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00f0b825 BS |
1 | Memory Resource Controller |
2 | ||
3 | NOTE: The Memory Resource Controller has been generically been referred | |
4 | to as the memory controller in this document. Do not confuse memory controller | |
5 | used here with the memory controller that is used in hardware. | |
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6 | |
7 | Salient features | |
8 | ||
9 | a. Enable control of both RSS (mapped) and Page Cache (unmapped) pages | |
10 | b. The infrastructure allows easy addition of other types of memory to control | |
11 | c. Provides *zero overhead* for non memory controller users | |
12 | d. Provides a double LRU: global memory pressure causes reclaim from the | |
13 | global LRU; a cgroup on hitting a limit, reclaims from the per | |
14 | cgroup LRU | |
15 | ||
dfc05c25 | 16 | NOTE: Swap Cache (unmapped) is not accounted now. |
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17 | |
18 | Benefits and Purpose of the memory controller | |
19 | ||
20 | The memory controller isolates the memory behaviour of a group of tasks | |
21 | from the rest of the system. The article on LWN [12] mentions some probable | |
22 | uses of the memory controller. The memory controller can be used to | |
23 | ||
24 | a. Isolate an application or a group of applications | |
25 | Memory hungry applications can be isolated and limited to a smaller | |
26 | amount of memory. | |
27 | b. Create a cgroup with limited amount of memory, this can be used | |
28 | as a good alternative to booting with mem=XXXX. | |
29 | c. Virtualization solutions can control the amount of memory they want | |
30 | to assign to a virtual machine instance. | |
31 | d. A CD/DVD burner could control the amount of memory used by the | |
32 | rest of the system to ensure that burning does not fail due to lack | |
33 | of available memory. | |
34 | e. There are several other use cases, find one or use the controller just | |
35 | for fun (to learn and hack on the VM subsystem). | |
36 | ||
37 | 1. History | |
38 | ||
39 | The memory controller has a long history. A request for comments for the memory | |
40 | controller was posted by Balbir Singh [1]. At the time the RFC was posted | |
41 | there were several implementations for memory control. The goal of the | |
42 | RFC was to build consensus and agreement for the minimal features required | |
43 | for memory control. The first RSS controller was posted by Balbir Singh[2] | |
44 | in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the | |
45 | RSS controller. At OLS, at the resource management BoF, everyone suggested | |
46 | that we handle both page cache and RSS together. Another request was raised | |
47 | to allow user space handling of OOM. The current memory controller is | |
48 | at version 6; it combines both mapped (RSS) and unmapped Page | |
49 | Cache Control [11]. | |
50 | ||
51 | 2. Memory Control | |
52 | ||
53 | Memory is a unique resource in the sense that it is present in a limited | |
54 | amount. If a task requires a lot of CPU processing, the task can spread | |
55 | its processing over a period of hours, days, months or years, but with | |
56 | memory, the same physical memory needs to be reused to accomplish the task. | |
57 | ||
58 | The memory controller implementation has been divided into phases. These | |
59 | are: | |
60 | ||
61 | 1. Memory controller | |
62 | 2. mlock(2) controller | |
63 | 3. Kernel user memory accounting and slab control | |
64 | 4. user mappings length controller | |
65 | ||
66 | The memory controller is the first controller developed. | |
67 | ||
68 | 2.1. Design | |
69 | ||
70 | The core of the design is a counter called the res_counter. The res_counter | |
71 | tracks the current memory usage and limit of the group of processes associated | |
72 | with the controller. Each cgroup has a memory controller specific data | |
73 | structure (mem_cgroup) associated with it. | |
74 | ||
75 | 2.2. Accounting | |
76 | ||
77 | +--------------------+ | |
78 | | mem_cgroup | | |
79 | | (res_counter) | | |
80 | +--------------------+ | |
81 | / ^ \ | |
82 | / | \ | |
83 | +---------------+ | +---------------+ | |
84 | | mm_struct | |.... | mm_struct | | |
85 | | | | | | | |
86 | +---------------+ | +---------------+ | |
87 | | | |
88 | + --------------+ | |
89 | | | |
90 | +---------------+ +------+--------+ | |
91 | | page +----------> page_cgroup| | |
92 | | | | | | |
93 | +---------------+ +---------------+ | |
94 | ||
95 | (Figure 1: Hierarchy of Accounting) | |
96 | ||
97 | ||
98 | Figure 1 shows the important aspects of the controller | |
99 | ||
100 | 1. Accounting happens per cgroup | |
101 | 2. Each mm_struct knows about which cgroup it belongs to | |
102 | 3. Each page has a pointer to the page_cgroup, which in turn knows the | |
103 | cgroup it belongs to | |
104 | ||
105 | The accounting is done as follows: mem_cgroup_charge() is invoked to setup | |
106 | the necessary data structures and check if the cgroup that is being charged | |
107 | is over its limit. If it is then reclaim is invoked on the cgroup. | |
108 | More details can be found in the reclaim section of this document. | |
109 | If everything goes well, a page meta-data-structure called page_cgroup is | |
110 | allocated and associated with the page. This routine also adds the page to | |
111 | the per cgroup LRU. | |
112 | ||
113 | 2.2.1 Accounting details | |
114 | ||
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115 | All mapped anon pages (RSS) and cache pages (Page Cache) are accounted. |
116 | (some pages which never be reclaimable and will not be on global LRU | |
117 | are not accounted. we just accounts pages under usual vm management.) | |
118 | ||
119 | RSS pages are accounted at page_fault unless they've already been accounted | |
120 | for earlier. A file page will be accounted for as Page Cache when it's | |
121 | inserted into inode (radix-tree). While it's mapped into the page tables of | |
122 | processes, duplicate accounting is carefully avoided. | |
123 | ||
124 | A RSS page is unaccounted when it's fully unmapped. A PageCache page is | |
125 | unaccounted when it's removed from radix-tree. | |
126 | ||
127 | At page migration, accounting information is kept. | |
128 | ||
129 | Note: we just account pages-on-lru because our purpose is to control amount | |
130 | of used pages. not-on-lru pages are tend to be out-of-control from vm view. | |
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131 | |
132 | 2.3 Shared Page Accounting | |
133 | ||
134 | Shared pages are accounted on the basis of the first touch approach. The | |
135 | cgroup that first touches a page is accounted for the page. The principle | |
136 | behind this approach is that a cgroup that aggressively uses a shared | |
137 | page will eventually get charged for it (once it is uncharged from | |
138 | the cgroup that brought it in -- this will happen on memory pressure). | |
139 | ||
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140 | Exception: If CONFIG_CGROUP_CGROUP_MEM_RES_CTLR_SWAP is not used.. |
141 | When you do swapoff and make swapped-out pages of shmem(tmpfs) to | |
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142 | be backed into memory in force, charges for pages are accounted against the |
143 | caller of swapoff rather than the users of shmem. | |
144 | ||
145 | ||
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146 | 2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP) |
147 | Swap Extension allows you to record charge for swap. A swapped-in page is | |
148 | charged back to original page allocator if possible. | |
149 | ||
150 | When swap is accounted, following files are added. | |
151 | - memory.memsw.usage_in_bytes. | |
152 | - memory.memsw.limit_in_bytes. | |
153 | ||
154 | usage of mem+swap is limited by memsw.limit_in_bytes. | |
155 | ||
156 | Note: why 'mem+swap' rather than swap. | |
157 | The global LRU(kswapd) can swap out arbitrary pages. Swap-out means | |
158 | to move account from memory to swap...there is no change in usage of | |
159 | mem+swap. | |
160 | ||
161 | In other words, when we want to limit the usage of swap without affecting | |
162 | global LRU, mem+swap limit is better than just limiting swap from OS point | |
163 | of view. | |
164 | ||
165 | 2.5 Reclaim | |
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166 | |
167 | Each cgroup maintains a per cgroup LRU that consists of an active | |
168 | and inactive list. When a cgroup goes over its limit, we first try | |
169 | to reclaim memory from the cgroup so as to make space for the new | |
170 | pages that the cgroup has touched. If the reclaim is unsuccessful, | |
171 | an OOM routine is invoked to select and kill the bulkiest task in the | |
172 | cgroup. | |
173 | ||
174 | The reclaim algorithm has not been modified for cgroups, except that | |
175 | pages that are selected for reclaiming come from the per cgroup LRU | |
176 | list. | |
177 | ||
178 | 2. Locking | |
179 | ||
180 | The memory controller uses the following hierarchy | |
181 | ||
182 | 1. zone->lru_lock is used for selecting pages to be isolated | |
dfc05c25 | 183 | 2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone) |
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184 | 3. lock_page_cgroup() is used to protect page->page_cgroup |
185 | ||
186 | 3. User Interface | |
187 | ||
188 | 0. Configuration | |
189 | ||
190 | a. Enable CONFIG_CGROUPS | |
191 | b. Enable CONFIG_RESOURCE_COUNTERS | |
00f0b825 | 192 | c. Enable CONFIG_CGROUP_MEM_RES_CTLR |
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193 | |
194 | 1. Prepare the cgroups | |
195 | # mkdir -p /cgroups | |
196 | # mount -t cgroup none /cgroups -o memory | |
197 | ||
198 | 2. Make the new group and move bash into it | |
199 | # mkdir /cgroups/0 | |
200 | # echo $$ > /cgroups/0/tasks | |
201 | ||
202 | Since now we're in the 0 cgroup, | |
203 | We can alter the memory limit: | |
fb78922c | 204 | # echo 4M > /cgroups/0/memory.limit_in_bytes |
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205 | |
206 | NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo, | |
207 | mega or gigabytes. | |
208 | ||
209 | # cat /cgroups/0/memory.limit_in_bytes | |
2324c5dd | 210 | 4194304 |
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211 | |
212 | NOTE: The interface has now changed to display the usage in bytes | |
213 | instead of pages | |
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214 | |
215 | We can check the usage: | |
0eea1030 | 216 | # cat /cgroups/0/memory.usage_in_bytes |
2324c5dd | 217 | 1216512 |
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218 | |
219 | A successful write to this file does not guarantee a successful set of | |
220 | this limit to the value written into the file. This can be due to a | |
221 | number of factors, such as rounding up to page boundaries or the total | |
222 | availability of memory on the system. The user is required to re-read | |
223 | this file after a write to guarantee the value committed by the kernel. | |
224 | ||
fb78922c | 225 | # echo 1 > memory.limit_in_bytes |
0eea1030 | 226 | # cat memory.limit_in_bytes |
2324c5dd | 227 | 4096 |
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228 | |
229 | The memory.failcnt field gives the number of times that the cgroup limit was | |
230 | exceeded. | |
231 | ||
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232 | The memory.stat file gives accounting information. Now, the number of |
233 | caches, RSS and Active pages/Inactive pages are shown. | |
234 | ||
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235 | 4. Testing |
236 | ||
237 | Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11]. | |
238 | Apart from that v6 has been tested with several applications and regular | |
239 | daily use. The controller has also been tested on the PPC64, x86_64 and | |
240 | UML platforms. | |
241 | ||
242 | 4.1 Troubleshooting | |
243 | ||
244 | Sometimes a user might find that the application under a cgroup is | |
245 | terminated. There are several causes for this: | |
246 | ||
247 | 1. The cgroup limit is too low (just too low to do anything useful) | |
248 | 2. The user is using anonymous memory and swap is turned off or too low | |
249 | ||
250 | A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of | |
251 | some of the pages cached in the cgroup (page cache pages). | |
252 | ||
253 | 4.2 Task migration | |
254 | ||
255 | When a task migrates from one cgroup to another, it's charge is not | |
256 | carried forward. The pages allocated from the original cgroup still | |
257 | remain charged to it, the charge is dropped when the page is freed or | |
258 | reclaimed. | |
259 | ||
260 | 4.3 Removing a cgroup | |
261 | ||
262 | A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a | |
263 | cgroup might have some charge associated with it, even though all | |
f817ed48 | 264 | tasks have migrated away from it. |
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265 | Such charges are freed(at default) or moved to its parent. When moved, |
266 | both of RSS and CACHES are moved to parent. | |
267 | If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also. | |
1b6df3aa | 268 | |
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269 | Charges recorded in swap information is not updated at removal of cgroup. |
270 | Recorded information is discarded and a cgroup which uses swap (swapcache) | |
271 | will be charged as a new owner of it. | |
272 | ||
273 | ||
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274 | 5. Misc. interfaces. |
275 | ||
276 | 5.1 force_empty | |
277 | memory.force_empty interface is provided to make cgroup's memory usage empty. | |
278 | You can use this interface only when the cgroup has no tasks. | |
279 | When writing anything to this | |
280 | ||
281 | # echo 0 > memory.force_empty | |
282 | ||
283 | Almost all pages tracked by this memcg will be unmapped and freed. Some of | |
284 | pages cannot be freed because it's locked or in-use. Such pages are moved | |
285 | to parent and this cgroup will be empty. But this may return -EBUSY in | |
286 | some too busy case. | |
287 | ||
288 | Typical use case of this interface is that calling this before rmdir(). | |
289 | Because rmdir() moves all pages to parent, some out-of-use page caches can be | |
290 | moved to the parent. If you want to avoid that, force_empty will be useful. | |
291 | ||
52bc0d82 | 292 | 6. Hierarchy support |
c1e862c1 | 293 | |
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294 | The memory controller supports a deep hierarchy and hierarchical accounting. |
295 | The hierarchy is created by creating the appropriate cgroups in the | |
296 | cgroup filesystem. Consider for example, the following cgroup filesystem | |
297 | hierarchy | |
298 | ||
299 | root | |
300 | / | \ | |
301 | / | \ | |
302 | a b c | |
303 | | \ | |
304 | | \ | |
305 | d e | |
306 | ||
307 | In the diagram above, with hierarchical accounting enabled, all memory | |
308 | usage of e, is accounted to its ancestors up until the root (i.e, c and root), | |
309 | that has memory.use_hierarchy enabled. If one of the ancestors goes over its | |
310 | limit, the reclaim algorithm reclaims from the tasks in the ancestor and the | |
311 | children of the ancestor. | |
312 | ||
313 | 6.1 Enabling hierarchical accounting and reclaim | |
314 | ||
315 | The memory controller by default disables the hierarchy feature. Support | |
316 | can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup | |
317 | ||
318 | # echo 1 > memory.use_hierarchy | |
319 | ||
320 | The feature can be disabled by | |
321 | ||
322 | # echo 0 > memory.use_hierarchy | |
323 | ||
324 | NOTE1: Enabling/disabling will fail if the cgroup already has other | |
325 | cgroups created below it. | |
326 | ||
327 | NOTE2: This feature can be enabled/disabled per subtree. | |
328 | ||
329 | 7. TODO | |
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330 | |
331 | 1. Add support for accounting huge pages (as a separate controller) | |
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332 | 2. Make per-cgroup scanner reclaim not-shared pages first |
333 | 3. Teach controller to account for shared-pages | |
628f4235 | 334 | 4. Start reclamation in the background when the limit is |
1b6df3aa | 335 | not yet hit but the usage is getting closer |
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336 | |
337 | Summary | |
338 | ||
339 | Overall, the memory controller has been a stable controller and has been | |
340 | commented and discussed quite extensively in the community. | |
341 | ||
342 | References | |
343 | ||
344 | 1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/ | |
345 | 2. Singh, Balbir. Memory Controller (RSS Control), | |
346 | http://lwn.net/Articles/222762/ | |
347 | 3. Emelianov, Pavel. Resource controllers based on process cgroups | |
348 | http://lkml.org/lkml/2007/3/6/198 | |
349 | 4. Emelianov, Pavel. RSS controller based on process cgroups (v2) | |
2324c5dd | 350 | http://lkml.org/lkml/2007/4/9/78 |
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351 | 5. Emelianov, Pavel. RSS controller based on process cgroups (v3) |
352 | http://lkml.org/lkml/2007/5/30/244 | |
353 | 6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/ | |
354 | 7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control | |
355 | subsystem (v3), http://lwn.net/Articles/235534/ | |
2324c5dd | 356 | 8. Singh, Balbir. RSS controller v2 test results (lmbench), |
1b6df3aa | 357 | http://lkml.org/lkml/2007/5/17/232 |
2324c5dd | 358 | 9. Singh, Balbir. RSS controller v2 AIM9 results |
1b6df3aa | 359 | http://lkml.org/lkml/2007/5/18/1 |
2324c5dd | 360 | 10. Singh, Balbir. Memory controller v6 test results, |
1b6df3aa | 361 | http://lkml.org/lkml/2007/8/19/36 |
2324c5dd LZ |
362 | 11. Singh, Balbir. Memory controller introduction (v6), |
363 | http://lkml.org/lkml/2007/8/17/69 | |
1b6df3aa BS |
364 | 12. Corbet, Jonathan, Controlling memory use in cgroups, |
365 | http://lwn.net/Articles/243795/ |