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- dirty_background_bytes 25- dirty_background_ratio 26- dirty_bytes 27- dirty_expire_centisecs 28- dirty_ratio 29- dirty_writeback_centisecs 30- drop_caches 31- extfrag_threshold 32- extra_free_kbytes 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_memory 54- overcommit_ratio 55- page-cluster 56- panic_on_oom 57- percpu_pagelist_fraction 58- stat_interval 59- swappiness 60- user_reserve_kbytes 61- vfs_cache_pressure 62- zone_reclaim_mode 63 64============================================================== 65 66admin_reserve_kbytes 67 68The amount of free memory in the system that should be reserved for users 69with the capability cap_sys_admin. 70 71admin_reserve_kbytes defaults to min(3% of free pages, 8MB) 72 73That should provide enough for the admin to log in and kill a process, 74if necessary, under the default overcommit 'guess' mode. 75 76Systems running under overcommit 'never' should increase this to account 77for the full Virtual Memory Size of programs used to recover. Otherwise, 78root may not be able to log in to recover the system. 79 80How do you calculate a minimum useful reserve? 81 82sshd or login + bash (or some other shell) + top (or ps, kill, etc.) 83 84For overcommit 'guess', we can sum resident set sizes (RSS). 85On x86_64 this is about 8MB. 86 87For overcommit 'never', we can take the max of their virtual sizes (VSZ) 88and add the sum of their RSS. 89On x86_64 this is about 128MB. 90 91Changing this takes effect whenever an application requests memory. 92 93============================================================== 94 95block_dump 96 97block_dump enables block I/O debugging when set to a nonzero value. More 98information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. 99 100============================================================== 101 102compact_memory 103 104Available only when CONFIG_COMPACTION is set. When 1 is written to the file, 105all zones are compacted such that free memory is available in contiguous 106blocks where possible. This can be important for example in the allocation of 107huge pages although processes will also directly compact memory as required. 108 109============================================================== 110 111dirty_background_bytes 112 113Contains the amount of dirty memory at which the background kernel 114flusher threads will start writeback. 115 116Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only 117one of them may be specified at a time. When one sysctl is written it is 118immediately taken into account to evaluate the dirty memory limits and the 119other appears as 0 when read. 120 121============================================================== 122 123dirty_background_ratio 124 125Contains, as a percentage of total system memory, the number of pages at which 126the background kernel flusher threads will start writing out dirty data. 127 128============================================================== 129 130dirty_bytes 131 132Contains the amount of dirty memory at which a process generating disk writes 133will itself start writeback. 134 135Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be 136specified at a time. When one sysctl is written it is immediately taken into 137account to evaluate the dirty memory limits and the other appears as 0 when 138read. 139 140Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any 141value lower than this limit will be ignored and the old configuration will be 142retained. 143 144============================================================== 145 146dirty_expire_centisecs 147 148This tunable is used to define when dirty data is old enough to be eligible 149for writeout by the kernel flusher threads. It is expressed in 100'ths 150of a second. Data which has been dirty in-memory for longer than this 151interval will be written out next time a flusher thread wakes up. 152 153============================================================== 154 155dirty_ratio 156 157Contains, as a percentage of total system memory, the number of pages at which 158a process which is generating disk writes will itself start writing out dirty 159data. 160 161============================================================== 162 163dirty_writeback_centisecs 164 165The kernel flusher threads will periodically wake up and write `old' data 166out to disk. This tunable expresses the interval between those wakeups, in 167100'ths of a second. 168 169Setting this to zero disables periodic writeback altogether. 170 171============================================================== 172 173drop_caches 174 175Writing to this will cause the kernel to drop clean caches, dentries and 176inodes from memory, causing that memory to become free. 177 178To free pagecache: 179 echo 1 > /proc/sys/vm/drop_caches 180To free dentries and inodes: 181 echo 2 > /proc/sys/vm/drop_caches 182To free pagecache, dentries and inodes: 183 echo 3 > /proc/sys/vm/drop_caches 184 185As this is a non-destructive operation and dirty objects are not freeable, the 186user should run `sync' first. 187 188============================================================== 189 190extfrag_threshold 191 192This parameter affects whether the kernel will compact memory or direct 193reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what 194the fragmentation index for each order is in each zone in the system. Values 195tending towards 0 imply allocations would fail due to lack of memory, 196values towards 1000 imply failures are due to fragmentation and -1 implies 197that the allocation will succeed as long as watermarks are met. 198 199The kernel will not compact memory in a zone if the 200fragmentation index is <= extfrag_threshold. The default value is 500. 201 202============================================================== 203 204extra_free_kbytes 205 206This parameter tells the VM to keep extra free memory between the threshold 207where background reclaim (kswapd) kicks in, and the threshold where direct 208reclaim (by allocating processes) kicks in. 209 210This is useful for workloads that require low latency memory allocations 211and have a bounded burstiness in memory allocations, for example a 212realtime application that receives and transmits network traffic 213(causing in-kernel memory allocations) with a maximum total message burst 214size of 200MB may need 200MB of extra free memory to avoid direct reclaim 215related latencies. 216 217============================================================== 218 219hugepages_treat_as_movable 220 221This parameter is only useful when kernelcore= is specified at boot time to 222create ZONE_MOVABLE for pages that may be reclaimed or migrated. Huge pages 223are not movable so are not normally allocated from ZONE_MOVABLE. A non-zero 224value written to hugepages_treat_as_movable allows huge pages to be allocated 225from ZONE_MOVABLE. 226 227Once enabled, the ZONE_MOVABLE is treated as an area of memory the huge 228pages pool can easily grow or shrink within. Assuming that applications are 229not running that mlock() a lot of memory, it is likely the huge pages pool 230can grow to the size of ZONE_MOVABLE by repeatedly entering the desired value 231into nr_hugepages and triggering page reclaim. 232 233============================================================== 234 235hugetlb_shm_group 236 237hugetlb_shm_group contains group id that is allowed to create SysV 238shared memory segment using hugetlb page. 239 240============================================================== 241 242laptop_mode 243 244laptop_mode is a knob that controls "laptop mode". All the things that are 245controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. 246 247============================================================== 248 249legacy_va_layout 250 251If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel 252will use the legacy (2.4) layout for all processes. 253 254============================================================== 255 256lowmem_reserve_ratio 257 258For some specialised workloads on highmem machines it is dangerous for 259the kernel to allow process memory to be allocated from the "lowmem" 260zone. This is because that memory could then be pinned via the mlock() 261system call, or by unavailability of swapspace. 262 263And on large highmem machines this lack of reclaimable lowmem memory 264can be fatal. 265 266So the Linux page allocator has a mechanism which prevents allocations 267which _could_ use highmem from using too much lowmem. This means that 268a certain amount of lowmem is defended from the possibility of being 269captured into pinned user memory. 270 271(The same argument applies to the old 16 megabyte ISA DMA region. This 272mechanism will also defend that region from allocations which could use 273highmem or lowmem). 274 275The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is 276in defending these lower zones. 277 278If you have a machine which uses highmem or ISA DMA and your 279applications are using mlock(), or if you are running with no swap then 280you probably should change the lowmem_reserve_ratio setting. 281 282The lowmem_reserve_ratio is an array. You can see them by reading this file. 283- 284% cat /proc/sys/vm/lowmem_reserve_ratio 285256 256 32 286- 287Note: # of this elements is one fewer than number of zones. Because the highest 288 zone's value is not necessary for following calculation. 289 290But, these values are not used directly. The kernel calculates # of protection 291pages for each zones from them. These are shown as array of protection pages 292in /proc/zoneinfo like followings. (This is an example of x86-64 box). 293Each zone has an array of protection pages like this. 294 295- 296Node 0, zone DMA 297 pages free 1355 298 min 3 299 low 3 300 high 4 301 : 302 : 303 numa_other 0 304 protection: (0, 2004, 2004, 2004) 305 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 306 pagesets 307 cpu: 0 pcp: 0 308 : 309- 310These protections are added to score to judge whether this zone should be used 311for page allocation or should be reclaimed. 312 313In this example, if normal pages (index=2) are required to this DMA zone and 314watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should 315not be used because pages_free(1355) is smaller than watermark + protection[2] 316(4 + 2004 = 2008). If this protection value is 0, this zone would be used for 317normal page requirement. If requirement is DMA zone(index=0), protection[0] 318(=0) is used. 319 320zone[i]'s protection[j] is calculated by following expression. 321 322(i < j): 323 zone[i]->protection[j] 324 = (total sums of present_pages from zone[i+1] to zone[j] on the node) 325 / lowmem_reserve_ratio[i]; 326(i = j): 327 (should not be protected. = 0; 328(i > j): 329 (not necessary, but looks 0) 330 331The default values of lowmem_reserve_ratio[i] are 332 256 (if zone[i] means DMA or DMA32 zone) 333 32 (others). 334As above expression, they are reciprocal number of ratio. 335256 means 1/256. # of protection pages becomes about "0.39%" of total present 336pages of higher zones on the node. 337 338If you would like to protect more pages, smaller values are effective. 339The minimum value is 1 (1/1 -> 100%). 340 341============================================================== 342 343max_map_count: 344 345This file contains the maximum number of memory map areas a process 346may have. Memory map areas are used as a side-effect of calling 347malloc, directly by mmap and mprotect, and also when loading shared 348libraries. 349 350While most applications need less than a thousand maps, certain 351programs, particularly malloc debuggers, may consume lots of them, 352e.g., up to one or two maps per allocation. 353 354The default value is 65536. 355 356============================================================= 357 358memory_failure_early_kill: 359 360Control how to kill processes when uncorrected memory error (typically 361a 2bit error in a memory module) is detected in the background by hardware 362that cannot be handled by the kernel. In some cases (like the page 363still having a valid copy on disk) the kernel will handle the failure 364transparently without affecting any applications. But if there is 365no other uptodate copy of the data it will kill to prevent any data 366corruptions from propagating. 367 3681: Kill all processes that have the corrupted and not reloadable page mapped 369as soon as the corruption is detected. Note this is not supported 370for a few types of pages, like kernel internally allocated data or 371the swap cache, but works for the majority of user pages. 372 3730: Only unmap the corrupted page from all processes and only kill a process 374who tries to access it. 375 376The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can 377handle this if they want to. 378 379This is only active on architectures/platforms with advanced machine 380check handling and depends on the hardware capabilities. 381 382Applications can override this setting individually with the PR_MCE_KILL prctl 383 384============================================================== 385 386memory_failure_recovery 387 388Enable memory failure recovery (when supported by the platform) 389 3901: Attempt recovery. 391 3920: Always panic on a memory failure. 393 394============================================================== 395 396min_free_kbytes: 397 398This is used to force the Linux VM to keep a minimum number 399of kilobytes free. The VM uses this number to compute a 400watermark[WMARK_MIN] value for each lowmem zone in the system. 401Each lowmem zone gets a number of reserved free pages based 402proportionally on its size. 403 404Some minimal amount of memory is needed to satisfy PF_MEMALLOC 405allocations; if you set this to lower than 1024KB, your system will 406become subtly broken, and prone to deadlock under high loads. 407 408Setting this too high will OOM your machine instantly. 409 410============================================================= 411 412min_slab_ratio: 413 414This is available only on NUMA kernels. 415 416A percentage of the total pages in each zone. On Zone reclaim 417(fallback from the local zone occurs) slabs will be reclaimed if more 418than this percentage of pages in a zone are reclaimable slab pages. 419This insures that the slab growth stays under control even in NUMA 420systems that rarely perform global reclaim. 421 422The default is 5 percent. 423 424Note that slab reclaim is triggered in a per zone / node fashion. 425The process of reclaiming slab memory is currently not node specific 426and may not be fast. 427 428============================================================= 429 430min_unmapped_ratio: 431 432This is available only on NUMA kernels. 433 434This is a percentage of the total pages in each zone. Zone reclaim will 435only occur if more than this percentage of pages are in a state that 436zone_reclaim_mode allows to be reclaimed. 437 438If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared 439against all file-backed unmapped pages including swapcache pages and tmpfs 440files. Otherwise, only unmapped pages backed by normal files but not tmpfs 441files and similar are considered. 442 443The default is 1 percent. 444 445============================================================== 446 447mmap_min_addr 448 449This file indicates the amount of address space which a user process will 450be restricted from mmapping. Since kernel null dereference bugs could 451accidentally operate based on the information in the first couple of pages 452of memory userspace processes should not be allowed to write to them. By 453default this value is set to 0 and no protections will be enforced by the 454security module. Setting this value to something like 64k will allow the 455vast majority of applications to work correctly and provide defense in depth 456against future potential kernel bugs. 457 458============================================================== 459 460mmap_rnd_bits: 461 462This value can be used to select the number of bits to use to 463determine the random offset to the base address of vma regions 464resulting from mmap allocations on architectures which support 465tuning address space randomization. This value will be bounded 466by the architecture's minimum and maximum supported values. 467 468This value can be changed after boot using the 469/proc/sys/vm/mmap_rnd_bits tunable 470 471============================================================== 472 473mmap_rnd_compat_bits: 474 475This value can be used to select the number of bits to use to 476determine the random offset to the base address of vma regions 477resulting from mmap allocations for applications run in 478compatibility mode on architectures which support tuning address 479space randomization. This value will be bounded by the 480architecture's minimum and maximum supported values. 481 482This value can be changed after boot using the 483/proc/sys/vm/mmap_rnd_compat_bits tunable 484 485============================================================== 486 487nr_hugepages 488 489Change the minimum size of the hugepage pool. 490 491See Documentation/vm/hugetlbpage.txt 492 493============================================================== 494 495nr_overcommit_hugepages 496 497Change the maximum size of the hugepage pool. The maximum is 498nr_hugepages + nr_overcommit_hugepages. 499 500See Documentation/vm/hugetlbpage.txt 501 502============================================================== 503 504nr_trim_pages 505 506This is available only on NOMMU kernels. 507 508This value adjusts the excess page trimming behaviour of power-of-2 aligned 509NOMMU mmap allocations. 510 511A value of 0 disables trimming of allocations entirely, while a value of 1 512trims excess pages aggressively. Any value >= 1 acts as the watermark where 513trimming of allocations is initiated. 514 515The default value is 1. 516 517See Documentation/nommu-mmap.txt for more information. 518 519============================================================== 520 521numa_zonelist_order 522 523This sysctl is only for NUMA. 524'where the memory is allocated from' is controlled by zonelists. 525(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. 526 you may be able to read ZONE_DMA as ZONE_DMA32...) 527 528In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. 529ZONE_NORMAL -> ZONE_DMA 530This means that a memory allocation request for GFP_KERNEL will 531get memory from ZONE_DMA only when ZONE_NORMAL is not available. 532 533In NUMA case, you can think of following 2 types of order. 534Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL 535 536(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL 537(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. 538 539Type(A) offers the best locality for processes on Node(0), but ZONE_DMA 540will be used before ZONE_NORMAL exhaustion. This increases possibility of 541out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. 542 543Type(B) cannot offer the best locality but is more robust against OOM of 544the DMA zone. 545 546Type(A) is called as "Node" order. Type (B) is "Zone" order. 547 548"Node order" orders the zonelists by node, then by zone within each node. 549Specify "[Nn]ode" for node order 550 551"Zone Order" orders the zonelists by zone type, then by node within each 552zone. Specify "[Zz]one" for zone order. 553 554Specify "[Dd]efault" to request automatic configuration. Autoconfiguration 555will select "node" order in following case. 556(1) if the DMA zone does not exist or 557(2) if the DMA zone comprises greater than 50% of the available memory or 558(3) if any node's DMA zone comprises greater than 60% of its local memory and 559 the amount of local memory is big enough. 560 561Otherwise, "zone" order will be selected. Default order is recommended unless 562this is causing problems for your system/application. 563 564============================================================== 565 566oom_dump_tasks 567 568Enables a system-wide task dump (excluding kernel threads) to be 569produced when the kernel performs an OOM-killing and includes such 570information as pid, uid, tgid, vm size, rss, nr_ptes, swapents, 571oom_score_adj score, and name. This is helpful to determine why the 572OOM killer was invoked, to identify the rogue task that caused it, 573and to determine why the OOM killer chose the task it did to kill. 574 575If this is set to zero, this information is suppressed. On very 576large systems with thousands of tasks it may not be feasible to dump 577the memory state information for each one. Such systems should not 578be forced to incur a performance penalty in OOM conditions when the 579information may not be desired. 580 581If this is set to non-zero, this information is shown whenever the 582OOM killer actually kills a memory-hogging task. 583 584The default value is 1 (enabled). 585 586============================================================== 587 588oom_kill_allocating_task 589 590This enables or disables killing the OOM-triggering task in 591out-of-memory situations. 592 593If this is set to zero, the OOM killer will scan through the entire 594tasklist and select a task based on heuristics to kill. This normally 595selects a rogue memory-hogging task that frees up a large amount of 596memory when killed. 597 598If this is set to non-zero, the OOM killer simply kills the task that 599triggered the out-of-memory condition. This avoids the expensive 600tasklist scan. 601 602If panic_on_oom is selected, it takes precedence over whatever value 603is used in oom_kill_allocating_task. 604 605The default value is 0. 606 607============================================================== 608 609overcommit_memory: 610 611This value contains a flag that enables memory overcommitment. 612 613When this flag is 0, the kernel attempts to estimate the amount 614of free memory left when userspace requests more memory. 615 616When this flag is 1, the kernel pretends there is always enough 617memory until it actually runs out. 618 619When this flag is 2, the kernel uses a "never overcommit" 620policy that attempts to prevent any overcommit of memory. 621Note that user_reserve_kbytes affects this policy. 622 623This feature can be very useful because there are a lot of 624programs that malloc() huge amounts of memory "just-in-case" 625and don't use much of it. 626 627The default value is 0. 628 629See Documentation/vm/overcommit-accounting and 630security/commoncap.c::cap_vm_enough_memory() for more information. 631 632============================================================== 633 634overcommit_ratio: 635 636When overcommit_memory is set to 2, the committed address 637space is not permitted to exceed swap plus this percentage 638of physical RAM. See above. 639 640============================================================== 641 642page-cluster 643 644page-cluster controls the number of pages up to which consecutive pages 645are read in from swap in a single attempt. This is the swap counterpart 646to page cache readahead. 647The mentioned consecutivity is not in terms of virtual/physical addresses, 648but consecutive on swap space - that means they were swapped out together. 649 650It is a logarithmic value - setting it to zero means "1 page", setting 651it to 1 means "2 pages", setting it to 2 means "4 pages", etc. 652Zero disables swap readahead completely. 653 654The default value is three (eight pages at a time). There may be some 655small benefits in tuning this to a different value if your workload is 656swap-intensive. 657 658Lower values mean lower latencies for initial faults, but at the same time 659extra faults and I/O delays for following faults if they would have been part of 660that consecutive pages readahead would have brought in. 661 662============================================================= 663 664panic_on_oom 665 666This enables or disables panic on out-of-memory feature. 667 668If this is set to 0, the kernel will kill some rogue process, 669called oom_killer. Usually, oom_killer can kill rogue processes and 670system will survive. 671 672If this is set to 1, the kernel panics when out-of-memory happens. 673However, if a process limits using nodes by mempolicy/cpusets, 674and those nodes become memory exhaustion status, one process 675may be killed by oom-killer. No panic occurs in this case. 676Because other nodes' memory may be free. This means system total status 677may be not fatal yet. 678 679If this is set to 2, the kernel panics compulsorily even on the 680above-mentioned. Even oom happens under memory cgroup, the whole 681system panics. 682 683The default value is 0. 6841 and 2 are for failover of clustering. Please select either 685according to your policy of failover. 686panic_on_oom=2+kdump gives you very strong tool to investigate 687why oom happens. You can get snapshot. 688 689============================================================= 690 691percpu_pagelist_fraction 692 693This is the fraction of pages at most (high mark pcp->high) in each zone that 694are allocated for each per cpu page list. The min value for this is 8. It 695means that we don't allow more than 1/8th of pages in each zone to be 696allocated in any single per_cpu_pagelist. This entry only changes the value 697of hot per cpu pagelists. User can specify a number like 100 to allocate 6981/100th of each zone to each per cpu page list. 699 700The batch value of each per cpu pagelist is also updated as a result. It is 701set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) 702 703The initial value is zero. Kernel does not use this value at boot time to set 704the high water marks for each per cpu page list. 705 706============================================================== 707 708stat_interval 709 710The time interval between which vm statistics are updated. The default 711is 1 second. 712 713============================================================== 714 715swappiness 716 717This control is used to define how aggressive the kernel will swap 718memory pages. Higher values will increase agressiveness, lower values 719decrease the amount of swap. 720 721The default value is 60. 722 723============================================================== 724 725- user_reserve_kbytes 726 727When overcommit_memory is set to 2, "never overommit" mode, reserve 728min(3% of current process size, user_reserve_kbytes) of free memory. 729This is intended to prevent a user from starting a single memory hogging 730process, such that they cannot recover (kill the hog). 731 732user_reserve_kbytes defaults to min(3% of the current process size, 128MB). 733 734If this is reduced to zero, then the user will be allowed to allocate 735all free memory with a single process, minus admin_reserve_kbytes. 736Any subsequent attempts to execute a command will result in 737"fork: Cannot allocate memory". 738 739Changing this takes effect whenever an application requests memory. 740 741============================================================== 742 743vfs_cache_pressure 744------------------ 745 746Controls the tendency of the kernel to reclaim the memory which is used for 747caching of directory and inode objects. 748 749At the default value of vfs_cache_pressure=100 the kernel will attempt to 750reclaim dentries and inodes at a "fair" rate with respect to pagecache and 751swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer 752to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will 753never reclaim dentries and inodes due to memory pressure and this can easily 754lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 755causes the kernel to prefer to reclaim dentries and inodes. 756 757============================================================== 758 759zone_reclaim_mode: 760 761Zone_reclaim_mode allows someone to set more or less aggressive approaches to 762reclaim memory when a zone runs out of memory. If it is set to zero then no 763zone reclaim occurs. Allocations will be satisfied from other zones / nodes 764in the system. 765 766This is value ORed together of 767 7681 = Zone reclaim on 7692 = Zone reclaim writes dirty pages out 7704 = Zone reclaim swaps pages 771 772zone_reclaim_mode is set during bootup to 1 if it is determined that pages 773from remote zones will cause a measurable performance reduction. The 774page allocator will then reclaim easily reusable pages (those page 775cache pages that are currently not used) before allocating off node pages. 776 777It may be beneficial to switch off zone reclaim if the system is 778used for a file server and all of memory should be used for caching files 779from disk. In that case the caching effect is more important than 780data locality. 781 782Allowing zone reclaim to write out pages stops processes that are 783writing large amounts of data from dirtying pages on other nodes. Zone 784reclaim will write out dirty pages if a zone fills up and so effectively 785throttle the process. This may decrease the performance of a single process 786since it cannot use all of system memory to buffer the outgoing writes 787anymore but it preserve the memory on other nodes so that the performance 788of other processes running on other nodes will not be affected. 789 790Allowing regular swap effectively restricts allocations to the local 791node unless explicitly overridden by memory policies or cpuset 792configurations. 793 794============ End of Document ================================= 795