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
25 - highmem_is_dirtyable (only if CONFIG_HIGHMEM set)
35 - oom_kill_allocating_task
39 - nr_overcommit_hugepages
41 ==============================================================
43 dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
44 dirty_writeback_centisecs, highmem_is_dirtyable,
45 vfs_cache_pressure, laptop_mode, block_dump, swap_token_timeout,
46 drop-caches, hugepages_treat_as_movable:
48 See Documentation/filesystems/proc.txt
50 ==============================================================
54 This value contains a flag that enables memory overcommitment.
56 When this flag is 0, the kernel attempts to estimate the amount
57 of free memory left when userspace requests more memory.
59 When this flag is 1, the kernel pretends there is always enough
60 memory until it actually runs out.
62 When this flag is 2, the kernel uses a "never overcommit"
63 policy that attempts to prevent any overcommit of memory.
65 This feature can be very useful because there are a lot of
66 programs that malloc() huge amounts of memory "just-in-case"
67 and don't use much of it.
69 The default value is 0.
71 See Documentation/vm/overcommit-accounting and
72 security/commoncap.c::cap_vm_enough_memory() for more information.
74 ==============================================================
78 When overcommit_memory is set to 2, the committed address
79 space is not permitted to exceed swap plus this percentage
80 of physical RAM. See above.
82 ==============================================================
86 The Linux VM subsystem avoids excessive disk seeks by reading
87 multiple pages on a page fault. The number of pages it reads
88 is dependent on the amount of memory in your machine.
90 The number of pages the kernel reads in at once is equal to
91 2 ^ page-cluster. Values above 2 ^ 5 don't make much sense
92 for swap because we only cluster swap data in 32-page groups.
94 ==============================================================
98 This file contains the maximum number of memory map areas a process
99 may have. Memory map areas are used as a side-effect of calling
100 malloc, directly by mmap and mprotect, and also when loading shared
103 While most applications need less than a thousand maps, certain
104 programs, particularly malloc debuggers, may consume lots of them,
105 e.g., up to one or two maps per allocation.
107 The default value is 65536.
109 ==============================================================
113 This is used to force the Linux VM to keep a minimum number
114 of kilobytes free. The VM uses this number to compute a pages_min
115 value for each lowmem zone in the system. Each lowmem zone gets
116 a number of reserved free pages based proportionally on its size.
118 Some minimal ammount of memory is needed to satisfy PF_MEMALLOC
119 allocations; if you set this to lower than 1024KB, your system will
120 become subtly broken, and prone to deadlock under high loads.
122 Setting this too high will OOM your machine instantly.
124 ==============================================================
126 percpu_pagelist_fraction
128 This is the fraction of pages at most (high mark pcp->high) in each zone that
129 are allocated for each per cpu page list. The min value for this is 8. It
130 means that we don't allow more than 1/8th of pages in each zone to be
131 allocated in any single per_cpu_pagelist. This entry only changes the value
132 of hot per cpu pagelists. User can specify a number like 100 to allocate
133 1/100th of each zone to each per cpu page list.
135 The batch value of each per cpu pagelist is also updated as a result. It is
136 set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
138 The initial value is zero. Kernel does not use this value at boot time to set
139 the high water marks for each per cpu page list.
141 ===============================================================
145 Zone_reclaim_mode allows someone to set more or less aggressive approaches to
146 reclaim memory when a zone runs out of memory. If it is set to zero then no
147 zone reclaim occurs. Allocations will be satisfied from other zones / nodes
150 This is value ORed together of
153 2 = Zone reclaim writes dirty pages out
154 4 = Zone reclaim swaps pages
156 zone_reclaim_mode is set during bootup to 1 if it is determined that pages
157 from remote zones will cause a measurable performance reduction. The
158 page allocator will then reclaim easily reusable pages (those page
159 cache pages that are currently not used) before allocating off node pages.
161 It may be beneficial to switch off zone reclaim if the system is
162 used for a file server and all of memory should be used for caching files
163 from disk. In that case the caching effect is more important than
166 Allowing zone reclaim to write out pages stops processes that are
167 writing large amounts of data from dirtying pages on other nodes. Zone
168 reclaim will write out dirty pages if a zone fills up and so effectively
169 throttle the process. This may decrease the performance of a single process
170 since it cannot use all of system memory to buffer the outgoing writes
171 anymore but it preserve the memory on other nodes so that the performance
172 of other processes running on other nodes will not be affected.
174 Allowing regular swap effectively restricts allocations to the local
175 node unless explicitly overridden by memory policies or cpuset
178 =============================================================
182 This is available only on NUMA kernels.
184 A percentage of the total pages in each zone. Zone reclaim will only
185 occur if more than this percentage of pages are file backed and unmapped.
186 This is to insure that a minimal amount of local pages is still available for
187 file I/O even if the node is overallocated.
189 The default is 1 percent.
191 =============================================================
195 This is available only on NUMA kernels.
197 A percentage of the total pages in each zone. On Zone reclaim
198 (fallback from the local zone occurs) slabs will be reclaimed if more
199 than this percentage of pages in a zone are reclaimable slab pages.
200 This insures that the slab growth stays under control even in NUMA
201 systems that rarely perform global reclaim.
203 The default is 5 percent.
205 Note that slab reclaim is triggered in a per zone / node fashion.
206 The process of reclaiming slab memory is currently not node specific
209 =============================================================
213 This enables or disables panic on out-of-memory feature.
215 If this is set to 0, the kernel will kill some rogue process,
216 called oom_killer. Usually, oom_killer can kill rogue processes and
219 If this is set to 1, the kernel panics when out-of-memory happens.
220 However, if a process limits using nodes by mempolicy/cpusets,
221 and those nodes become memory exhaustion status, one process
222 may be killed by oom-killer. No panic occurs in this case.
223 Because other nodes' memory may be free. This means system total status
224 may be not fatal yet.
226 If this is set to 2, the kernel panics compulsorily even on the
229 The default value is 0.
230 1 and 2 are for failover of clustering. Please select either
231 according to your policy of failover.
233 =============================================================
235 oom_kill_allocating_task
237 This enables or disables killing the OOM-triggering task in
238 out-of-memory situations.
240 If this is set to zero, the OOM killer will scan through the entire
241 tasklist and select a task based on heuristics to kill. This normally
242 selects a rogue memory-hogging task that frees up a large amount of
245 If this is set to non-zero, the OOM killer simply kills the task that
246 triggered the out-of-memory condition. This avoids the expensive
249 If panic_on_oom is selected, it takes precedence over whatever value
250 is used in oom_kill_allocating_task.
252 The default value is 0.
254 ==============================================================
258 This file indicates the amount of address space which a user process will
259 be restricted from mmaping. Since kernel null dereference bugs could
260 accidentally operate based on the information in the first couple of pages
261 of memory userspace processes should not be allowed to write to them. By
262 default this value is set to 0 and no protections will be enforced by the
263 security module. Setting this value to something like 64k will allow the
264 vast majority of applications to work correctly and provide defense in depth
265 against future potential kernel bugs.
267 ==============================================================
271 This sysctl is only for NUMA.
272 'where the memory is allocated from' is controlled by zonelists.
273 (This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
274 you may be able to read ZONE_DMA as ZONE_DMA32...)
276 In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
277 ZONE_NORMAL -> ZONE_DMA
278 This means that a memory allocation request for GFP_KERNEL will
279 get memory from ZONE_DMA only when ZONE_NORMAL is not available.
281 In NUMA case, you can think of following 2 types of order.
282 Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
284 (A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
285 (B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
287 Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
288 will be used before ZONE_NORMAL exhaustion. This increases possibility of
289 out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
291 Type(B) cannot offer the best locality but is more robust against OOM of
294 Type(A) is called as "Node" order. Type (B) is "Zone" order.
296 "Node order" orders the zonelists by node, then by zone within each node.
297 Specify "[Nn]ode" for zone order
299 "Zone Order" orders the zonelists by zone type, then by node within each
300 zone. Specify "[Zz]one"for zode order.
302 Specify "[Dd]efault" to request automatic configuration. Autoconfiguration
303 will select "node" order in following case.
304 (1) if the DMA zone does not exist or
305 (2) if the DMA zone comprises greater than 50% of the available memory or
306 (3) if any node's DMA zone comprises greater than 60% of its local memory and
307 the amount of local memory is big enough.
309 Otherwise, "zone" order will be selected. Default order is recommended unless
310 this is causing problems for your system/application.
312 ==============================================================
316 Change the minimum size of the hugepage pool.
318 See Documentation/vm/hugetlbpage.txt
320 ==============================================================
322 nr_overcommit_hugepages
324 Change the maximum size of the hugepage pool. The maximum is
325 nr_hugepages + nr_overcommit_hugepages.
327 See Documentation/vm/hugetlbpage.txt