914 lines
33 KiB
Plaintext
914 lines
33 KiB
Plaintext
Documentation for /proc/sys/vm/* kernel version 2.6.29
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(c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
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(c) 2008 Peter W. Morreale <pmorreale@novell.com>
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For general info and legal blurb, please look in README.
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==============================================================
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This file contains the documentation for the sysctl files in
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/proc/sys/vm and is valid for Linux kernel version 2.6.29.
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The files in this directory can be used to tune the operation
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of the virtual memory (VM) subsystem of the Linux kernel and
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the writeout of dirty data to disk.
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Default values and initialization routines for most of these
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files can be found in mm/swap.c.
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Currently, these files are in /proc/sys/vm:
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- admin_reserve_kbytes
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- block_dump
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- compact_memory
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- compact_unevictable_allowed
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- dirty_background_bytes
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- dirty_background_ratio
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- dirty_bytes
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- dirty_expire_centisecs
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- dirty_ratio
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- dirtytime_expire_seconds
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- dirty_writeback_centisecs
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- drop_caches
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- extfrag_threshold
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- hugetlb_shm_group
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- laptop_mode
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- legacy_va_layout
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- lowmem_reserve_ratio
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- max_map_count
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- memory_failure_early_kill
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- memory_failure_recovery
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- min_free_kbytes
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- min_slab_ratio
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- min_unmapped_ratio
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- mmap_min_addr
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- mmap_rnd_bits
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- mmap_rnd_compat_bits
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- nr_hugepages
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- nr_hugepages_mempolicy
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- nr_overcommit_hugepages
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- nr_trim_pages (only if CONFIG_MMU=n)
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- numa_zonelist_order
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- oom_dump_tasks
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- oom_kill_allocating_task
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- overcommit_kbytes
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- overcommit_memory
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- overcommit_ratio
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- page-cluster
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- panic_on_oom
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- percpu_pagelist_fraction
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- stat_interval
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- stat_refresh
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- numa_stat
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- swappiness
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- user_reserve_kbytes
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- vfs_cache_pressure
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- watermark_scale_factor
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- zone_reclaim_mode
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==============================================================
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admin_reserve_kbytes
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The amount of free memory in the system that should be reserved for users
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with the capability cap_sys_admin.
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admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
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That should provide enough for the admin to log in and kill a process,
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if necessary, under the default overcommit 'guess' mode.
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Systems running under overcommit 'never' should increase this to account
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for the full Virtual Memory Size of programs used to recover. Otherwise,
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root may not be able to log in to recover the system.
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How do you calculate a minimum useful reserve?
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sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
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For overcommit 'guess', we can sum resident set sizes (RSS).
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On x86_64 this is about 8MB.
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For overcommit 'never', we can take the max of their virtual sizes (VSZ)
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and add the sum of their RSS.
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On x86_64 this is about 128MB.
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Changing this takes effect whenever an application requests memory.
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==============================================================
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block_dump
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block_dump enables block I/O debugging when set to a nonzero value. More
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information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.
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==============================================================
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compact_memory
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Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
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all zones are compacted such that free memory is available in contiguous
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blocks where possible. This can be important for example in the allocation of
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huge pages although processes will also directly compact memory as required.
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==============================================================
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compact_unevictable_allowed
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Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
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allowed to examine the unevictable lru (mlocked pages) for pages to compact.
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This should be used on systems where stalls for minor page faults are an
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acceptable trade for large contiguous free memory. Set to 0 to prevent
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compaction from moving pages that are unevictable. Default value is 1.
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==============================================================
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dirty_background_bytes
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Contains the amount of dirty memory at which the background kernel
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flusher threads will start writeback.
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Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
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one of them may be specified at a time. When one sysctl is written it is
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immediately taken into account to evaluate the dirty memory limits and the
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other appears as 0 when read.
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==============================================================
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dirty_background_ratio
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Contains, as a percentage of total available memory that contains free pages
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and reclaimable pages, the number of pages at which the background kernel
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flusher threads will start writing out dirty data.
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The total available memory is not equal to total system memory.
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==============================================================
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dirty_bytes
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Contains the amount of dirty memory at which a process generating disk writes
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will itself start writeback.
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Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
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specified at a time. When one sysctl is written it is immediately taken into
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account to evaluate the dirty memory limits and the other appears as 0 when
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read.
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Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
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value lower than this limit will be ignored and the old configuration will be
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retained.
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==============================================================
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dirty_expire_centisecs
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This tunable is used to define when dirty data is old enough to be eligible
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for writeout by the kernel flusher threads. It is expressed in 100'ths
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of a second. Data which has been dirty in-memory for longer than this
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interval will be written out next time a flusher thread wakes up.
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==============================================================
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dirty_ratio
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Contains, as a percentage of total available memory that contains free pages
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and reclaimable pages, the number of pages at which a process which is
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generating disk writes will itself start writing out dirty data.
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The total available memory is not equal to total system memory.
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==============================================================
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dirtytime_expire_seconds
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When a lazytime inode is constantly having its pages dirtied, the inode with
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an updated timestamp will never get chance to be written out. And, if the
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only thing that has happened on the file system is a dirtytime inode caused
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by an atime update, a worker will be scheduled to make sure that inode
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eventually gets pushed out to disk. This tunable is used to define when dirty
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inode is old enough to be eligible for writeback by the kernel flusher threads.
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And, it is also used as the interval to wakeup dirtytime_writeback thread.
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==============================================================
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dirty_writeback_centisecs
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The kernel flusher threads will periodically wake up and write `old' data
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out to disk. This tunable expresses the interval between those wakeups, in
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100'ths of a second.
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Setting this to zero disables periodic writeback altogether.
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==============================================================
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drop_caches
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Writing to this will cause the kernel to drop clean caches, as well as
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reclaimable slab objects like dentries and inodes. Once dropped, their
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memory becomes free.
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To free pagecache:
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echo 1 > /proc/sys/vm/drop_caches
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To free reclaimable slab objects (includes dentries and inodes):
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echo 2 > /proc/sys/vm/drop_caches
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To free slab objects and pagecache:
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echo 3 > /proc/sys/vm/drop_caches
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This is a non-destructive operation and will not free any dirty objects.
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To increase the number of objects freed by this operation, the user may run
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`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
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number of dirty objects on the system and create more candidates to be
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dropped.
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This file is not a means to control the growth of the various kernel caches
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(inodes, dentries, pagecache, etc...) These objects are automatically
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reclaimed by the kernel when memory is needed elsewhere on the system.
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Use of this file can cause performance problems. Since it discards cached
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objects, it may cost a significant amount of I/O and CPU to recreate the
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dropped objects, especially if they were under heavy use. Because of this,
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use outside of a testing or debugging environment is not recommended.
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You may see informational messages in your kernel log when this file is
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used:
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cat (1234): drop_caches: 3
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These are informational only. They do not mean that anything is wrong
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with your system. To disable them, echo 4 (bit 3) into drop_caches.
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==============================================================
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extfrag_threshold
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This parameter affects whether the kernel will compact memory or direct
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reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
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debugfs shows what the fragmentation index for each order is in each zone in
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the system. Values tending towards 0 imply allocations would fail due to lack
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of memory, values towards 1000 imply failures are due to fragmentation and -1
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implies that the allocation will succeed as long as watermarks are met.
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The kernel will not compact memory in a zone if the
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fragmentation index is <= extfrag_threshold. The default value is 500.
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==============================================================
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highmem_is_dirtyable
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Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
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This parameter controls whether the high memory is considered for dirty
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writers throttling. This is not the case by default which means that
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only the amount of memory directly visible/usable by the kernel can
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be dirtied. As a result, on systems with a large amount of memory and
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lowmem basically depleted writers might be throttled too early and
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streaming writes can get very slow.
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Changing the value to non zero would allow more memory to be dirtied
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and thus allow writers to write more data which can be flushed to the
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storage more effectively. Note this also comes with a risk of pre-mature
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OOM killer because some writers (e.g. direct block device writes) can
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only use the low memory and they can fill it up with dirty data without
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any throttling.
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==============================================================
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hugetlb_shm_group
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hugetlb_shm_group contains group id that is allowed to create SysV
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shared memory segment using hugetlb page.
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==============================================================
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laptop_mode
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laptop_mode is a knob that controls "laptop mode". All the things that are
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controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.
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==============================================================
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legacy_va_layout
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If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
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will use the legacy (2.4) layout for all processes.
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==============================================================
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lowmem_reserve_ratio
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For some specialised workloads on highmem machines it is dangerous for
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the kernel to allow process memory to be allocated from the "lowmem"
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zone. This is because that memory could then be pinned via the mlock()
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system call, or by unavailability of swapspace.
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And on large highmem machines this lack of reclaimable lowmem memory
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can be fatal.
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So the Linux page allocator has a mechanism which prevents allocations
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which _could_ use highmem from using too much lowmem. This means that
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a certain amount of lowmem is defended from the possibility of being
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captured into pinned user memory.
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(The same argument applies to the old 16 megabyte ISA DMA region. This
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mechanism will also defend that region from allocations which could use
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highmem or lowmem).
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The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
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in defending these lower zones.
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If you have a machine which uses highmem or ISA DMA and your
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applications are using mlock(), or if you are running with no swap then
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you probably should change the lowmem_reserve_ratio setting.
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The lowmem_reserve_ratio is an array. You can see them by reading this file.
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-
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% cat /proc/sys/vm/lowmem_reserve_ratio
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256 256 32
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-
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But, these values are not used directly. The kernel calculates # of protection
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pages for each zones from them. These are shown as array of protection pages
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in /proc/zoneinfo like followings. (This is an example of x86-64 box).
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Each zone has an array of protection pages like this.
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-
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Node 0, zone DMA
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pages free 1355
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min 3
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low 3
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high 4
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:
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:
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numa_other 0
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protection: (0, 2004, 2004, 2004)
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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pagesets
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cpu: 0 pcp: 0
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:
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-
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These protections are added to score to judge whether this zone should be used
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for page allocation or should be reclaimed.
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In this example, if normal pages (index=2) are required to this DMA zone and
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watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
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not be used because pages_free(1355) is smaller than watermark + protection[2]
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(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
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normal page requirement. If requirement is DMA zone(index=0), protection[0]
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(=0) is used.
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zone[i]'s protection[j] is calculated by following expression.
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(i < j):
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zone[i]->protection[j]
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= (total sums of managed_pages from zone[i+1] to zone[j] on the node)
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/ lowmem_reserve_ratio[i];
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(i = j):
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(should not be protected. = 0;
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(i > j):
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(not necessary, but looks 0)
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The default values of lowmem_reserve_ratio[i] are
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256 (if zone[i] means DMA or DMA32 zone)
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32 (others).
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As above expression, they are reciprocal number of ratio.
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256 means 1/256. # of protection pages becomes about "0.39%" of total managed
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pages of higher zones on the node.
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If you would like to protect more pages, smaller values are effective.
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The minimum value is 1 (1/1 -> 100%). The value less than 1 completely
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disables protection of the pages.
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==============================================================
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max_map_count:
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This file contains the maximum number of memory map areas a process
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may have. Memory map areas are used as a side-effect of calling
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malloc, directly by mmap, mprotect, and madvise, and also when loading
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shared libraries.
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While most applications need less than a thousand maps, certain
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programs, particularly malloc debuggers, may consume lots of them,
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e.g., up to one or two maps per allocation.
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The default value is 65536.
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=============================================================
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memory_failure_early_kill:
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Control how to kill processes when uncorrected memory error (typically
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a 2bit error in a memory module) is detected in the background by hardware
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that cannot be handled by the kernel. In some cases (like the page
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still having a valid copy on disk) the kernel will handle the failure
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transparently without affecting any applications. But if there is
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no other uptodate copy of the data it will kill to prevent any data
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corruptions from propagating.
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1: Kill all processes that have the corrupted and not reloadable page mapped
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as soon as the corruption is detected. Note this is not supported
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for a few types of pages, like kernel internally allocated data or
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the swap cache, but works for the majority of user pages.
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0: Only unmap the corrupted page from all processes and only kill a process
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who tries to access it.
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The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
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handle this if they want to.
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This is only active on architectures/platforms with advanced machine
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check handling and depends on the hardware capabilities.
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Applications can override this setting individually with the PR_MCE_KILL prctl
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==============================================================
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memory_failure_recovery
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Enable memory failure recovery (when supported by the platform)
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1: Attempt recovery.
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0: Always panic on a memory failure.
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==============================================================
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min_free_kbytes:
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This is used to force the Linux VM to keep a minimum number
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of kilobytes free. The VM uses this number to compute a
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watermark[WMARK_MIN] value for each lowmem zone in the system.
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Each lowmem zone gets a number of reserved free pages based
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proportionally on its size.
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Some minimal amount of memory is needed to satisfy PF_MEMALLOC
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allocations; if you set this to lower than 1024KB, your system will
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become subtly broken, and prone to deadlock under high loads.
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Setting this too high will OOM your machine instantly.
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=============================================================
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min_slab_ratio:
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This is available only on NUMA kernels.
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A percentage of the total pages in each zone. On Zone reclaim
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(fallback from the local zone occurs) slabs will be reclaimed if more
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than this percentage of pages in a zone are reclaimable slab pages.
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This insures that the slab growth stays under control even in NUMA
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systems that rarely perform global reclaim.
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The default is 5 percent.
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Note that slab reclaim is triggered in a per zone / node fashion.
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The process of reclaiming slab memory is currently not node specific
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and may not be fast.
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=============================================================
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min_unmapped_ratio:
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This is available only on NUMA kernels.
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This is a percentage of the total pages in each zone. Zone reclaim will
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only occur if more than this percentage of pages are in a state that
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zone_reclaim_mode allows to be reclaimed.
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If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
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against all file-backed unmapped pages including swapcache pages and tmpfs
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files. Otherwise, only unmapped pages backed by normal files but not tmpfs
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files and similar are considered.
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The default is 1 percent.
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==============================================================
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mmap_min_addr
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This file indicates the amount of address space which a user process will
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be restricted from mmapping. Since kernel null dereference bugs could
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accidentally operate based on the information in the first couple of pages
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of memory userspace processes should not be allowed to write to them. By
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default this value is set to 0 and no protections will be enforced by the
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security module. Setting this value to something like 64k will allow the
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vast majority of applications to work correctly and provide defense in depth
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against future potential kernel bugs.
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==============================================================
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mmap_rnd_bits:
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This value can be used to select the number of bits to use to
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determine the random offset to the base address of vma regions
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resulting from mmap allocations on architectures which support
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tuning address space randomization. This value will be bounded
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by the architecture's minimum and maximum supported values.
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This value can be changed after boot using the
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/proc/sys/vm/mmap_rnd_bits tunable
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==============================================================
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mmap_rnd_compat_bits:
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This value can be used to select the number of bits to use to
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determine the random offset to the base address of vma regions
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resulting from mmap allocations for applications run in
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compatibility mode on architectures which support tuning address
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space randomization. This value will be bounded by the
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architecture's minimum and maximum supported values.
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This value can be changed after boot using the
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/proc/sys/vm/mmap_rnd_compat_bits tunable
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==============================================================
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nr_hugepages
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Change the minimum size of the hugepage pool.
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See Documentation/admin-guide/mm/hugetlbpage.rst
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|
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==============================================================
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nr_hugepages_mempolicy
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Change the size of the hugepage pool at run-time on a specific
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set of NUMA nodes.
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See Documentation/admin-guide/mm/hugetlbpage.rst
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==============================================================
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nr_overcommit_hugepages
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Change the maximum size of the hugepage pool. The maximum is
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nr_hugepages + nr_overcommit_hugepages.
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See Documentation/admin-guide/mm/hugetlbpage.rst
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==============================================================
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nr_trim_pages
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This is available only on NOMMU kernels.
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This value adjusts the excess page trimming behaviour of power-of-2 aligned
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NOMMU mmap allocations.
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A value of 0 disables trimming of allocations entirely, while a value of 1
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trims excess pages aggressively. Any value >= 1 acts as the watermark where
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trimming of allocations is initiated.
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The default value is 1.
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See Documentation/nommu-mmap.txt for more information.
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==============================================================
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numa_zonelist_order
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This sysctl is only for NUMA and it is deprecated. Anything but
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Node order will fail!
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'where the memory is allocated from' is controlled by zonelists.
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(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
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you may be able to read ZONE_DMA as ZONE_DMA32...)
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In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
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ZONE_NORMAL -> ZONE_DMA
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This means that a memory allocation request for GFP_KERNEL will
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get memory from ZONE_DMA only when ZONE_NORMAL is not available.
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In NUMA case, you can think of following 2 types of order.
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Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
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(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
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(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
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Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
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will be used before ZONE_NORMAL exhaustion. This increases possibility of
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out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
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Type(B) cannot offer the best locality but is more robust against OOM of
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the DMA zone.
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Type(A) is called as "Node" order. Type (B) is "Zone" order.
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"Node order" orders the zonelists by node, then by zone within each node.
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Specify "[Nn]ode" for node order
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"Zone Order" orders the zonelists by zone type, then by node within each
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zone. Specify "[Zz]one" for zone order.
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Specify "[Dd]efault" to request automatic configuration.
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On 32-bit, the Normal zone needs to be preserved for allocations accessible
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by the kernel, so "zone" order will be selected.
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On 64-bit, devices that require DMA32/DMA are relatively rare, so "node"
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order will be selected.
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Default order is recommended unless this is causing problems for your
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system/application.
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==============================================================
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oom_dump_tasks
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Enables a system-wide task dump (excluding kernel threads) to be produced
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when the kernel performs an OOM-killing and includes such information as
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pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj
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score, and name. This is helpful to determine why the OOM killer was
|
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invoked, to identify the rogue task that caused it, and to determine why
|
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the OOM killer chose the task it did to kill.
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If this is set to zero, this information is suppressed. On very
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large systems with thousands of tasks it may not be feasible to dump
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the memory state information for each one. Such systems should not
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be forced to incur a performance penalty in OOM conditions when the
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information may not be desired.
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If this is set to non-zero, this information is shown whenever the
|
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OOM killer actually kills a memory-hogging task.
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The default value is 1 (enabled).
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|
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==============================================================
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oom_kill_allocating_task
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This enables or disables killing the OOM-triggering task in
|
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out-of-memory situations.
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If this is set to zero, the OOM killer will scan through the entire
|
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tasklist and select a task based on heuristics to kill. This normally
|
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selects a rogue memory-hogging task that frees up a large amount of
|
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memory when killed.
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If this is set to non-zero, the OOM killer simply kills the task that
|
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triggered the out-of-memory condition. This avoids the expensive
|
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tasklist scan.
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|
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If panic_on_oom is selected, it takes precedence over whatever value
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is used in oom_kill_allocating_task.
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The default value is 0.
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|
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==============================================================
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overcommit_kbytes:
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|
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When overcommit_memory is set to 2, the committed address space is not
|
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permitted to exceed swap plus this amount of physical RAM. See below.
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Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one
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of them may be specified at a time. Setting one disables the other (which
|
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then appears as 0 when read).
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|
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==============================================================
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overcommit_memory:
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This value contains a flag that enables memory overcommitment.
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|
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When this flag is 0, the kernel attempts to estimate the amount
|
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of free memory left when userspace requests more memory.
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|
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When this flag is 1, the kernel pretends there is always enough
|
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memory until it actually runs out.
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When this flag is 2, the kernel uses a "never overcommit"
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policy that attempts to prevent any overcommit of memory.
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Note that user_reserve_kbytes affects this policy.
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This feature can be very useful because there are a lot of
|
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programs that malloc() huge amounts of memory "just-in-case"
|
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and don't use much of it.
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The default value is 0.
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See Documentation/vm/overcommit-accounting.rst and
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mm/util.c::__vm_enough_memory() for more information.
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|
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==============================================================
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overcommit_ratio:
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|
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When overcommit_memory is set to 2, the committed address
|
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space is not permitted to exceed swap plus this percentage
|
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of physical RAM. See above.
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|
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==============================================================
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page-cluster
|
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|
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page-cluster controls the number of pages up to which consecutive pages
|
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are read in from swap in a single attempt. This is the swap counterpart
|
|
to page cache readahead.
|
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The mentioned consecutivity is not in terms of virtual/physical addresses,
|
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but consecutive on swap space - that means they were swapped out together.
|
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|
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It is a logarithmic value - setting it to zero means "1 page", setting
|
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it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
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Zero disables swap readahead completely.
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|
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The default value is three (eight pages at a time). There may be some
|
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small benefits in tuning this to a different value if your workload is
|
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swap-intensive.
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Lower values mean lower latencies for initial faults, but at the same time
|
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extra faults and I/O delays for following faults if they would have been part of
|
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that consecutive pages readahead would have brought in.
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|
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=============================================================
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panic_on_oom
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|
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This enables or disables panic on out-of-memory feature.
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|
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If this is set to 0, the kernel will kill some rogue process,
|
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called oom_killer. Usually, oom_killer can kill rogue processes and
|
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system will survive.
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|
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If this is set to 1, the kernel panics when out-of-memory happens.
|
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However, if a process limits using nodes by mempolicy/cpusets,
|
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and those nodes become memory exhaustion status, one process
|
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may be killed by oom-killer. No panic occurs in this case.
|
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Because other nodes' memory may be free. This means system total status
|
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may be not fatal yet.
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|
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If this is set to 2, the kernel panics compulsorily even on the
|
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above-mentioned. Even oom happens under memory cgroup, the whole
|
|
system panics.
|
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|
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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.
|
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|
|
=============================================================
|
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|
|
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.
|
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|
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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)
|
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|
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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.
|
|
|
|
==============================================================
|
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|
|
stat_interval
|
|
|
|
The time interval between which vm statistics are updated. The default
|
|
is 1 second.
|
|
|
|
==============================================================
|
|
|
|
stat_refresh
|
|
|
|
Any read or write (by root only) flushes all the per-cpu vm statistics
|
|
into their global totals, for more accurate reports when testing
|
|
e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
|
|
|
|
As a side-effect, it also checks for negative totals (elsewhere reported
|
|
as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
|
|
(At time of writing, a few stats are known sometimes to be found negative,
|
|
with no ill effects: errors and warnings on these stats are suppressed.)
|
|
|
|
==============================================================
|
|
|
|
numa_stat
|
|
|
|
This interface allows runtime configuration of numa statistics.
|
|
|
|
When page allocation performance becomes a bottleneck and you can tolerate
|
|
some possible tool breakage and decreased numa counter precision, you can
|
|
do:
|
|
echo 0 > /proc/sys/vm/numa_stat
|
|
|
|
When page allocation performance is not a bottleneck and you want all
|
|
tooling to work, you can do:
|
|
echo 1 > /proc/sys/vm/numa_stat
|
|
|
|
==============================================================
|
|
|
|
swappiness
|
|
|
|
This control is used to define how aggressive the kernel will swap
|
|
memory pages. Higher values will increase aggressiveness, 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.
|
|
|
|
=============================================================
|
|
|
|
watermark_scale_factor:
|
|
|
|
This factor controls the aggressiveness of kswapd. It defines the
|
|
amount of memory left in a node/system before kswapd is woken up and
|
|
how much memory needs to be free before kswapd goes back to sleep.
|
|
|
|
The unit is in fractions of 10,000. The default value of 10 means the
|
|
distances between watermarks are 0.1% of the available memory in the
|
|
node/system. The maximum value is 1000, or 10% of memory.
|
|
|
|
A high rate of threads entering direct reclaim (allocstall) or kswapd
|
|
going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate
|
|
that the number of free pages kswapd maintains for latency reasons is
|
|
too small for the allocation bursts occurring in the system. This knob
|
|
can then be used to tune kswapd aggressiveness accordingly.
|
|
|
|
==============================================================
|
|
|
|
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 =================================
|