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diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt new file mode 100644 index 000000000..9832ec52f --- /dev/null +++ b/Documentation/sysctl/vm.txt @@ -0,0 +1,815 @@ +Documentation for /proc/sys/vm/* kernel version 2.6.29 + (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> + (c) 2008 Peter W. Morreale <pmorreale@novell.com> + +For general info and legal blurb, please look in README. + +============================================================== + +This file contains the documentation for the sysctl files in +/proc/sys/vm and is valid for Linux kernel version 2.6.29. + +The files in this directory can be used to tune the operation +of the virtual memory (VM) subsystem of the Linux kernel and +the writeout of dirty data to disk. + +Default values and initialization routines for most of these +files can be found in mm/swap.c. + +Currently, these files are in /proc/sys/vm: + +- admin_reserve_kbytes +- block_dump +- compact_memory +- compact_unevictable_allowed +- dirty_background_bytes +- dirty_background_ratio +- dirty_bytes +- dirty_expire_centisecs +- dirty_ratio +- dirty_writeback_centisecs +- drop_caches +- extfrag_threshold +- hugepages_treat_as_movable +- hugetlb_shm_group +- laptop_mode +- legacy_va_layout +- lowmem_reserve_ratio +- max_map_count +- memory_failure_early_kill +- memory_failure_recovery +- min_free_kbytes +- min_slab_ratio +- min_unmapped_ratio +- mmap_min_addr +- nr_hugepages +- nr_overcommit_hugepages +- nr_trim_pages (only if CONFIG_MMU=n) +- numa_zonelist_order +- oom_dump_tasks +- oom_kill_allocating_task +- overcommit_kbytes +- overcommit_memory +- overcommit_ratio +- page-cluster +- panic_on_oom +- percpu_pagelist_fraction +- stat_interval +- swappiness +- user_reserve_kbytes +- vfs_cache_pressure +- zone_reclaim_mode + +============================================================== + +admin_reserve_kbytes + +The amount of free memory in the system that should be reserved for users +with the capability cap_sys_admin. + +admin_reserve_kbytes defaults to min(3% of free pages, 8MB) + +That should provide enough for the admin to log in and kill a process, +if necessary, under the default overcommit 'guess' mode. + +Systems running under overcommit 'never' should increase this to account +for the full Virtual Memory Size of programs used to recover. Otherwise, +root may not be able to log in to recover the system. + +How do you calculate a minimum useful reserve? + +sshd or login + bash (or some other shell) + top (or ps, kill, etc.) + +For overcommit 'guess', we can sum resident set sizes (RSS). +On x86_64 this is about 8MB. + +For overcommit 'never', we can take the max of their virtual sizes (VSZ) +and add the sum of their RSS. +On x86_64 this is about 128MB. + +Changing this takes effect whenever an application requests memory. + +============================================================== + +block_dump + +block_dump enables block I/O debugging when set to a nonzero value. More +information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. + +============================================================== + +compact_memory + +Available only when CONFIG_COMPACTION is set. When 1 is written to the file, +all zones are compacted such that free memory is available in contiguous +blocks where possible. This can be important for example in the allocation of +huge pages although processes will also directly compact memory as required. + +============================================================== + +compact_unevictable_allowed + +Available only when CONFIG_COMPACTION is set. When set to 1, compaction is +allowed to examine the unevictable lru (mlocked pages) for pages to compact. +This should be used on systems where stalls for minor page faults are an +acceptable trade for large contiguous free memory. Set to 0 to prevent +compaction from moving pages that are unevictable. Default value is 1. + +============================================================== + +dirty_background_bytes + +Contains the amount of dirty memory at which the background kernel +flusher threads will start writeback. + +Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only +one of them may be specified at a time. When one sysctl is written it is +immediately taken into account to evaluate the dirty memory limits and the +other appears as 0 when read. + +============================================================== + +dirty_background_ratio + +Contains, as a percentage of total available memory that contains free pages +and reclaimable pages, the number of pages at which the background kernel +flusher threads will start writing out dirty data. + +The total avaiable memory is not equal to total system memory. + +============================================================== + +dirty_bytes + +Contains the amount of dirty memory at which a process generating disk writes +will itself start writeback. + +Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be +specified at a time. When one sysctl is written it is immediately taken into +account to evaluate the dirty memory limits and the other appears as 0 when +read. + +Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any +value lower than this limit will be ignored and the old configuration will be +retained. + +============================================================== + +dirty_expire_centisecs + +This tunable is used to define when dirty data is old enough to be eligible +for writeout by the kernel flusher threads. It is expressed in 100'ths +of a second. Data which has been dirty in-memory for longer than this +interval will be written out next time a flusher thread wakes up. + +============================================================== + +dirty_ratio + +Contains, as a percentage of total available memory that contains free pages +and reclaimable pages, the number of pages at which a process which is +generating disk writes will itself start writing out dirty data. + +The total avaiable memory is not equal to total system memory. + +============================================================== + +dirty_writeback_centisecs + +The kernel flusher threads will periodically wake up and write `old' data +out to disk. This tunable expresses the interval between those wakeups, in +100'ths of a second. + +Setting this to zero disables periodic writeback altogether. + +============================================================== + +drop_caches + +Writing to this will cause the kernel to drop clean caches, as well as +reclaimable slab objects like dentries and inodes. Once dropped, their +memory becomes free. + +To free pagecache: + echo 1 > /proc/sys/vm/drop_caches +To free reclaimable slab objects (includes dentries and inodes): + echo 2 > /proc/sys/vm/drop_caches +To free slab objects and pagecache: + echo 3 > /proc/sys/vm/drop_caches + +This is a non-destructive operation and will not free any dirty objects. +To increase the number of objects freed by this operation, the user may run +`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the +number of dirty objects on the system and create more candidates to be +dropped. + +This file is not a means to control the growth of the various kernel caches +(inodes, dentries, pagecache, etc...) These objects are automatically +reclaimed by the kernel when memory is needed elsewhere on the system. + +Use of this file can cause performance problems. Since it discards cached +objects, it may cost a significant amount of I/O and CPU to recreate the +dropped objects, especially if they were under heavy use. Because of this, +use outside of a testing or debugging environment is not recommended. + +You may see informational messages in your kernel log when this file is +used: + + cat (1234): drop_caches: 3 + +These are informational only. They do not mean that anything is wrong +with your system. To disable them, echo 4 (bit 3) into drop_caches. + +============================================================== + +extfrag_threshold + +This parameter affects whether the kernel will compact memory or direct +reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what +the fragmentation index for each order is in each zone in the system. Values +tending towards 0 imply allocations would fail due to lack of memory, +values towards 1000 imply failures are due to fragmentation and -1 implies +that the allocation will succeed as long as watermarks are met. + +The kernel will not compact memory in a zone if the +fragmentation index is <= extfrag_threshold. The default value is 500. + +============================================================== + +hugepages_treat_as_movable + +This parameter controls whether we can allocate hugepages from ZONE_MOVABLE +or not. If set to non-zero, hugepages can be allocated from ZONE_MOVABLE. +ZONE_MOVABLE is created when kernel boot parameter kernelcore= is specified, +so this parameter has no effect if used without kernelcore=. + +Hugepage migration is now available in some situations which depend on the +architecture and/or the hugepage size. If a hugepage supports migration, +allocation from ZONE_MOVABLE is always enabled for the hugepage regardless +of the value of this parameter. +IOW, this parameter affects only non-migratable hugepages. + +Assuming that hugepages are not migratable in your system, one usecase of +this parameter is that users can make hugepage pool more extensible by +enabling the allocation from ZONE_MOVABLE. This is because on ZONE_MOVABLE +page reclaim/migration/compaction work more and you can get contiguous +memory more likely. Note that using ZONE_MOVABLE for non-migratable +hugepages can do harm to other features like memory hotremove (because +memory hotremove expects that memory blocks on ZONE_MOVABLE are always +removable,) so it's a trade-off responsible for the users. + +============================================================== + +hugetlb_shm_group + +hugetlb_shm_group contains group id that is allowed to create SysV +shared memory segment using hugetlb page. + +============================================================== + +laptop_mode + +laptop_mode is a knob that controls "laptop mode". All the things that are +controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. + +============================================================== + +legacy_va_layout + +If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel +will use the legacy (2.4) layout for all processes. + +============================================================== + +lowmem_reserve_ratio + +For some specialised workloads on highmem machines it is dangerous for +the kernel to allow process memory to be allocated from the "lowmem" +zone. This is because that memory could then be pinned via the mlock() +system call, or by unavailability of swapspace. + +And on large highmem machines this lack of reclaimable lowmem memory +can be fatal. + +So the Linux page allocator has a mechanism which prevents allocations +which _could_ use highmem from using too much lowmem. This means that +a certain amount of lowmem is defended from the possibility of being +captured into pinned user memory. + +(The same argument applies to the old 16 megabyte ISA DMA region. This +mechanism will also defend that region from allocations which could use +highmem or lowmem). + +The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is +in defending these lower zones. + +If you have a machine which uses highmem or ISA DMA and your +applications are using mlock(), or if you are running with no swap then +you probably should change the lowmem_reserve_ratio setting. + +The lowmem_reserve_ratio is an array. You can see them by reading this file. +- +% cat /proc/sys/vm/lowmem_reserve_ratio +256 256 32 +- +Note: # of this elements is one fewer than number of zones. Because the highest + zone's value is not necessary for following calculation. + +But, these values are not used directly. The kernel calculates # of protection +pages for each zones from them. These are shown as array of protection pages +in /proc/zoneinfo like followings. (This is an example of x86-64 box). +Each zone has an array of protection pages like this. + +- +Node 0, zone DMA + pages free 1355 + min 3 + low 3 + high 4 + : + : + numa_other 0 + protection: (0, 2004, 2004, 2004) + ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + pagesets + cpu: 0 pcp: 0 + : +- +These protections are added to score to judge whether this zone should be used +for page allocation or should be reclaimed. + +In this example, if normal pages (index=2) are required to this DMA zone and +watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should +not be used because pages_free(1355) is smaller than watermark + protection[2] +(4 + 2004 = 2008). If this protection value is 0, this zone would be used for +normal page requirement. If requirement is DMA zone(index=0), protection[0] +(=0) is used. + +zone[i]'s protection[j] is calculated by following expression. + +(i < j): + zone[i]->protection[j] + = (total sums of present_pages from zone[i+1] to zone[j] on the node) + / lowmem_reserve_ratio[i]; +(i = j): + (should not be protected. = 0; +(i > j): + (not necessary, but looks 0) + +The default values of lowmem_reserve_ratio[i] are + 256 (if zone[i] means DMA or DMA32 zone) + 32 (others). +As above expression, they are reciprocal number of ratio. +256 means 1/256. # of protection pages becomes about "0.39%" of total present +pages of higher zones on the node. + +If you would like to protect more pages, smaller values are effective. +The minimum value is 1 (1/1 -> 100%). + +============================================================== + +max_map_count: + +This file contains the maximum number of memory map areas a process +may have. Memory map areas are used as a side-effect of calling +malloc, directly by mmap and mprotect, and also when loading shared +libraries. + +While most applications need less than a thousand maps, certain +programs, particularly malloc debuggers, may consume lots of them, +e.g., up to one or two maps per allocation. + +The default value is 65536. + +============================================================= + +memory_failure_early_kill: + +Control how to kill processes when uncorrected memory error (typically +a 2bit error in a memory module) is detected in the background by hardware +that cannot be handled by the kernel. In some cases (like the page +still having a valid copy on disk) the kernel will handle the failure +transparently without affecting any applications. But if there is +no other uptodate copy of the data it will kill to prevent any data +corruptions from propagating. + +1: Kill all processes that have the corrupted and not reloadable page mapped +as soon as the corruption is detected. Note this is not supported +for a few types of pages, like kernel internally allocated data or +the swap cache, but works for the majority of user pages. + +0: Only unmap the corrupted page from all processes and only kill a process +who tries to access it. + +The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can +handle this if they want to. + +This is only active on architectures/platforms with advanced machine +check handling and depends on the hardware capabilities. + +Applications can override this setting individually with the PR_MCE_KILL prctl + +============================================================== + +memory_failure_recovery + +Enable memory failure recovery (when supported by the platform) + +1: Attempt recovery. + +0: Always panic on a memory failure. + +============================================================== + +min_free_kbytes: + +This is used to force the Linux VM to keep a minimum number +of kilobytes free. The VM uses this number to compute a +watermark[WMARK_MIN] value for each lowmem zone in the system. +Each lowmem zone gets a number of reserved free pages based +proportionally on its size. + +Some minimal amount of memory is needed to satisfy PF_MEMALLOC +allocations; if you set this to lower than 1024KB, your system will +become subtly broken, and prone to deadlock under high loads. + +Setting this too high will OOM your machine instantly. + +============================================================= + +min_slab_ratio: + +This is available only on NUMA kernels. + +A percentage of the total pages in each zone. On Zone reclaim +(fallback from the local zone occurs) slabs will be reclaimed if more +than this percentage of pages in a zone are reclaimable slab pages. +This insures that the slab growth stays under control even in NUMA +systems that rarely perform global reclaim. + +The default is 5 percent. + +Note that slab reclaim is triggered in a per zone / node fashion. +The process of reclaiming slab memory is currently not node specific +and may not be fast. + +============================================================= + +min_unmapped_ratio: + +This is available only on NUMA kernels. + +This is a percentage of the total pages in each zone. Zone reclaim will +only occur if more than this percentage of pages are in a state that +zone_reclaim_mode allows to be reclaimed. + +If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared +against all file-backed unmapped pages including swapcache pages and tmpfs +files. Otherwise, only unmapped pages backed by normal files but not tmpfs +files and similar are considered. + +The default is 1 percent. + +============================================================== + +mmap_min_addr + +This file indicates the amount of address space which a user process will +be restricted from mmapping. Since kernel null dereference bugs could +accidentally operate based on the information in the first couple of pages +of memory userspace processes should not be allowed to write to them. By +default this value is set to 0 and no protections will be enforced by the +security module. Setting this value to something like 64k will allow the +vast majority of applications to work correctly and provide defense in depth +against future potential kernel bugs. + +============================================================== + +nr_hugepages + +Change the minimum size of the hugepage pool. + +See Documentation/vm/hugetlbpage.txt + +============================================================== + +nr_overcommit_hugepages + +Change the maximum size of the hugepage pool. The maximum is +nr_hugepages + nr_overcommit_hugepages. + +See Documentation/vm/hugetlbpage.txt + +============================================================== + +nr_trim_pages + +This is available only on NOMMU kernels. + +This value adjusts the excess page trimming behaviour of power-of-2 aligned +NOMMU mmap allocations. + +A value of 0 disables trimming of allocations entirely, while a value of 1 +trims excess pages aggressively. Any value >= 1 acts as the watermark where +trimming of allocations is initiated. + +The default value is 1. + +See Documentation/nommu-mmap.txt for more information. + +============================================================== + +numa_zonelist_order + +This sysctl is only for NUMA. +'where the memory is allocated from' is controlled by zonelists. +(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation. + you may be able to read ZONE_DMA as ZONE_DMA32...) + +In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following. +ZONE_NORMAL -> ZONE_DMA +This means that a memory allocation request for GFP_KERNEL will +get memory from ZONE_DMA only when ZONE_NORMAL is not available. + +In NUMA case, you can think of following 2 types of order. +Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL + +(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL +(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA. + +Type(A) offers the best locality for processes on Node(0), but ZONE_DMA +will be used before ZONE_NORMAL exhaustion. This increases possibility of +out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small. + +Type(B) cannot offer the best locality but is more robust against OOM of +the DMA zone. + +Type(A) is called as "Node" order. Type (B) is "Zone" order. + +"Node order" orders the zonelists by node, then by zone within each node. +Specify "[Nn]ode" for node order + +"Zone Order" orders the zonelists by zone type, then by node within each +zone. Specify "[Zz]one" for zone order. + +Specify "[Dd]efault" to request automatic configuration. Autoconfiguration +will select "node" order in following case. +(1) if the DMA zone does not exist or +(2) if the DMA zone comprises greater than 50% of the available memory or +(3) if any node's DMA zone comprises greater than 70% of its local memory and + the amount of local memory is big enough. + +Otherwise, "zone" order will be selected. Default order is recommended unless +this is causing problems for your system/application. + +============================================================== + +oom_dump_tasks + +Enables a system-wide task dump (excluding kernel threads) to be produced +when the kernel performs an OOM-killing and includes such information as +pid, uid, tgid, vm size, rss, nr_ptes, nr_pmds, swapents, oom_score_adj +score, and name. This is helpful to determine why the OOM killer was +invoked, to identify the rogue task that caused it, and to determine why +the OOM killer chose the task it did to kill. + +If this is set to zero, this information is suppressed. On very +large systems with thousands of tasks it may not be feasible to dump +the memory state information for each one. Such systems should not +be forced to incur a performance penalty in OOM conditions when the +information may not be desired. + +If this is set to non-zero, this information is shown whenever the +OOM killer actually kills a memory-hogging task. + +The default value is 1 (enabled). + +============================================================== + +oom_kill_allocating_task + +This enables or disables killing the OOM-triggering task in +out-of-memory situations. + +If this is set to zero, the OOM killer will scan through the entire +tasklist and select a task based on heuristics to kill. This normally +selects a rogue memory-hogging task that frees up a large amount of +memory when killed. + +If this is set to non-zero, the OOM killer simply kills the task that +triggered the out-of-memory condition. This avoids the expensive +tasklist scan. + +If panic_on_oom is selected, it takes precedence over whatever value +is used in oom_kill_allocating_task. + +The default value is 0. + +============================================================== + +overcommit_kbytes: + +When overcommit_memory is set to 2, the committed address space is not +permitted to exceed swap plus this amount of physical RAM. See below. + +Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one +of them may be specified at a time. Setting one disables the other (which +then appears as 0 when read). + +============================================================== + +overcommit_memory: + +This value contains a flag that enables memory overcommitment. + +When this flag is 0, the kernel attempts to estimate the amount +of free memory left when userspace requests more memory. + +When this flag is 1, the kernel pretends there is always enough +memory until it actually runs out. + +When this flag is 2, the kernel uses a "never overcommit" +policy that attempts to prevent any overcommit of memory. +Note that user_reserve_kbytes affects this policy. + +This feature can be very useful because there are a lot of +programs that malloc() huge amounts of memory "just-in-case" +and don't use much of it. + +The default value is 0. + +See Documentation/vm/overcommit-accounting and +security/commoncap.c::cap_vm_enough_memory() for more information. + +============================================================== + +overcommit_ratio: + +When overcommit_memory is set to 2, the committed address +space is not permitted to exceed swap plus this percentage +of physical RAM. See above. + +============================================================== + +page-cluster + +page-cluster controls the number of pages up to which consecutive pages +are read in from swap in a single attempt. This is the swap counterpart +to page cache readahead. +The mentioned consecutivity is not in terms of virtual/physical addresses, +but consecutive on swap space - that means they were swapped out together. + +It is a logarithmic value - setting it to zero means "1 page", setting +it to 1 means "2 pages", setting it to 2 means "4 pages", etc. +Zero disables swap readahead completely. + +The default value is three (eight pages at a time). There may be some +small benefits in tuning this to a different value if your workload is +swap-intensive. + +Lower values mean lower latencies for initial faults, but at the same time +extra faults and I/O delays for following faults if they would have been part of +that consecutive pages readahead would have brought in. + +============================================================= + +panic_on_oom + +This enables or disables panic on out-of-memory feature. + +If this is set to 0, the kernel will kill some rogue process, +called oom_killer. Usually, oom_killer can kill rogue processes and +system will survive. + +If this is set to 1, the kernel panics when out-of-memory happens. +However, if a process limits using nodes by mempolicy/cpusets, +and those nodes become memory exhaustion status, one process +may be killed by oom-killer. No panic occurs in this case. +Because other nodes' memory may be free. This means system total status +may be not fatal yet. + +If this is set to 2, the kernel panics compulsorily even on the +above-mentioned. Even oom happens under memory cgroup, the whole +system panics. + +The default value is 0. +1 and 2 are for failover of clustering. Please select either +according to your policy of failover. +panic_on_oom=2+kdump gives you very strong tool to investigate +why oom happens. You can get snapshot. + +============================================================= + +percpu_pagelist_fraction + +This is the fraction of pages at most (high mark pcp->high) in each zone that +are allocated for each per cpu page list. The min value for this is 8. It +means that we don't allow more than 1/8th of pages in each zone to be +allocated in any single per_cpu_pagelist. This entry only changes the value +of hot per cpu pagelists. User can specify a number like 100 to allocate +1/100th of each zone to each per cpu page list. + +The batch value of each per cpu pagelist is also updated as a result. It is +set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8) + +The initial value is zero. Kernel does not use this value at boot time to set +the high water marks for each per cpu page list. If the user writes '0' to this +sysctl, it will revert to this default behavior. + +============================================================== + +stat_interval + +The time interval between which vm statistics are updated. The default +is 1 second. + +============================================================== + +swappiness + +This control is used to define how aggressive the kernel will swap +memory pages. Higher values will increase agressiveness, lower values +decrease the amount of swap. A value of 0 instructs the kernel not to +initiate swap until the amount of free and file-backed pages is less +than the high water mark in a zone. + +The default value is 60. + +============================================================== + +- user_reserve_kbytes + +When overcommit_memory is set to 2, "never overcommit" mode, reserve +min(3% of current process size, user_reserve_kbytes) of free memory. +This is intended to prevent a user from starting a single memory hogging +process, such that they cannot recover (kill the hog). + +user_reserve_kbytes defaults to min(3% of the current process size, 128MB). + +If this is reduced to zero, then the user will be allowed to allocate +all free memory with a single process, minus admin_reserve_kbytes. +Any subsequent attempts to execute a command will result in +"fork: Cannot allocate memory". + +Changing this takes effect whenever an application requests memory. + +============================================================== + +vfs_cache_pressure +------------------ + +This percentage value controls the tendency of the kernel to reclaim +the memory which is used for caching of directory and inode objects. + +At the default value of vfs_cache_pressure=100 the kernel will attempt to +reclaim dentries and inodes at a "fair" rate with respect to pagecache and +swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer +to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will +never reclaim dentries and inodes due to memory pressure and this can easily +lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 +causes the kernel to prefer to reclaim dentries and inodes. + +Increasing vfs_cache_pressure significantly beyond 100 may have negative +performance impact. Reclaim code needs to take various locks to find freeable +directory and inode objects. With vfs_cache_pressure=1000, it will look for +ten times more freeable objects than there are. + +============================================================== + +zone_reclaim_mode: + +Zone_reclaim_mode allows someone to set more or less aggressive approaches to +reclaim memory when a zone runs out of memory. If it is set to zero then no +zone reclaim occurs. Allocations will be satisfied from other zones / nodes +in the system. + +This is value ORed together of + +1 = Zone reclaim on +2 = Zone reclaim writes dirty pages out +4 = Zone reclaim swaps pages + +zone_reclaim_mode is disabled by default. For file servers or workloads +that benefit from having their data cached, zone_reclaim_mode should be +left disabled as the caching effect is likely to be more important than +data locality. + +zone_reclaim may be enabled if it's known that the workload is partitioned +such that each partition fits within a NUMA node and that accessing remote +memory would cause a measurable performance reduction. The page allocator +will then reclaim easily reusable pages (those page cache pages that are +currently not used) before allocating off node pages. + +Allowing zone reclaim to write out pages stops processes that are +writing large amounts of data from dirtying pages on other nodes. Zone +reclaim will write out dirty pages if a zone fills up and so effectively +throttle the process. This may decrease the performance of a single process +since it cannot use all of system memory to buffer the outgoing writes +anymore but it preserve the memory on other nodes so that the performance +of other processes running on other nodes will not be affected. + +Allowing regular swap effectively restricts allocations to the local +node unless explicitly overridden by memory policies or cpuset +configurations. + +============ End of Document ================================= |