linux/Documentation/sysctl/vm.txt
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   1Documentation for /proc/sys/vm/*        kernel version 2.6.29
   2        (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
   3        (c) 2008         Peter W. Morreale <pmorreale@novell.com>
   4
   5For general info and legal blurb, please look in README.
   6
   7==============================================================
   8
   9This file contains the documentation for the sysctl files in
  10/proc/sys/vm and is valid for Linux kernel version 2.6.29.
  11
  12The files in this directory can be used to tune the operation
  13of the virtual memory (VM) subsystem of the Linux kernel and
  14the writeout of dirty data to disk.
  15
  16Default values and initialization routines for most of these
  17files can be found in mm/swap.c.
  18
  19Currently, 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
  69admin_reserve_kbytes
  70
  71The amount of free memory in the system that should be reserved for users
  72with the capability cap_sys_admin.
  73
  74admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
  75
  76That should provide enough for the admin to log in and kill a process,
  77if necessary, under the default overcommit 'guess' mode.
  78
  79Systems running under overcommit 'never' should increase this to account
  80for the full Virtual Memory Size of programs used to recover. Otherwise,
  81root may not be able to log in to recover the system.
  82
  83How do you calculate a minimum useful reserve?
  84
  85sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
  86
  87For overcommit 'guess', we can sum resident set sizes (RSS).
  88On x86_64 this is about 8MB.
  89
  90For overcommit 'never', we can take the max of their virtual sizes (VSZ)
  91and add the sum of their RSS.
  92On x86_64 this is about 128MB.
  93
  94Changing this takes effect whenever an application requests memory.
  95
  96==============================================================
  97
  98block_dump
  99
 100block_dump enables block I/O debugging when set to a nonzero value. More
 101information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
 102
 103==============================================================
 104
 105compact_memory
 106
 107Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
 108all zones are compacted such that free memory is available in contiguous
 109blocks where possible. This can be important for example in the allocation of
 110huge pages although processes will also directly compact memory as required.
 111
 112==============================================================
 113
 114compact_unevictable_allowed
 115
 116Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
 117allowed to examine the unevictable lru (mlocked pages) for pages to compact.
 118This should be used on systems where stalls for minor page faults are an
 119acceptable trade for large contiguous free memory.  Set to 0 to prevent
 120compaction from moving pages that are unevictable.  Default value is 1.
 121
 122==============================================================
 123
 124dirty_background_bytes
 125
 126Contains the amount of dirty memory at which the background kernel
 127flusher threads will start writeback.
 128
 129Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
 130one of them may be specified at a time. When one sysctl is written it is
 131immediately taken into account to evaluate the dirty memory limits and the
 132other appears as 0 when read.
 133
 134==============================================================
 135
 136dirty_background_ratio
 137
 138Contains, as a percentage of total available memory that contains free pages
 139and reclaimable pages, the number of pages at which the background kernel
 140flusher threads will start writing out dirty data.
 141
 142The total available memory is not equal to total system memory.
 143
 144==============================================================
 145
 146dirty_bytes
 147
 148Contains the amount of dirty memory at which a process generating disk writes
 149will itself start writeback.
 150
 151Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
 152specified at a time. When one sysctl is written it is immediately taken into
 153account to evaluate the dirty memory limits and the other appears as 0 when
 154read.
 155
 156Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
 157value lower than this limit will be ignored and the old configuration will be
 158retained.
 159
 160==============================================================
 161
 162dirty_expire_centisecs
 163
 164This tunable is used to define when dirty data is old enough to be eligible
 165for writeout by the kernel flusher threads.  It is expressed in 100'ths
 166of a second.  Data which has been dirty in-memory for longer than this
 167interval will be written out next time a flusher thread wakes up.
 168
 169==============================================================
 170
 171dirty_ratio
 172
 173Contains, as a percentage of total available memory that contains free pages
 174and reclaimable pages, the number of pages at which a process which is
 175generating disk writes will itself start writing out dirty data.
 176
 177The total available memory is not equal to total system memory.
 178
 179==============================================================
 180
 181dirty_writeback_centisecs
 182
 183The kernel flusher threads will periodically wake up and write `old' data
 184out to disk.  This tunable expresses the interval between those wakeups, in
 185100'ths of a second.
 186
 187Setting this to zero disables periodic writeback altogether.
 188
 189==============================================================
 190
 191drop_caches
 192
 193Writing to this will cause the kernel to drop clean caches, as well as
 194reclaimable slab objects like dentries and inodes.  Once dropped, their
 195memory becomes free.
 196
 197To free pagecache:
 198        echo 1 > /proc/sys/vm/drop_caches
 199To free reclaimable slab objects (includes dentries and inodes):
 200        echo 2 > /proc/sys/vm/drop_caches
 201To free slab objects and pagecache:
 202        echo 3 > /proc/sys/vm/drop_caches
 203
 204This is a non-destructive operation and will not free any dirty objects.
 205To 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
 207number of dirty objects on the system and create more candidates to be
 208dropped.
 209
 210This file is not a means to control the growth of the various kernel caches
 211(inodes, dentries, pagecache, etc...)  These objects are automatically
 212reclaimed by the kernel when memory is needed elsewhere on the system.
 213
 214Use of this file can cause performance problems.  Since it discards cached
 215objects, it may cost a significant amount of I/O and CPU to recreate the
 216dropped objects, especially if they were under heavy use.  Because of this,
 217use outside of a testing or debugging environment is not recommended.
 218
 219You may see informational messages in your kernel log when this file is
 220used:
 221
 222        cat (1234): drop_caches: 3
 223
 224These are informational only.  They do not mean that anything is wrong
 225with your system.  To disable them, echo 4 (bit 3) into drop_caches.
 226
 227==============================================================
 228
 229extfrag_threshold
 230
 231This parameter affects whether the kernel will compact memory or direct
 232reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
 233debugfs shows what the fragmentation index for each order is in each zone in
 234the system. Values tending towards 0 imply allocations would fail due to lack
 235of memory, values towards 1000 imply failures are due to fragmentation and -1
 236implies that the allocation will succeed as long as watermarks are met.
 237
 238The kernel will not compact memory in a zone if the
 239fragmentation index is <= extfrag_threshold. The default value is 500.
 240
 241==============================================================
 242
 243hugepages_treat_as_movable
 244
 245This parameter controls whether we can allocate hugepages from ZONE_MOVABLE
 246or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE.
 247ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified,
 248so this parameter has no effect if used without kernelcore=.
 249
 250Hugepage migration is now available in some situations which depend on the
 251architecture and/or the hugepage size. If a hugepage supports migration,
 252allocation from ZONE_MOVABLE is always enabled for the hugepage regardless
 253of the value of this parameter.
 254IOW, this parameter affects only non-migratable hugepages.
 255
 256Assuming that hugepages are not migratable in your system, one usecase of
 257this parameter is that users can make hugepage pool more extensible by
 258enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE
 259page reclaim/migration/compaction work more and you can get contiguous
 260memory more likely. Note that using ZONE_MOVABLE for non-migratable
 261hugepages can do harm to other features like memory hotremove (because
 262memory hotremove expects that memory blocks on ZONE_MOVABLE are always
 263removable,) so it's a trade-off responsible for the users.
 264
 265==============================================================
 266
 267hugetlb_shm_group
 268
 269hugetlb_shm_group contains group id that is allowed to create SysV
 270shared memory segment using hugetlb page.
 271
 272==============================================================
 273
 274laptop_mode
 275
 276laptop_mode is a knob that controls "laptop mode". All the things that are
 277controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
 278
 279==============================================================
 280
 281legacy_va_layout
 282
 283If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
 284will use the legacy (2.4) layout for all processes.
 285
 286==============================================================
 287
 288lowmem_reserve_ratio
 289
 290For some specialised workloads on highmem machines it is dangerous for
 291the kernel to allow process memory to be allocated from the "lowmem"
 292zone.  This is because that memory could then be pinned via the mlock()
 293system call, or by unavailability of swapspace.
 294
 295And on large highmem machines this lack of reclaimable lowmem memory
 296can be fatal.
 297
 298So the Linux page allocator has a mechanism which prevents allocations
 299which _could_ use highmem from using too much lowmem.  This means that
 300a certain amount of lowmem is defended from the possibility of being
 301captured into pinned user memory.
 302
 303(The same argument applies to the old 16 megabyte ISA DMA region.  This
 304mechanism will also defend that region from allocations which could use
 305highmem or lowmem).
 306
 307The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
 308in defending these lower zones.
 309
 310If you have a machine which uses highmem or ISA DMA and your
 311applications are using mlock(), or if you are running with no swap then
 312you probably should change the lowmem_reserve_ratio setting.
 313
 314The lowmem_reserve_ratio is an array. You can see them by reading this file.
 315-
 316% cat /proc/sys/vm/lowmem_reserve_ratio
 317256     256     32
 318-
 319Note: # of this elements is one fewer than number of zones. Because the highest
 320      zone's value is not necessary for following calculation.
 321
 322But, these values are not used directly. The kernel calculates # of protection
 323pages for each zones from them. These are shown as array of protection pages
 324in /proc/zoneinfo like followings. (This is an example of x86-64 box).
 325Each zone has an array of protection pages like this.
 326
 327-
 328Node 0, zone      DMA
 329  pages free     1355
 330        min      3
 331        low      3
 332        high     4
 333        :
 334        :
 335    numa_other   0
 336        protection: (0, 2004, 2004, 2004)
 337        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 338  pagesets
 339    cpu: 0 pcp: 0
 340        :
 341-
 342These protections are added to score to judge whether this zone should be used
 343for page allocation or should be reclaimed.
 344
 345In this example, if normal pages (index=2) are required to this DMA zone and
 346watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
 347not be used because pages_free(1355) is smaller than watermark + protection[2]
 348(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
 349normal page requirement. If requirement is DMA zone(index=0), protection[0]
 350(=0) is used.
 351
 352zone[i]'s protection[j] is calculated by following expression.
 353
 354(i < j):
 355  zone[i]->protection[j]
 356  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
 357    / lowmem_reserve_ratio[i];
 358(i = j):
 359   (should not be protected. = 0;
 360(i > j):
 361   (not necessary, but looks 0)
 362
 363The default values of lowmem_reserve_ratio[i] are
 364    256 (if zone[i] means DMA or DMA32 zone)
 365    32  (others).
 366As above expression, they are reciprocal number of ratio.
 367256 means 1/256. # of protection pages becomes about "0.39%" of total managed
 368pages of higher zones on the node.
 369
 370If you would like to protect more pages, smaller values are effective.
 371The minimum value is 1 (1/1 -> 100%).
 372
 373==============================================================
 374
 375max_map_count:
 376
 377This file contains the maximum number of memory map areas a process
 378may have. Memory map areas are used as a side-effect of calling
 379malloc, directly by mmap and mprotect, and also when loading shared
 380libraries.
 381
 382While most applications need less than a thousand maps, certain
 383programs, particularly malloc debuggers, may consume lots of them,
 384e.g., up to one or two maps per allocation.
 385
 386The default value is 65536.
 387
 388=============================================================
 389
 390memory_failure_early_kill:
 391
 392Control how to kill processes when uncorrected memory error (typically
 393a 2bit error in a memory module) is detected in the background by hardware
 394that cannot be handled by the kernel. In some cases (like the page
 395still having a valid copy on disk) the kernel will handle the failure
 396transparently without affecting any applications. But if there is
 397no other uptodate copy of the data it will kill to prevent any data
 398corruptions from propagating.
 399
 4001: Kill all processes that have the corrupted and not reloadable page mapped
 401as soon as the corruption is detected.  Note this is not supported
 402for a few types of pages, like kernel internally allocated data or
 403the swap cache, but works for the majority of user pages.
 404
 4050: Only unmap the corrupted page from all processes and only kill a process
 406who tries to access it.
 407
 408The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
 409handle this if they want to.
 410
 411This is only active on architectures/platforms with advanced machine
 412check handling and depends on the hardware capabilities.
 413
 414Applications can override this setting individually with the PR_MCE_KILL prctl
 415
 416==============================================================
 417
 418memory_failure_recovery
 419
 420Enable memory failure recovery (when supported by the platform)
 421
 4221: Attempt recovery.
 423
 4240: Always panic on a memory failure.
 425
 426==============================================================
 427
 428min_free_kbytes:
 429
 430This is used to force the Linux VM to keep a minimum number
 431of kilobytes free.  The VM uses this number to compute a
 432watermark[WMARK_MIN] value for each lowmem zone in the system.
 433Each lowmem zone gets a number of reserved free pages based
 434proportionally on its size.
 435
 436Some minimal amount of memory is needed to satisfy PF_MEMALLOC
 437allocations; if you set this to lower than 1024KB, your system will
 438become subtly broken, and prone to deadlock under high loads.
 439
 440Setting this too high will OOM your machine instantly.
 441
 442=============================================================
 443
 444min_slab_ratio:
 445
 446This is available only on NUMA kernels.
 447
 448A percentage of the total pages in each zone.  On Zone reclaim
 449(fallback from the local zone occurs) slabs will be reclaimed if more
 450than this percentage of pages in a zone are reclaimable slab pages.
 451This insures that the slab growth stays under control even in NUMA
 452systems that rarely perform global reclaim.
 453
 454The default is 5 percent.
 455
 456Note that slab reclaim is triggered in a per zone / node fashion.
 457The process of reclaiming slab memory is currently not node specific
 458and may not be fast.
 459
 460=============================================================
 461
 462min_unmapped_ratio:
 463
 464This is available only on NUMA kernels.
 465
 466This is a percentage of the total pages in each zone. Zone reclaim will
 467only occur if more than this percentage of pages are in a state that
 468zone_reclaim_mode allows to be reclaimed.
 469
 470If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
 471against all file-backed unmapped pages including swapcache pages and tmpfs
 472files. Otherwise, only unmapped pages backed by normal files but not tmpfs
 473files and similar are considered.
 474
 475The default is 1 percent.
 476
 477==============================================================
 478
 479mmap_min_addr
 480
 481This file indicates the amount of address space  which a user process will
 482be restricted from mmapping.  Since kernel null dereference bugs could
 483accidentally operate based on the information in the first couple of pages
 484of memory userspace processes should not be allowed to write to them.  By
 485default this value is set to 0 and no protections will be enforced by the
 486security module.  Setting this value to something like 64k will allow the
 487vast majority of applications to work correctly and provide defense in depth
 488against future potential kernel bugs.
 489
 490==============================================================
 491
 492mmap_rnd_bits:
 493
 494This value can be used to select the number of bits to use to
 495determine the random offset to the base address of vma regions
 496resulting from mmap allocations on architectures which support
 497tuning address space randomization.  This value will be bounded
 498by the architecture's minimum and maximum supported values.
 499
 500This value can be changed after boot using the
 501/proc/sys/vm/mmap_rnd_bits tunable
 502
 503==============================================================
 504
 505mmap_rnd_compat_bits:
 506
 507This value can be used to select the number of bits to use to
 508determine the random offset to the base address of vma regions
 509resulting from mmap allocations for applications run in
 510compatibility mode on architectures which support tuning address
 511space randomization.  This value will be bounded by the
 512architecture's minimum and maximum supported values.
 513
 514This value can be changed after boot using the
 515/proc/sys/vm/mmap_rnd_compat_bits tunable
 516
 517==============================================================
 518
 519nr_hugepages
 520
 521Change the minimum size of the hugepage pool.
 522
 523See Documentation/vm/hugetlbpage.txt
 524
 525==============================================================
 526
 527nr_overcommit_hugepages
 528
 529Change the maximum size of the hugepage pool. The maximum is
 530nr_hugepages + nr_overcommit_hugepages.
 531
 532See Documentation/vm/hugetlbpage.txt
 533
 534==============================================================
 535
 536nr_trim_pages
 537
 538This is available only on NOMMU kernels.
 539
 540This value adjusts the excess page trimming behaviour of power-of-2 aligned
 541NOMMU mmap allocations.
 542
 543A value of 0 disables trimming of allocations entirely, while a value of 1
 544trims excess pages aggressively. Any value >= 1 acts as the watermark where
 545trimming of allocations is initiated.
 546
 547The default value is 1.
 548
 549See Documentation/nommu-mmap.txt for more information.
 550
 551==============================================================
 552
 553numa_zonelist_order
 554
 555This sysctl is only for NUMA.
 556'where the memory is allocated from' is controlled by zonelists.
 557(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
 558 you may be able to read ZONE_DMA as ZONE_DMA32...)
 559
 560In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
 561ZONE_NORMAL -> ZONE_DMA
 562This means that a memory allocation request for GFP_KERNEL will
 563get memory from ZONE_DMA only when ZONE_NORMAL is not available.
 564
 565In NUMA case, you can think of following 2 types of order.
 566Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
 567
 568(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
 569(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
 570
 571Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
 572will be used before ZONE_NORMAL exhaustion. This increases possibility of
 573out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
 574
 575Type(B) cannot offer the best locality but is more robust against OOM of
 576the DMA zone.
 577
 578Type(A) is called as "Node" order. Type (B) is "Zone" order.
 579
 580"Node order" orders the zonelists by node, then by zone within each node.
 581Specify "[Nn]ode" for node order
 582
 583"Zone Order" orders the zonelists by zone type, then by node within each
 584zone.  Specify "[Zz]one" for zone order.
 585
 586Specify "[Dd]efault" to request automatic configuration.
 587
 588On 32-bit, the Normal zone needs to be preserved for allocations accessible
 589by the kernel, so "zone" order will be selected.
 590
 591On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
 592order will be selected.
 593
 594Default order is recommended unless this is causing problems for your
 595system/application.
 596
 597==============================================================
 598
 599oom_dump_tasks
 600
 601Enables a system-wide task dump (excluding kernel threads) to be produced
 602when the kernel performs an OOM-killing and includes such information as
 603pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj
 604score, and name.  This is helpful to determine why the OOM killer was
 605invoked, to identify the rogue task that caused it, and to determine why
 606the OOM killer chose the task it did to kill.
 607
 608If this is set to zero, this information is suppressed.  On very
 609large systems with thousands of tasks it may not be feasible to dump
 610the memory state information for each one.  Such systems should not
 611be forced to incur a performance penalty in OOM conditions when the
 612information may not be desired.
 613
 614If this is set to non-zero, this information is shown whenever the
 615OOM killer actually kills a memory-hogging task.
 616
 617The default value is 1 (enabled).
 618
 619==============================================================
 620
 621oom_kill_allocating_task
 622
 623This enables or disables killing the OOM-triggering task in
 624out-of-memory situations.
 625
 626If this is set to zero, the OOM killer will scan through the entire
 627tasklist and select a task based on heuristics to kill.  This normally
 628selects a rogue memory-hogging task that frees up a large amount of
 629memory when killed.
 630
 631If this is set to non-zero, the OOM killer simply kills the task that
 632triggered the out-of-memory condition.  This avoids the expensive
 633tasklist scan.
 634
 635If panic_on_oom is selected, it takes precedence over whatever value
 636is used in oom_kill_allocating_task.
 637
 638The default value is 0.
 639
 640==============================================================
 641
 642overcommit_kbytes:
 643
 644When overcommit_memory is set to 2, the committed address space is not
 645permitted to exceed swap plus this amount of physical RAM. See below.
 646
 647Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
 648of them may be specified at a time. Setting one disables the other (which
 649then appears as 0 when read).
 650
 651==============================================================
 652
 653overcommit_memory:
 654
 655This value contains a flag that enables memory overcommitment.
 656
 657When this flag is 0, the kernel attempts to estimate the amount
 658of free memory left when userspace requests more memory.
 659
 660When this flag is 1, the kernel pretends there is always enough
 661memory until it actually runs out.
 662
 663When this flag is 2, the kernel uses a "never overcommit"
 664policy that attempts to prevent any overcommit of memory.
 665Note that user_reserve_kbytes affects this policy.
 666
 667This feature can be very useful because there are a lot of
 668programs that malloc() huge amounts of memory "just-in-case"
 669and don't use much of it.
 670
 671The default value is 0.
 672
 673See Documentation/vm/overcommit-accounting and
 674mm/mmap.c::__vm_enough_memory() for more information.
 675
 676==============================================================
 677
 678overcommit_ratio:
 679
 680When overcommit_memory is set to 2, the committed address
 681space is not permitted to exceed swap plus this percentage
 682of physical RAM.  See above.
 683
 684==============================================================
 685
 686page-cluster
 687
 688page-cluster controls the number of pages up to which consecutive pages
 689are read in from swap in a single attempt. This is the swap counterpart
 690to page cache readahead.
 691The mentioned consecutivity is not in terms of virtual/physical addresses,
 692but consecutive on swap space - that means they were swapped out together.
 693
 694It is a logarithmic value - setting it to zero means "1 page", setting
 695it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
 696Zero disables swap readahead completely.
 697
 698The default value is three (eight pages at a time).  There may be some
 699small benefits in tuning this to a different value if your workload is
 700swap-intensive.
 701
 702Lower values mean lower latencies for initial faults, but at the same time
 703extra faults and I/O delays for following faults if they would have been part of
 704that consecutive pages readahead would have brought in.
 705
 706=============================================================
 707
 708panic_on_oom
 709
 710This enables or disables panic on out-of-memory feature.
 711
 712If this is set to 0, the kernel will kill some rogue process,
 713called oom_killer.  Usually, oom_killer can kill rogue processes and
 714system will survive.
 715
 716If this is set to 1, the kernel panics when out-of-memory happens.
 717However, if a process limits using nodes by mempolicy/cpusets,
 718and those nodes become memory exhaustion status, one process
 719may be killed by oom-killer. No panic occurs in this case.
 720Because other nodes' memory may be free. This means system total status
 721may be not fatal yet.
 722
 723If this is set to 2, the kernel panics compulsorily even on the
 724above-mentioned. Even oom happens under memory cgroup, the whole
 725system panics.
 726
 727The default value is 0.
 7281 and 2 are for failover of clustering. Please select either
 729according to your policy of failover.
 730panic_on_oom=2+kdump gives you very strong tool to investigate
 731why oom happens. You can get snapshot.
 732
 733=============================================================
 734
 735percpu_pagelist_fraction
 736
 737This is the fraction of pages at most (high mark pcp->high) in each zone that
 738are allocated for each per cpu page list.  The min value for this is 8.  It
 739means that we don't allow more than 1/8th of pages in each zone to be
 740allocated in any single per_cpu_pagelist.  This entry only changes the value
 741of hot per cpu pagelists.  User can specify a number like 100 to allocate
 7421/100th of each zone to each per cpu page list.
 743
 744The batch value of each per cpu pagelist is also updated as a result.  It is
 745set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
 746
 747The initial value is zero.  Kernel does not use this value at boot time to set
 748the high water marks for each per cpu page list.  If the user writes '0' to this
 749sysctl, it will revert to this default behavior.
 750
 751==============================================================
 752
 753stat_interval
 754
 755The time interval between which vm statistics are updated.  The default
 756is 1 second.
 757
 758==============================================================
 759
 760stat_refresh
 761
 762Any read or write (by root only) flushes all the per-cpu vm statistics
 763into their global totals, for more accurate reports when testing
 764e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
 765
 766As a side-effect, it also checks for negative totals (elsewhere reported
 767as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
 768(At time of writing, a few stats are known sometimes to be found negative,
 769with no ill effects: errors and warnings on these stats are suppressed.)
 770
 771==============================================================
 772
 773swappiness
 774
 775This control is used to define how aggressive the kernel will swap
 776memory pages.  Higher values will increase agressiveness, lower values
 777decrease the amount of swap.  A value of 0 instructs the kernel not to
 778initiate swap until the amount of free and file-backed pages is less
 779than the high water mark in a zone.
 780
 781The default value is 60.
 782
 783==============================================================
 784
 785- user_reserve_kbytes
 786
 787When overcommit_memory is set to 2, "never overcommit" mode, reserve
 788min(3% of current process size, user_reserve_kbytes) of free memory.
 789This is intended to prevent a user from starting a single memory hogging
 790process, such that they cannot recover (kill the hog).
 791
 792user_reserve_kbytes defaults to min(3% of the current process size, 128MB).
 793
 794If this is reduced to zero, then the user will be allowed to allocate
 795all free memory with a single process, minus admin_reserve_kbytes.
 796Any subsequent attempts to execute a command will result in
 797"fork: Cannot allocate memory".
 798
 799Changing this takes effect whenever an application requests memory.
 800
 801==============================================================
 802
 803vfs_cache_pressure
 804------------------
 805
 806This percentage value controls the tendency of the kernel to reclaim
 807the memory which is used for caching of directory and inode objects.
 808
 809At the default value of vfs_cache_pressure=100 the kernel will attempt to
 810reclaim dentries and inodes at a "fair" rate with respect to pagecache and
 811swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
 812to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
 813never reclaim dentries and inodes due to memory pressure and this can easily
 814lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
 815causes the kernel to prefer to reclaim dentries and inodes.
 816
 817Increasing vfs_cache_pressure significantly beyond 100 may have negative
 818performance impact. Reclaim code needs to take various locks to find freeable
 819directory and inode objects. With vfs_cache_pressure=1000, it will look for
 820ten times more freeable objects than there are.
 821
 822=============================================================
 823
 824watermark_scale_factor:
 825
 826This factor controls the aggressiveness of kswapd. It defines the
 827amount of memory left in a node/system before kswapd is woken up and
 828how much memory needs to be free before kswapd goes back to sleep.
 829
 830The unit is in fractions of 10,000. The default value of 10 means the
 831distances between watermarks are 0.1% of the available memory in the
 832node/system. The maximum value is 1000, or 10% of memory.
 833
 834A high rate of threads entering direct reclaim (allocstall) or kswapd
 835going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
 836that the number of free pages kswapd maintains for latency reasons is
 837too small for the allocation bursts occurring in the system. This knob
 838can then be used to tune kswapd aggressiveness accordingly.
 839
 840==============================================================
 841
 842zone_reclaim_mode:
 843
 844Zone_reclaim_mode allows someone to set more or less aggressive approaches to
 845reclaim memory when a zone runs out of memory. If it is set to zero then no
 846zone reclaim occurs. Allocations will be satisfied from other zones / nodes
 847in the system.
 848
 849This is value ORed together of
 850
 8511       = Zone reclaim on
 8522       = Zone reclaim writes dirty pages out
 8534       = Zone reclaim swaps pages
 854
 855zone_reclaim_mode is disabled by default.  For file servers or workloads
 856that benefit from having their data cached, zone_reclaim_mode should be
 857left disabled as the caching effect is likely to be more important than
 858data locality.
 859
 860zone_reclaim may be enabled if it's known that the workload is partitioned
 861such that each partition fits within a NUMA node and that accessing remote
 862memory would cause a measurable performance reduction.  The page allocator
 863will then reclaim easily reusable pages (those page cache pages that are
 864currently not used) before allocating off node pages.
 865
 866Allowing zone reclaim to write out pages stops processes that are
 867writing large amounts of data from dirtying pages on other nodes. Zone
 868reclaim will write out dirty pages if a zone fills up and so effectively
 869throttle the process. This may decrease the performance of a single process
 870since it cannot use all of system memory to buffer the outgoing writes
 871anymore but it preserve the memory on other nodes so that the performance
 872of other processes running on other nodes will not be affected.
 873
 874Allowing regular swap effectively restricts allocations to the local
 875node unless explicitly overridden by memory policies or cpuset
 876configurations.
 877
 878============ End of Document =================================
 879
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