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1 | Documentation for /proc/sys/vm/* kernel version 2.2.10 |
2 | (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> | |
3 | ||
4 | For general info and legal blurb, please look in README. | |
5 | ||
6 | ============================================================== | |
7 | ||
8 | This file contains the documentation for the sysctl files in | |
9 | /proc/sys/vm and is valid for Linux kernel version 2.2. | |
10 | ||
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. | |
14 | ||
15 | Default values and initialization routines for most of these | |
16 | files can be found in mm/swap.c. | |
17 | ||
18 | Currently, these files are in /proc/sys/vm: | |
19 | - overcommit_memory | |
20 | - page-cluster | |
21 | - dirty_ratio | |
22 | - dirty_background_ratio | |
23 | - dirty_expire_centisecs | |
24 | - dirty_writeback_centisecs | |
195cf453 | 25 | - highmem_is_dirtyable (only if CONFIG_HIGHMEM set) |
1da177e4 LT |
26 | - max_map_count |
27 | - min_free_kbytes | |
28 | - laptop_mode | |
29 | - block_dump | |
9d0243bc | 30 | - drop-caches |
1743660b | 31 | - zone_reclaim_mode |
9614634f | 32 | - min_unmapped_ratio |
0ff38490 | 33 | - min_slab_ratio |
fadd8fbd | 34 | - panic_on_oom |
fe071d7e | 35 | - oom_kill_allocating_task |
ed032189 | 36 | - mmap_min_address |
f0c0b2b8 | 37 | - numa_zonelist_order |
d5dbac87 NA |
38 | - nr_hugepages |
39 | - nr_overcommit_hugepages | |
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40 | |
41 | ============================================================== | |
42 | ||
43 | dirty_ratio, dirty_background_ratio, dirty_expire_centisecs, | |
195cf453 BG |
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: | |
1da177e4 LT |
47 | |
48 | See Documentation/filesystems/proc.txt | |
49 | ||
50 | ============================================================== | |
51 | ||
52 | overcommit_memory: | |
53 | ||
54 | This value contains a flag that enables memory overcommitment. | |
55 | ||
56 | When this flag is 0, the kernel attempts to estimate the amount | |
57 | of free memory left when userspace requests more memory. | |
58 | ||
59 | When this flag is 1, the kernel pretends there is always enough | |
60 | memory until it actually runs out. | |
61 | ||
62 | When this flag is 2, the kernel uses a "never overcommit" | |
63 | policy that attempts to prevent any overcommit of memory. | |
64 | ||
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. | |
68 | ||
69 | The default value is 0. | |
70 | ||
71 | See Documentation/vm/overcommit-accounting and | |
72 | security/commoncap.c::cap_vm_enough_memory() for more information. | |
73 | ||
74 | ============================================================== | |
75 | ||
76 | overcommit_ratio: | |
77 | ||
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. | |
81 | ||
82 | ============================================================== | |
83 | ||
84 | page-cluster: | |
85 | ||
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. | |
89 | ||
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. | |
93 | ||
94 | ============================================================== | |
95 | ||
96 | max_map_count: | |
97 | ||
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 | |
101 | libraries. | |
102 | ||
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. | |
106 | ||
107 | The default value is 65536. | |
108 | ||
109 | ============================================================== | |
110 | ||
111 | min_free_kbytes: | |
112 | ||
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. | |
8ad4b1fb | 117 | |
24950898 PM |
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. | |
121 | ||
122 | Setting this too high will OOM your machine instantly. | |
123 | ||
8ad4b1fb RS |
124 | ============================================================== |
125 | ||
126 | percpu_pagelist_fraction | |
127 | ||
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. | |
134 | ||
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) | |
137 | ||
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. | |
1743660b CL |
140 | |
141 | =============================================================== | |
142 | ||
143 | zone_reclaim_mode: | |
144 | ||
5d3f083d | 145 | Zone_reclaim_mode allows someone to set more or less aggressive approaches to |
1b2ffb78 CL |
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 | |
148 | in the system. | |
149 | ||
150 | This is value ORed together of | |
151 | ||
152 | 1 = Zone reclaim on | |
153 | 2 = Zone reclaim writes dirty pages out | |
154 | 4 = Zone reclaim swaps pages | |
155 | ||
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 | |
1743660b | 158 | page allocator will then reclaim easily reusable pages (those page |
1b2ffb78 CL |
159 | cache pages that are currently not used) before allocating off node pages. |
160 | ||
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 | |
164 | data locality. | |
165 | ||
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. | |
1743660b | 173 | |
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174 | Allowing regular swap effectively restricts allocations to the local |
175 | node unless explicitly overridden by memory policies or cpuset | |
176 | configurations. | |
1743660b | 177 | |
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178 | ============================================================= |
179 | ||
9614634f CL |
180 | min_unmapped_ratio: |
181 | ||
182 | This is available only on NUMA kernels. | |
183 | ||
0ff38490 | 184 | A percentage of the total pages in each zone. Zone reclaim will only |
9614634f CL |
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. | |
188 | ||
189 | The default is 1 percent. | |
190 | ||
191 | ============================================================= | |
192 | ||
0ff38490 CL |
193 | min_slab_ratio: |
194 | ||
195 | This is available only on NUMA kernels. | |
196 | ||
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. | |
202 | ||
203 | The default is 5 percent. | |
204 | ||
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 | |
207 | and may not be fast. | |
208 | ||
209 | ============================================================= | |
210 | ||
fadd8fbd KH |
211 | panic_on_oom |
212 | ||
2b744c01 | 213 | This enables or disables panic on out-of-memory feature. |
fadd8fbd | 214 | |
2b744c01 YG |
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 | |
217 | system will survive. | |
218 | ||
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. | |
fadd8fbd | 225 | |
2b744c01 YG |
226 | If this is set to 2, the kernel panics compulsorily even on the |
227 | above-mentioned. | |
228 | ||
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. | |
ed032189 | 232 | |
fe071d7e DR |
233 | ============================================================= |
234 | ||
235 | oom_kill_allocating_task | |
236 | ||
237 | This enables or disables killing the OOM-triggering task in | |
238 | out-of-memory situations. | |
239 | ||
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 | |
243 | memory when killed. | |
244 | ||
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 | |
247 | tasklist scan. | |
248 | ||
249 | If panic_on_oom is selected, it takes precedence over whatever value | |
250 | is used in oom_kill_allocating_task. | |
251 | ||
252 | The default value is 0. | |
253 | ||
ed032189 EP |
254 | ============================================================== |
255 | ||
256 | mmap_min_addr | |
257 | ||
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. | |
266 | ||
f0c0b2b8 KH |
267 | ============================================================== |
268 | ||
269 | numa_zonelist_order | |
270 | ||
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...) | |
275 | ||
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. | |
280 | ||
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 | |
283 | ||
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. | |
286 | ||
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. | |
290 | ||
291 | Type(B) cannot offer the best locality but is more robust against OOM of | |
292 | the DMA zone. | |
293 | ||
294 | Type(A) is called as "Node" order. Type (B) is "Zone" order. | |
295 | ||
296 | "Node order" orders the zonelists by node, then by zone within each node. | |
297 | Specify "[Nn]ode" for zone order | |
298 | ||
299 | "Zone Order" orders the zonelists by zone type, then by node within each | |
300 | zone. Specify "[Zz]one"for zode order. | |
301 | ||
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. | |
308 | ||
309 | Otherwise, "zone" order will be selected. Default order is recommended unless | |
310 | this is causing problems for your system/application. | |
d5dbac87 NA |
311 | |
312 | ============================================================== | |
313 | ||
314 | nr_hugepages | |
315 | ||
316 | Change the minimum size of the hugepage pool. | |
317 | ||
318 | See Documentation/vm/hugetlbpage.txt | |
319 | ||
320 | ============================================================== | |
321 | ||
322 | nr_overcommit_hugepages | |
323 | ||
324 | Change the maximum size of the hugepage pool. The maximum is | |
325 | nr_hugepages + nr_overcommit_hugepages. | |
326 | ||
327 | See Documentation/vm/hugetlbpage.txt |