1 Documentation for /proc/sys/vm/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
4 For general info and legal blurb, please look in README.
6 ==============================================================
8 This file contains the documentation for the sysctl files in
9 /proc/sys/vm and is valid for Linux kernel version 2.2.
11 The files in this directory can be used to tune the operation
12 of the virtual memory (VM) subsystem of the Linux kernel and
13 the writeout of dirty data to disk.
15 Default values and initialization routines for most of these
16 files can be found in mm/swap.c.
18 Currently, these files are in /proc/sys/vm:
22 - dirty_background_ratio
23 - dirty_expire_centisecs
24 - dirty_writeback_centisecs
31 - zone_reclaim_interval
33 ==============================================================
35 dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
36 dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode,
37 block_dump, swap_token_timeout, drop-caches:
39 See Documentation/filesystems/proc.txt
41 ==============================================================
45 This value contains a flag that enables memory overcommitment.
47 When this flag is 0, the kernel attempts to estimate the amount
48 of free memory left when userspace requests more memory.
50 When this flag is 1, the kernel pretends there is always enough
51 memory until it actually runs out.
53 When this flag is 2, the kernel uses a "never overcommit"
54 policy that attempts to prevent any overcommit of memory.
56 This feature can be very useful because there are a lot of
57 programs that malloc() huge amounts of memory "just-in-case"
58 and don't use much of it.
60 The default value is 0.
62 See Documentation/vm/overcommit-accounting and
63 security/commoncap.c::cap_vm_enough_memory() for more information.
65 ==============================================================
69 When overcommit_memory is set to 2, the committed address
70 space is not permitted to exceed swap plus this percentage
71 of physical RAM. See above.
73 ==============================================================
77 The Linux VM subsystem avoids excessive disk seeks by reading
78 multiple pages on a page fault. The number of pages it reads
79 is dependent on the amount of memory in your machine.
81 The number of pages the kernel reads in at once is equal to
82 2 ^ page-cluster. Values above 2 ^ 5 don't make much sense
83 for swap because we only cluster swap data in 32-page groups.
85 ==============================================================
89 This file contains the maximum number of memory map areas a process
90 may have. Memory map areas are used as a side-effect of calling
91 malloc, directly by mmap and mprotect, and also when loading shared
94 While most applications need less than a thousand maps, certain
95 programs, particularly malloc debuggers, may consume lots of them,
96 e.g., up to one or two maps per allocation.
98 The default value is 65536.
100 ==============================================================
104 This is used to force the Linux VM to keep a minimum number
105 of kilobytes free. The VM uses this number to compute a pages_min
106 value for each lowmem zone in the system. Each lowmem zone gets
107 a number of reserved free pages based proportionally on its size.
109 ==============================================================
111 percpu_pagelist_fraction
113 This is the fraction of pages at most (high mark pcp->high) in each zone that
114 are allocated for each per cpu page list. The min value for this is 8. It
115 means that we don't allow more than 1/8th of pages in each zone to be
116 allocated in any single per_cpu_pagelist. This entry only changes the value
117 of hot per cpu pagelists. User can specify a number like 100 to allocate
118 1/100th of each zone to each per cpu page list.
120 The batch value of each per cpu pagelist is also updated as a result. It is
121 set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
123 The initial value is zero. Kernel does not use this value at boot time to set
124 the high water marks for each per cpu page list.
126 ===============================================================
130 This is set during bootup to 1 if it is determined that pages from
131 remote zones will cause a significant performance reduction. The
132 page allocator will then reclaim easily reusable pages (those page
133 cache pages that are currently not used) before going off node.
135 The user can override this setting. It may be beneficial to switch
136 off zone reclaim if the system is used for a file server and all
137 of memory should be used for caching files from disk.
139 It may be beneficial to switch this on if one wants to do zone
140 reclaim regardless of the numa distances in the system.
141 ================================================================
143 zone_reclaim_interval:
145 The time allowed for off node allocations after zone reclaim
146 has failed to reclaim enough pages to allow a local allocation.
148 Time is set in seconds and set by default to 30 seconds.
150 Reduce the interval if undesired off node allocations occur. However, too
151 frequent scans will have a negative impact onoff node allocation performance.