diff options
Diffstat (limited to 'Documentation/sysctl/vm.txt')
| -rw-r--r-- | Documentation/sysctl/vm.txt | 93 |
1 files changed, 70 insertions, 23 deletions
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt index 79a797eb3e8..4415aa91568 100644 --- a/Documentation/sysctl/vm.txt +++ b/Documentation/sysctl/vm.txt @@ -47,6 +47,7 @@ Currently, these files are in /proc/sys/vm: - numa_zonelist_order - oom_dump_tasks - oom_kill_allocating_task +- overcommit_kbytes - overcommit_memory - overcommit_ratio - page-cluster @@ -119,8 +120,11 @@ other appears as 0 when read. dirty_background_ratio -Contains, as a percentage of total system memory, the number of pages at which -the background kernel flusher threads will start writing out dirty data. +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. ============================================================== @@ -151,9 +155,11 @@ interval will be written out next time a flusher thread wakes up. dirty_ratio -Contains, as a percentage of total system memory, the number of pages at which -a process which is generating disk writes will itself start writing out dirty -data. +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. ============================================================== @@ -169,18 +175,39 @@ Setting this to zero disables periodic writeback altogether. drop_caches -Writing to this will cause the kernel to drop clean caches, dentries and -inodes from memory, causing that memory to become free. +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 dentries and inodes: +To free reclaimable slab objects (includes dentries and inodes): echo 2 > /proc/sys/vm/drop_caches -To free pagecache, dentries and inodes: +To free slab objects and pagecache: echo 3 > /proc/sys/vm/drop_caches -As this is a non-destructive operation and dirty objects are not freeable, the -user should run `sync' first. +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. ============================================================== @@ -569,6 +596,17 @@ 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. @@ -664,7 +702,8 @@ 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. +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. ============================================================== @@ -679,7 +718,9 @@ 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. +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. @@ -706,8 +747,8 @@ Changing this takes effect whenever an application requests memory. vfs_cache_pressure ------------------ -Controls the tendency of the kernel to reclaim the memory which is used for -caching of directory and inode objects. +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 @@ -717,6 +758,11 @@ 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: @@ -732,16 +778,17 @@ This is value ORed together of 2 = Zone reclaim writes dirty pages out 4 = Zone reclaim swaps pages -zone_reclaim_mode is set during bootup to 1 if it is determined that pages -from remote zones will 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. - -It may be beneficial to switch off zone reclaim if the system is -used for a file server and all of memory should be used for caching files -from disk. In that case the caching effect is more important than +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 |