, the kernel panics compulsorily even on the 744 above-mentioned. Even oom happens under memory cgroup, the whole 745 system panics. 746 747 The default value is 0. 748 1 and 2 are for failover of clustering. Please select either 749 according to your policy of failover. 750 panic_on_oom=2+kdump gives you very strong tool to investigate 751 why oom happens. You can get snapshot. 752 753 ============================================================= 754 755 percpu_pagelist_fraction 756 757 This is the fraction of pages at most (high mark pcp->high) in each zone that 758 are allocated for each per cpu page list. The min value for this is 8. It 759 means that we don't allow more than 1/8th of pages in each zone to be 760 allocated in any single per_cpu_pagelist. This entry only changes the value 761 of hot per cpu pagelists. User can specify a number like 100 to allocate 762 1/100th of each zone to each per cpu page list. 763 764 The batch value of each per cpu pagelist is also updated as a result. It is 765 set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) 766 767 The initial value is zero. Kernel does not use this value at boot time to set 768 the high water marks for each per cpu page list. If the user writes '0' to this 769 sysctl, it will revert to this default behavior. 770 771 ============================================================== 772 773 stat_interval 774 775 The time interval between which vm statistics are updated. The default 776 is 1 second. 777 778 ============================================================== 779 780 stat_refresh 781 782 Any read or write (by root only) flushes all the per-cpu vm statistics 783 into their global totals, for more accurate reports when testing 784 e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo 785 786 As a side-effect, it also checks for negative totals (elsewhere reported 787 as 0) and "fails" with EINVAL if any are found, with a warning in dmesg. 788 (At time of writing, a few stats are known sometimes to be found negative, 789 with no ill effects: errors and warnings on these stats are suppressed.) 790 791 ============================================================== 792 793 swappiness 794 795 This control is used to define how aggressive the kernel will swap 796 memory pages. Higher values will increase agressiveness, lower values 797 decrease the amount of swap. A value of 0 instructs the kernel not to 798 initiate swap until the amount of free and file-backed pages is less 799 than the high water mark in a zone. 800 801 The default value is 60. 802 803 ============================================================== 804 805 - user_reserve_kbytes 806 807 When overcommit_memory is set to 2, "never overcommit" mode, reserve 808 min(3% of current process size, user_reserve_kbytes) of free memory. 809 This is intended to prevent a user from starting a single memory hogging 810 process, such that they cannot recover (kill the hog). 811 812 user_reserve_kbytes defaults to min(3% of the current process size, 128MB). 813 814 If this is reduced to zero, then the user will be allowed to allocate 815 all free memory with a single process, minus admin_reserve_kbytes. 816 Any subsequent attempts to execute a command will result in 817 "fork: Cannot allocate memory". 818 819 Changing this takes effect whenever an application requests memory. 820 821 ============================================================== 822 823 vfs_cache_pressure 824 ------------------ 825 826 This percentage value controls the tendency of the kernel to reclaim 827 the memory which is used for caching of directory and inode objects. 828 829 At the default value of vfs_cache_pressure=100 the kernel will attempt to 830 reclaim dentries and inodes at a "fair" rate with respect to pagecache and 831 swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer 832 to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will 833 never reclaim dentries and inodes due to memory pressure and this can easily 834 lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 835 causes the kernel to prefer to reclaim dentries and inodes. 836 837 Increasing vfs_cache_pressure significantly beyond 100 may have negative 838 performance impact. Reclaim code needs to take various locks to find freeable 839 directory and inode objects. With vfs_cache_pressure=1000, it will look for 840 ten times more freeable objects than there are. 841 842 ============================================================= 843 844 watermark_scale_factor: 845 846 This factor controls the aggressiveness of kswapd. It defines the 847 amount of memory left in a node/system before kswapd is woken up and 848 how much memory needs to be free before kswapd goes back to sleep. 849 850 The unit is in fractions of 10,000. The default value of 10 means the 851 distances between watermarks are 0.1% of the available memory in the 852 node/system. The maximum value is 1000, or 10% of memory. 853 854 A high rate of threads entering direct reclaim (allocstall) or kswapd 855 going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate 856 that the number of free pages kswapd maintains for latency reasons is 857 too small for the allocation bursts occurring in the system. This knob 858 can then be used to tune kswapd aggressiveness accordingly. 859 860 ============================================================== 861 862 zone_reclaim_mode: 863 864 Zone_reclaim_mode allows someone to set more or less aggressive approaches to 865 reclaim memory when a zone runs out of memory. If it is set to zero then no 866 zone reclaim occurs. Allocations will be satisfied from other zones / nodes 867 in the system. 868 869 This is value ORed together of 870 871 1 = Zone reclaim on 872 2 = Zone reclaim writes dirty pages out 873 4 = Zone reclaim swaps pages 874 875 zone_reclaim_mode is disabled by default. For file servers or workloads 876 that benefit from having their data cached, zone_reclaim_mode should be 877 left disabled as the caching effect is likely to be more important than 878 data locality. 879 880 zone_reclaim may be enabled if it's known that the workload is partitioned 881 such that each partition fits within a NUMA node and that accessing remote 882 memory would cause a measurable performance reduction. The page allocator 883 will then reclaim easily reusable pages (those page cache pages that are 884 currently not used) before allocating off node pages. 885 886 Allowing zone reclaim to write out pages stops processes that are 887 writing large amounts of data from dirtying pages on other nodes. Zone 888 reclaim will write out dirty pages if a zone fills up and so effectively 889 throttle the process. This may decrease the performance of a single process 890 since it cannot use all of system memory to buffer the outgoing writes 891 anymore but it preserve the memory on other nodes so that the performance 892 of other processes running on other nodes will not be affected. 893 894 Allowing regular swap effectively restricts allocations to the local 895 node unless explicitly overridden by memory policies or cpuset 896 configurations. 897 898 ============ End of Document =================================