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 =================================