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997 lines
34 KiB
997 lines
34 KiB
=============================== |
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Documentation for /proc/sys/vm/ |
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=============================== |
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kernel version 2.6.29 |
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Copyright (c) 1998, 1999, Rik van Riel <[email protected]> |
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Copyright (c) 2008 Peter W. Morreale <[email protected]> |
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For general info and legal blurb, please look in index.rst. |
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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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- compact_memory |
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- compaction_proactiveness |
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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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- highmem_is_dirtyable |
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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_high_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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- unprivileged_userfaultfd |
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- user_reserve_kbytes |
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- vfs_cache_pressure |
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- watermark_boost_factor |
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- watermark_scale_factor |
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- zone_reclaim_mode |
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admin_reserve_kbytes |
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==================== |
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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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compact_memory |
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============== |
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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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compaction_proactiveness |
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======================== |
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This tunable takes a value in the range [0, 100] with a default value of |
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20. This tunable determines how aggressively compaction is done in the |
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background. Write of a non zero value to this tunable will immediately |
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trigger the proactive compaction. Setting it to 0 disables proactive compaction. |
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Note that compaction has a non-trivial system-wide impact as pages |
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belonging to different processes are moved around, which could also lead |
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to latency spikes in unsuspecting applications. The kernel employs |
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various heuristics to avoid wasting CPU cycles if it detects that |
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proactive compaction is not being effective. |
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Be careful when setting it to extreme values like 100, as that may |
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cause excessive background compaction activity. |
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compact_unevictable_allowed |
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=========================== |
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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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On CONFIG_PREEMPT_RT the default value is 0 in order to avoid a page fault, due |
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to compaction, which would block the task from becoming active until the fault |
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is resolved. |
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dirty_background_bytes |
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====================== |
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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: |
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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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dirty_background_ratio |
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====================== |
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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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dirty_bytes |
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=========== |
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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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dirty_expire_centisecs |
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====================== |
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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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dirty_ratio |
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=========== |
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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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dirtytime_expire_seconds |
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======================== |
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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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dirty_writeback_centisecs |
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========================= |
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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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drop_caches |
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=========== |
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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 2) into drop_caches. |
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extfrag_threshold |
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================= |
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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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highmem_is_dirtyable |
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==================== |
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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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hugetlb_shm_group |
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================= |
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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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laptop_mode |
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=========== |
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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/admin-guide/laptops/laptop-mode.rst. |
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legacy_va_layout |
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================ |
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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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lowmem_reserve_ratio |
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==================== |
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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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% cat /proc/sys/vm/lowmem_reserve_ratio |
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256 256 32 |
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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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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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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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=== ==================================== |
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256 (if zone[i] means DMA or DMA32 zone) |
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32 (others) |
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=== ==================================== |
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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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max_map_count: |
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============== |
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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 65530. |
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memory_failure_early_kill: |
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========================== |
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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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memory_failure_recovery |
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======================= |
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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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min_free_kbytes |
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=============== |
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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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min_slab_ratio |
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============== |
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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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min_unmapped_ratio |
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================== |
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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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mmap_min_addr |
|
============= |
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|
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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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mmap_rnd_bits |
|
============= |
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|
|
This value can be used to select the number of bits to use to |
|
determine the random offset to the base address of vma regions |
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resulting from mmap allocations on architectures which support |
|
tuning address space randomization. This value will be bounded |
|
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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mmap_rnd_compat_bits |
|
==================== |
|
|
|
This value can be used to select the number of bits to use to |
|
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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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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nr_hugepages_mempolicy |
|
====================== |
|
|
|
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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|
nr_overcommit_hugepages |
|
======================= |
|
|
|
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 |
|
============= |
|
|
|
This is available only on NOMMU kernels. |
|
|
|
This value adjusts the excess page trimming behaviour of power-of-2 aligned |
|
NOMMU mmap allocations. |
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|
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 |
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trimming of allocations is initiated. |
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|
The default value is 1. |
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|
|
See Documentation/admin-guide/mm/nommu-mmap.rst for more information. |
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|
|
numa_zonelist_order |
|
=================== |
|
|
|
This sysctl is only for NUMA and it is deprecated. Anything but |
|
Node order will fail! |
|
|
|
'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. |
|
|
|
On 32-bit, the Normal zone needs to be preserved for allocations accessible |
|
by the kernel, so "zone" order will be selected. |
|
|
|
On 64-bit, devices that require DMA32/DMA are relatively rare, so "node" |
|
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, pgtables_bytes, swapents, oom_score_adj |
|
score, and name. This is helpful to determine why the OOM killer was |
|
invoked, to identify the rogue task that caused it, and to determine why |
|
the OOM killer chose the task it did to kill. |
|
|
|
If this is set to zero, this information is suppressed. On very |
|
large systems with thousands of tasks it may not be feasible to dump |
|
the memory state information for each one. Such systems should not |
|
be forced to incur a performance penalty in OOM conditions when the |
|
information may not be desired. |
|
|
|
If this is set to non-zero, this information is shown whenever the |
|
OOM killer actually kills a memory-hogging task. |
|
|
|
The default value is 1 (enabled). |
|
|
|
|
|
oom_kill_allocating_task |
|
======================== |
|
|
|
This enables or disables killing the OOM-triggering task in |
|
out-of-memory situations. |
|
|
|
If this is set to zero, the OOM killer will scan through the entire |
|
tasklist and select a task based on heuristics to kill. This normally |
|
selects a rogue memory-hogging task that frees up a large amount of |
|
memory when killed. |
|
|
|
If this is set to non-zero, the OOM killer simply kills the task that |
|
triggered the out-of-memory condition. This avoids the expensive |
|
tasklist scan. |
|
|
|
If panic_on_oom is selected, it takes precedence over whatever value |
|
is used in oom_kill_allocating_task. |
|
|
|
The default value is 0. |
|
|
|
|
|
overcommit_kbytes |
|
================= |
|
|
|
When overcommit_memory is set to 2, the committed address space is not |
|
permitted to exceed swap plus this amount of physical RAM. See below. |
|
|
|
Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one |
|
of them may be specified at a time. Setting one disables the other (which |
|
then appears as 0 when read). |
|
|
|
|
|
overcommit_memory |
|
================= |
|
|
|
This value contains a flag that enables memory overcommitment. |
|
|
|
When this flag is 0, the kernel attempts to estimate the amount |
|
of free memory left when userspace requests more memory. |
|
|
|
When this flag is 1, the kernel pretends there is always enough |
|
memory until it actually runs out. |
|
|
|
When this flag is 2, the kernel uses a "never overcommit" |
|
policy that attempts to prevent any overcommit of memory. |
|
Note that user_reserve_kbytes affects this policy. |
|
|
|
This feature can be very useful because there are a lot of |
|
programs that malloc() huge amounts of memory "just-in-case" |
|
and don't use much of it. |
|
|
|
The default value is 0. |
|
|
|
See Documentation/vm/overcommit-accounting.rst and |
|
mm/util.c::__vm_enough_memory() for more information. |
|
|
|
|
|
overcommit_ratio |
|
================ |
|
|
|
When overcommit_memory is set to 2, the committed address |
|
space is not permitted to exceed swap plus this percentage |
|
of physical RAM. See above. |
|
|
|
|
|
page-cluster |
|
============ |
|
|
|
page-cluster controls the number of pages up to which consecutive pages |
|
are read in from swap in a single attempt. This is the swap counterpart |
|
to page cache readahead. |
|
The mentioned consecutivity is not in terms of virtual/physical addresses, |
|
but consecutive on swap space - that means they were swapped out together. |
|
|
|
It is a logarithmic value - setting it to zero means "1 page", setting |
|
it to 1 means "2 pages", setting it to 2 means "4 pages", etc. |
|
Zero disables swap readahead completely. |
|
|
|
The default value is three (eight pages at a time). There may be some |
|
small benefits in tuning this to a different value if your workload is |
|
swap-intensive. |
|
|
|
Lower values mean lower latencies for initial faults, but at the same time |
|
extra faults and I/O delays for following faults if they would have been part of |
|
that consecutive pages readahead would have brought in. |
|
|
|
|
|
panic_on_oom |
|
============ |
|
|
|
This enables or disables panic on out-of-memory feature. |
|
|
|
If this is set to 0, the kernel will kill some rogue process, |
|
called oom_killer. Usually, oom_killer can kill rogue processes and |
|
system will survive. |
|
|
|
If this is set to 1, the kernel panics when out-of-memory happens. |
|
However, if a process limits using nodes by mempolicy/cpusets, |
|
and those nodes become memory exhaustion status, one process |
|
may be killed by oom-killer. No panic occurs in this case. |
|
Because other nodes' memory may be free. This means system total status |
|
may be not fatal yet. |
|
|
|
If this is set to 2, the kernel panics compulsorily even on the |
|
above-mentioned. Even oom happens under memory cgroup, the whole |
|
system panics. |
|
|
|
The default value is 0. |
|
|
|
1 and 2 are for failover of clustering. Please select either |
|
according to your policy of failover. |
|
|
|
panic_on_oom=2+kdump gives you very strong tool to investigate |
|
why oom happens. You can get snapshot. |
|
|
|
|
|
percpu_pagelist_high_fraction |
|
============================= |
|
|
|
This is the fraction of pages in each zone that are can be stored to |
|
per-cpu page lists. It is an upper boundary that is divided depending |
|
on the number of online CPUs. The min value for this is 8 which means |
|
that we do not allow more than 1/8th of pages in each zone to be stored |
|
on per-cpu page lists. This entry only changes the value of hot per-cpu |
|
page lists. A user can specify a number like 100 to allocate 1/100th of |
|
each zone between per-cpu lists. |
|
|
|
The batch value of each per-cpu page list remains the same regardless of |
|
the value of the high fraction so allocation latencies are unaffected. |
|
|
|
The initial value is zero. Kernel uses this value to set the high pcp->high |
|
mark based on the low watermark for the zone and the number of local |
|
online CPUs. If the user writes '0' to this sysctl, it will revert to |
|
this default behavior. |
|
|
|
|
|
stat_interval |
|
============= |
|
|
|
The time interval between which vm statistics are updated. The default |
|
is 1 second. |
|
|
|
|
|
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 the rough relative IO cost of swapping |
|
and filesystem paging, as a value between 0 and 200. At 100, the VM |
|
assumes equal IO cost and will thus apply memory pressure to the page |
|
cache and swap-backed pages equally; lower values signify more |
|
expensive swap IO, higher values indicates cheaper. |
|
|
|
Keep in mind that filesystem IO patterns under memory pressure tend to |
|
be more efficient than swap's random IO. An optimal value will require |
|
experimentation and will also be workload-dependent. |
|
|
|
The default value is 60. |
|
|
|
For in-memory swap, like zram or zswap, as well as hybrid setups that |
|
have swap on faster devices than the filesystem, values beyond 100 can |
|
be considered. For example, if the random IO against the swap device |
|
is on average 2x faster than IO from the filesystem, swappiness should |
|
be 133 (x + 2x = 200, 2x = 133.33). |
|
|
|
At 0, the kernel will not initiate swap until the amount of free and |
|
file-backed pages is less than the high watermark in a zone. |
|
|
|
|
|
unprivileged_userfaultfd |
|
======================== |
|
|
|
This flag controls the mode in which unprivileged users can use the |
|
userfaultfd system calls. Set this to 0 to restrict unprivileged users |
|
to handle page faults in user mode only. In this case, users without |
|
SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to |
|
succeed. Prohibiting use of userfaultfd for handling faults from kernel |
|
mode may make certain vulnerabilities more difficult to exploit. |
|
|
|
Set this to 1 to allow unprivileged users to use the userfaultfd system |
|
calls without any restrictions. |
|
|
|
The default value is 0. |
|
|
|
|
|
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_boost_factor |
|
====================== |
|
|
|
This factor controls the level of reclaim when memory is being fragmented. |
|
It defines the percentage of the high watermark of a zone that will be |
|
reclaimed if pages of different mobility are being mixed within pageblocks. |
|
The intent is that compaction has less work to do in the future and to |
|
increase the success rate of future high-order allocations such as SLUB |
|
allocations, THP and hugetlbfs pages. |
|
|
|
To make it sensible with respect to the watermark_scale_factor |
|
parameter, the unit is in fractions of 10,000. The default value of |
|
15,000 means that up to 150% of the high watermark will be reclaimed in the |
|
event of a pageblock being mixed due to fragmentation. The level of reclaim |
|
is determined by the number of fragmentation events that occurred in the |
|
recent past. If this value is smaller than a pageblock then a pageblocks |
|
worth of pages will be reclaimed (e.g. 2MB on 64-bit x86). A boost factor |
|
of 0 will disable the feature. |
|
|
|
|
|
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 3000, or 30% 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 OR'ed 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. |
|
|
|
Consider enabling one or more zone_reclaim mode bits 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 take additional actions 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.
|
|
|