308 lines
11 KiB
Plaintext
308 lines
11 KiB
Plaintext
Documentation for /proc/sys/vm/* kernel version 2.2.10
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(c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
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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 2.2.
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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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- overcommit_memory
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- page-cluster
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- dirty_ratio
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- dirty_background_ratio
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- dirty_expire_centisecs
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- dirty_writeback_centisecs
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- max_map_count
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- min_free_kbytes
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- laptop_mode
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- block_dump
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- drop-caches
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- zone_reclaim_mode
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- min_unmapped_ratio
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- min_slab_ratio
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- panic_on_oom
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- oom_kill_allocating_task
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- mmap_min_address
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- numa_zonelist_order
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==============================================================
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dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
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dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode,
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block_dump, swap_token_timeout, drop-caches,
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hugepages_treat_as_movable:
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See Documentation/filesystems/proc.txt
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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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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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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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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 and
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security/commoncap.c::cap_vm_enough_memory() for more information.
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==============================================================
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overcommit_ratio:
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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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page-cluster:
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The Linux VM subsystem avoids excessive disk seeks by reading
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multiple pages on a page fault. The number of pages it reads
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is dependent on the amount of memory in your machine.
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The number of pages the kernel reads in at once is equal to
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2 ^ page-cluster. Values above 2 ^ 5 don't make much sense
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for swap because we only cluster swap data in 32-page groups.
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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 and mprotect, and also when loading shared
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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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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 pages_min
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value for each lowmem zone in the system. Each lowmem zone gets
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a number of reserved free pages based proportionally on its size.
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Some minimal ammount 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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percpu_pagelist_fraction
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This is the fraction of pages at most (high mark pcp->high) in each zone that
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are allocated for each per cpu page list. The min value for this is 8. It
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means that we don't allow more than 1/8th of pages in each zone to be
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allocated in any single per_cpu_pagelist. This entry only changes the value
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of hot per cpu pagelists. User can specify a number like 100 to allocate
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1/100th of each zone to each per cpu page list.
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The batch value of each per cpu pagelist is also updated as a result. It is
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set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
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The initial value is zero. Kernel does not use this value at boot time to set
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the high water marks for each per cpu page list.
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===============================================================
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zone_reclaim_mode:
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Zone_reclaim_mode allows someone to set more or less aggressive approaches to
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reclaim memory when a zone runs out of memory. If it is set to zero then no
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zone reclaim occurs. Allocations will be satisfied from other zones / nodes
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in the system.
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This is value ORed together of
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1 = Zone reclaim on
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2 = Zone reclaim writes dirty pages out
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4 = Zone reclaim swaps pages
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zone_reclaim_mode is set during bootup to 1 if it is determined that pages
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from remote zones will cause a measurable performance reduction. The
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page allocator will then reclaim easily reusable pages (those page
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cache pages that are currently not used) before allocating off node pages.
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It may be beneficial to switch off zone reclaim if the system is
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used for a file server and all of memory should be used for caching files
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from disk. In that case the caching effect is more important than
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data locality.
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Allowing zone reclaim to write out pages stops processes that are
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writing large amounts of data from dirtying pages on other nodes. Zone
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reclaim will write out dirty pages if a zone fills up and so effectively
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throttle the process. This may decrease the performance of a single process
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since it cannot use all of system memory to buffer the outgoing writes
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anymore but it preserve the memory on other nodes so that the performance
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of other processes running on other nodes will not be affected.
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Allowing regular swap effectively restricts allocations to the local
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node unless explicitly overridden by memory policies or cpuset
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configurations.
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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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A percentage of the total pages in each zone. Zone reclaim will only
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occur if more than this percentage of pages are file backed and unmapped.
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This is to insure that a minimal amount of local pages is still available for
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file I/O even if the node is overallocated.
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The default is 1 percent.
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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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panic_on_oom
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This enables or disables panic on out-of-memory feature.
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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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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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If this is set to 2, the kernel panics compulsorily even on the
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above-mentioned.
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The default value is 0.
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1 and 2 are for failover of clustering. Please select either
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according to your policy of failover.
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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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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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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 mmaping. 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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numa_zonelist_order
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This sysctl is only for NUMA.
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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 zone 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 zode order.
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Specify "[Dd]efault" to request automatic configuration. Autoconfiguration
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will select "node" order in following case.
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(1) if the DMA zone does not exist or
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(2) if the DMA zone comprises greater than 50% of the available memory or
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(3) if any node's DMA zone comprises greater than 60% of its local memory and
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the amount of local memory is big enough.
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Otherwise, "zone" order will be selected. Default order is recommended unless
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this is causing problems for your system/application.
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