| Documentation for /proc/sys/vm/* kernel version 2.2.10 |
| (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 |
| /proc/sys/vm and is valid for Linux kernel version 2.2. |
| |
| The files in this directory can be used to tune the operation |
| of the virtual memory (VM) subsystem of the Linux kernel and |
| the writeout of dirty data to disk. |
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| Default values and initialization routines for most of these |
| files can be found in mm/swap.c. |
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| Currently, these files are in /proc/sys/vm: |
| - overcommit_memory |
| - page-cluster |
| - dirty_ratio |
| - dirty_background_ratio |
| - dirty_expire_centisecs |
| - dirty_writeback_centisecs |
| - max_map_count |
| - min_free_kbytes |
| - laptop_mode |
| - block_dump |
| - drop-caches |
| - zone_reclaim_mode |
| - panic_on_oom |
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| ============================================================== |
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| dirty_ratio, dirty_background_ratio, dirty_expire_centisecs, |
| dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode, |
| block_dump, swap_token_timeout, drop-caches: |
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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 |
| 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 |
| memory until it actually runs out. |
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| When this flag is 2, the kernel uses a "never overcommit" |
| 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 |
| programs that malloc() huge amounts of memory "just-in-case" |
| and don't use much of it. |
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| The default value is 0. |
| |
| See Documentation/vm/overcommit-accounting and |
| 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 |
| space is not permitted to exceed swap plus this percentage |
| 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 |
| multiple pages on a page fault. The number of pages it reads |
| 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 |
| 2 ^ page-cluster. Values above 2 ^ 5 don't make much sense |
| 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 |
| may have. Memory map areas are used as a side-effect of calling |
| malloc, directly by mmap and mprotect, and also when loading shared |
| libraries. |
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| While most applications need less than a thousand maps, certain |
| programs, particularly malloc debuggers, may consume lots of them, |
| e.g., up to one or two maps per allocation. |
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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 |
| of kilobytes free. The VM uses this number to compute a pages_min |
| value for each lowmem zone in the system. Each lowmem zone gets |
| a number of reserved free pages based proportionally on its size. |
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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 |
| 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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| 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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| 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. |
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| =============================================================== |
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| zone_reclaim_mode: |
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| Zone_reclaim_mode allows to set more or less agressive 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. |
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| This is value ORed together of |
| |
| 1 = Zone reclaim on |
| 2 = Zone reclaim writes dirty pages out |
| 4 = Zone reclaim swaps pages |
| 8 = Also do a global slab reclaim pass |
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| zone_reclaim_mode is set during bootup to 1 if it is determined that pages |
| from remote zones will cause a measurable performance reduction. The |
| page allocator will then reclaim easily reusable pages (those page |
| cache pages that are currently not used) before allocating off node pages. |
| |
| It may be beneficial to switch off zone reclaim if the system is |
| used for a file server and all of memory should be used for caching files |
| from disk. In that case the caching effect is more important than |
| data locality. |
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| 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. |
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| It may be advisable to allow slab reclaim if the system makes heavy |
| use of files and builds up large slab caches. However, the slab |
| shrink operation is global, may take a long time and free slabs |
| in all nodes of the system. |
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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. If this is set to 1, |
| the kernel panics when out-of-memory happens. 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 you want to panic the system |
| rather than killing rogue processes, set this to 1. |
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| The default value is 0. |
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