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-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:
-
-- block_dump
-- compact_memory
-- 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_pdflush_threads
-- nr_trim_pages (only if CONFIG_MMU=n)
-- numa_zonelist_order
-- oom_dump_tasks
-- oom_kill_allocating_task
-- overcommit_memory
-- overcommit_ratio
-- page-cluster
-- panic_on_oom
-- percpu_pagelist_fraction
-- stat_interval
-- swappiness
-- vfs_cache_pressure
-- zone_reclaim_mode
-
-==============================================================
-
-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.
-
-==============================================================
-
-dirty_background_bytes
-
-Contains the amount of dirty memory at which the pdflush background writeback
-daemon 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 system memory, the number of pages at which
-the pdflush background writeback daemon will start writing out dirty data.
-
-==============================================================
-
-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 pdflush daemons. 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 pdflush daemon 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.
-
-==============================================================
-
-dirty_writeback_centisecs
-
-The pdflush writeback daemons 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, dentries and
-inodes from memory, causing that memory to become free.
-
-To free pagecache:
- echo 1 > /proc/sys/vm/drop_caches
-To free dentries and inodes:
- echo 2 > /proc/sys/vm/drop_caches
-To free pagecache, dentries and inodes:
- 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.
-
-==============================================================
-
-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 is only useful when kernelcore= is specified at boot time to
-create ZONE_MOVABLE for pages that may be reclaimed or migrated. Huge pages
-are not movable so are not normally allocated from ZONE_MOVABLE. A non-zero
-value written to hugepages_treat_as_movable allows huge pages to be allocated
-from ZONE_MOVABLE.
-
-Once enabled, the ZONE_MOVABLE is treated as an area of memory the huge
-pages pool can easily grow or shrink within. Assuming that applications are
-not running that mlock() a lot of memory, it is likely the huge pages pool
-can grow to the size of ZONE_MOVABLE by repeatedly entering the desired value
-into nr_hugepages and triggering page reclaim.
-
-==============================================================
-
-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_pdflush_threads
-
-The current number of pdflush threads. This value is read-only.
-The value changes according to the number of dirty pages in the system.
-
-When necessary, additional pdflush threads are created, one per second, up to
-nr_pdflush_threads_max.
-
-==============================================================
-
-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 60% 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, cpu, oom_adj score, and
-name. This is helpful to determine why the OOM killer was invoked
-and to identify the rogue task that caused it.
-
-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_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.
-
-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 which are written to swap in
-a single attempt. The swap I/O size.
-
-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.
-
-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.
-
-=============================================================
-
-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.
-
-==============================================================
-
-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.
-
-The default value is 60.
-
-==============================================================
-
-vfs_cache_pressure
-------------------
-
-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.
-
-==============================================================
-
-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 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
-data locality.
-
-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 =================================