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00f0b825 BS |
1 | Memory Resource Controller |
2 | ||
1306a85a JW |
3 | NOTE: This document is hopelessly outdated and it asks for a complete |
4 | rewrite. It still contains a useful information so we are keeping it | |
5 | here but make sure to check the current code if you need a deeper | |
6 | understanding. | |
7 | ||
67de0162 JS |
8 | NOTE: The Memory Resource Controller has generically been referred to as the |
9 | memory controller in this document. Do not confuse memory controller | |
10 | used here with the memory controller that is used in hardware. | |
1b6df3aa | 11 | |
dc10e281 KH |
12 | (For editors) |
13 | In this document: | |
14 | When we mention a cgroup (cgroupfs's directory) with memory controller, | |
15 | we call it "memory cgroup". When you see git-log and source code, you'll | |
16 | see patch's title and function names tend to use "memcg". | |
17 | In this document, we avoid using it. | |
1b6df3aa | 18 | |
1b6df3aa BS |
19 | Benefits and Purpose of the memory controller |
20 | ||
21 | The memory controller isolates the memory behaviour of a group of tasks | |
22 | from the rest of the system. The article on LWN [12] mentions some probable | |
23 | uses of the memory controller. The memory controller can be used to | |
24 | ||
25 | a. Isolate an application or a group of applications | |
1939c557 | 26 | Memory-hungry applications can be isolated and limited to a smaller |
1b6df3aa | 27 | amount of memory. |
1939c557 | 28 | b. Create a cgroup with a limited amount of memory; this can be used |
1b6df3aa BS |
29 | as a good alternative to booting with mem=XXXX. |
30 | c. Virtualization solutions can control the amount of memory they want | |
31 | to assign to a virtual machine instance. | |
32 | d. A CD/DVD burner could control the amount of memory used by the | |
33 | rest of the system to ensure that burning does not fail due to lack | |
34 | of available memory. | |
1939c557 | 35 | e. There are several other use cases; find one or use the controller just |
1b6df3aa BS |
36 | for fun (to learn and hack on the VM subsystem). |
37 | ||
dc10e281 KH |
38 | Current Status: linux-2.6.34-mmotm(development version of 2010/April) |
39 | ||
40 | Features: | |
41 | - accounting anonymous pages, file caches, swap caches usage and limiting them. | |
6252efcc | 42 | - pages are linked to per-memcg LRU exclusively, and there is no global LRU. |
dc10e281 KH |
43 | - optionally, memory+swap usage can be accounted and limited. |
44 | - hierarchical accounting | |
45 | - soft limit | |
1939c557 | 46 | - moving (recharging) account at moving a task is selectable. |
dc10e281 | 47 | - usage threshold notifier |
70ddf637 | 48 | - memory pressure notifier |
dc10e281 KH |
49 | - oom-killer disable knob and oom-notifier |
50 | - Root cgroup has no limit controls. | |
51 | ||
1939c557 | 52 | Kernel memory support is a work in progress, and the current version provides |
65c64ce8 | 53 | basically functionality. (See Section 2.7) |
dc10e281 KH |
54 | |
55 | Brief summary of control files. | |
56 | ||
57 | tasks # attach a task(thread) and show list of threads | |
58 | cgroup.procs # show list of processes | |
59 | cgroup.event_control # an interface for event_fd() | |
3e32cb2e | 60 | memory.usage_in_bytes # show current usage for memory |
a111c966 | 61 | (See 5.5 for details) |
3e32cb2e | 62 | memory.memsw.usage_in_bytes # show current usage for memory+Swap |
a111c966 | 63 | (See 5.5 for details) |
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64 | memory.limit_in_bytes # set/show limit of memory usage |
65 | memory.memsw.limit_in_bytes # set/show limit of memory+Swap usage | |
66 | memory.failcnt # show the number of memory usage hits limits | |
67 | memory.memsw.failcnt # show the number of memory+Swap hits limits | |
68 | memory.max_usage_in_bytes # show max memory usage recorded | |
d66c1ce7 | 69 | memory.memsw.max_usage_in_bytes # show max memory+Swap usage recorded |
dc10e281 KH |
70 | memory.soft_limit_in_bytes # set/show soft limit of memory usage |
71 | memory.stat # show various statistics | |
72 | memory.use_hierarchy # set/show hierarchical account enabled | |
73 | memory.force_empty # trigger forced move charge to parent | |
70ddf637 | 74 | memory.pressure_level # set memory pressure notifications |
dc10e281 KH |
75 | memory.swappiness # set/show swappiness parameter of vmscan |
76 | (See sysctl's vm.swappiness) | |
77 | memory.move_charge_at_immigrate # set/show controls of moving charges | |
78 | memory.oom_control # set/show oom controls. | |
50c35e5b | 79 | memory.numa_stat # show the number of memory usage per numa node |
dc10e281 | 80 | |
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81 | memory.kmem.limit_in_bytes # set/show hard limit for kernel memory |
82 | memory.kmem.usage_in_bytes # show current kernel memory allocation | |
83 | memory.kmem.failcnt # show the number of kernel memory usage hits limits | |
84 | memory.kmem.max_usage_in_bytes # show max kernel memory usage recorded | |
85 | ||
3aaabe23 | 86 | memory.kmem.tcp.limit_in_bytes # set/show hard limit for tcp buf memory |
5a6dd343 | 87 | memory.kmem.tcp.usage_in_bytes # show current tcp buf memory allocation |
05a73ed2 WL |
88 | memory.kmem.tcp.failcnt # show the number of tcp buf memory usage hits limits |
89 | memory.kmem.tcp.max_usage_in_bytes # show max tcp buf memory usage recorded | |
e5671dfa | 90 | |
1b6df3aa BS |
91 | 1. History |
92 | ||
93 | The memory controller has a long history. A request for comments for the memory | |
94 | controller was posted by Balbir Singh [1]. At the time the RFC was posted | |
95 | there were several implementations for memory control. The goal of the | |
96 | RFC was to build consensus and agreement for the minimal features required | |
97 | for memory control. The first RSS controller was posted by Balbir Singh[2] | |
98 | in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the | |
99 | RSS controller. At OLS, at the resource management BoF, everyone suggested | |
100 | that we handle both page cache and RSS together. Another request was raised | |
101 | to allow user space handling of OOM. The current memory controller is | |
102 | at version 6; it combines both mapped (RSS) and unmapped Page | |
103 | Cache Control [11]. | |
104 | ||
105 | 2. Memory Control | |
106 | ||
107 | Memory is a unique resource in the sense that it is present in a limited | |
108 | amount. If a task requires a lot of CPU processing, the task can spread | |
109 | its processing over a period of hours, days, months or years, but with | |
110 | memory, the same physical memory needs to be reused to accomplish the task. | |
111 | ||
112 | The memory controller implementation has been divided into phases. These | |
113 | are: | |
114 | ||
115 | 1. Memory controller | |
116 | 2. mlock(2) controller | |
117 | 3. Kernel user memory accounting and slab control | |
118 | 4. user mappings length controller | |
119 | ||
120 | The memory controller is the first controller developed. | |
121 | ||
122 | 2.1. Design | |
123 | ||
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124 | The core of the design is a counter called the page_counter. The |
125 | page_counter tracks the current memory usage and limit of the group of | |
126 | processes associated with the controller. Each cgroup has a memory controller | |
127 | specific data structure (mem_cgroup) associated with it. | |
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128 | |
129 | 2.2. Accounting | |
130 | ||
131 | +--------------------+ | |
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132 | | mem_cgroup | |
133 | | (page_counter) | | |
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134 | +--------------------+ |
135 | / ^ \ | |
136 | / | \ | |
137 | +---------------+ | +---------------+ | |
138 | | mm_struct | |.... | mm_struct | | |
139 | | | | | | | |
140 | +---------------+ | +---------------+ | |
141 | | | |
142 | + --------------+ | |
143 | | | |
144 | +---------------+ +------+--------+ | |
145 | | page +----------> page_cgroup| | |
146 | | | | | | |
147 | +---------------+ +---------------+ | |
148 | ||
149 | (Figure 1: Hierarchy of Accounting) | |
150 | ||
151 | ||
152 | Figure 1 shows the important aspects of the controller | |
153 | ||
154 | 1. Accounting happens per cgroup | |
155 | 2. Each mm_struct knows about which cgroup it belongs to | |
156 | 3. Each page has a pointer to the page_cgroup, which in turn knows the | |
157 | cgroup it belongs to | |
158 | ||
348b4655 JL |
159 | The accounting is done as follows: mem_cgroup_charge_common() is invoked to |
160 | set up the necessary data structures and check if the cgroup that is being | |
161 | charged is over its limit. If it is, then reclaim is invoked on the cgroup. | |
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162 | More details can be found in the reclaim section of this document. |
163 | If everything goes well, a page meta-data-structure called page_cgroup is | |
dc10e281 KH |
164 | updated. page_cgroup has its own LRU on cgroup. |
165 | (*) page_cgroup structure is allocated at boot/memory-hotplug time. | |
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166 | |
167 | 2.2.1 Accounting details | |
168 | ||
5b4e655e | 169 | All mapped anon pages (RSS) and cache pages (Page Cache) are accounted. |
6252efcc | 170 | Some pages which are never reclaimable and will not be on the LRU |
dc10e281 | 171 | are not accounted. We just account pages under usual VM management. |
5b4e655e KH |
172 | |
173 | RSS pages are accounted at page_fault unless they've already been accounted | |
174 | for earlier. A file page will be accounted for as Page Cache when it's | |
175 | inserted into inode (radix-tree). While it's mapped into the page tables of | |
176 | processes, duplicate accounting is carefully avoided. | |
177 | ||
1939c557 | 178 | An RSS page is unaccounted when it's fully unmapped. A PageCache page is |
dc10e281 KH |
179 | unaccounted when it's removed from radix-tree. Even if RSS pages are fully |
180 | unmapped (by kswapd), they may exist as SwapCache in the system until they | |
1939c557 | 181 | are really freed. Such SwapCaches are also accounted. |
dc10e281 KH |
182 | A swapped-in page is not accounted until it's mapped. |
183 | ||
1939c557 | 184 | Note: The kernel does swapin-readahead and reads multiple swaps at once. |
dc10e281 KH |
185 | This means swapped-in pages may contain pages for other tasks than a task |
186 | causing page fault. So, we avoid accounting at swap-in I/O. | |
5b4e655e KH |
187 | |
188 | At page migration, accounting information is kept. | |
189 | ||
dc10e281 KH |
190 | Note: we just account pages-on-LRU because our purpose is to control amount |
191 | of used pages; not-on-LRU pages tend to be out-of-control from VM view. | |
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192 | |
193 | 2.3 Shared Page Accounting | |
194 | ||
195 | Shared pages are accounted on the basis of the first touch approach. The | |
196 | cgroup that first touches a page is accounted for the page. The principle | |
197 | behind this approach is that a cgroup that aggressively uses a shared | |
198 | page will eventually get charged for it (once it is uncharged from | |
199 | the cgroup that brought it in -- this will happen on memory pressure). | |
200 | ||
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201 | But see section 8.2: when moving a task to another cgroup, its pages may |
202 | be recharged to the new cgroup, if move_charge_at_immigrate has been chosen. | |
203 | ||
df7c6b99 | 204 | Exception: If CONFIG_MEMCG_SWAP is not used. |
8c7c6e34 | 205 | When you do swapoff and make swapped-out pages of shmem(tmpfs) to |
d13d1443 KH |
206 | be backed into memory in force, charges for pages are accounted against the |
207 | caller of swapoff rather than the users of shmem. | |
208 | ||
c255a458 | 209 | 2.4 Swap Extension (CONFIG_MEMCG_SWAP) |
dc10e281 | 210 | |
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211 | Swap Extension allows you to record charge for swap. A swapped-in page is |
212 | charged back to original page allocator if possible. | |
213 | ||
214 | When swap is accounted, following files are added. | |
215 | - memory.memsw.usage_in_bytes. | |
216 | - memory.memsw.limit_in_bytes. | |
217 | ||
dc10e281 KH |
218 | memsw means memory+swap. Usage of memory+swap is limited by |
219 | memsw.limit_in_bytes. | |
220 | ||
221 | Example: Assume a system with 4G of swap. A task which allocates 6G of memory | |
222 | (by mistake) under 2G memory limitation will use all swap. | |
223 | In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap. | |
1939c557 | 224 | By using the memsw limit, you can avoid system OOM which can be caused by swap |
dc10e281 | 225 | shortage. |
8c7c6e34 | 226 | |
dc10e281 | 227 | * why 'memory+swap' rather than swap. |
8c7c6e34 KH |
228 | The global LRU(kswapd) can swap out arbitrary pages. Swap-out means |
229 | to move account from memory to swap...there is no change in usage of | |
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230 | memory+swap. In other words, when we want to limit the usage of swap without |
231 | affecting global LRU, memory+swap limit is better than just limiting swap from | |
1939c557 | 232 | an OS point of view. |
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233 | |
234 | * What happens when a cgroup hits memory.memsw.limit_in_bytes | |
67de0162 | 235 | When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out |
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236 | in this cgroup. Then, swap-out will not be done by cgroup routine and file |
237 | caches are dropped. But as mentioned above, global LRU can do swapout memory | |
238 | from it for sanity of the system's memory management state. You can't forbid | |
239 | it by cgroup. | |
8c7c6e34 KH |
240 | |
241 | 2.5 Reclaim | |
1b6df3aa | 242 | |
dc10e281 KH |
243 | Each cgroup maintains a per cgroup LRU which has the same structure as |
244 | global VM. When a cgroup goes over its limit, we first try | |
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245 | to reclaim memory from the cgroup so as to make space for the new |
246 | pages that the cgroup has touched. If the reclaim is unsuccessful, | |
247 | an OOM routine is invoked to select and kill the bulkiest task in the | |
dc10e281 | 248 | cgroup. (See 10. OOM Control below.) |
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249 | |
250 | The reclaim algorithm has not been modified for cgroups, except that | |
1939c557 | 251 | pages that are selected for reclaiming come from the per-cgroup LRU |
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252 | list. |
253 | ||
4b3bde4c BS |
254 | NOTE: Reclaim does not work for the root cgroup, since we cannot set any |
255 | limits on the root cgroup. | |
256 | ||
daaf1e68 KH |
257 | Note2: When panic_on_oom is set to "2", the whole system will panic. |
258 | ||
9490ff27 KH |
259 | When oom event notifier is registered, event will be delivered. |
260 | (See oom_control section) | |
261 | ||
dc10e281 | 262 | 2.6 Locking |
1b6df3aa | 263 | |
dc10e281 KH |
264 | lock_page_cgroup()/unlock_page_cgroup() should not be called under |
265 | mapping->tree_lock. | |
1b6df3aa | 266 | |
dc10e281 KH |
267 | Other lock order is following: |
268 | PG_locked. | |
269 | mm->page_table_lock | |
270 | zone->lru_lock | |
271 | lock_page_cgroup. | |
272 | In many cases, just lock_page_cgroup() is called. | |
273 | per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by | |
274 | zone->lru_lock, it has no lock of its own. | |
1b6df3aa | 275 | |
c255a458 | 276 | 2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM) |
e5671dfa GC |
277 | |
278 | With the Kernel memory extension, the Memory Controller is able to limit | |
279 | the amount of kernel memory used by the system. Kernel memory is fundamentally | |
280 | different than user memory, since it can't be swapped out, which makes it | |
281 | possible to DoS the system by consuming too much of this precious resource. | |
282 | ||
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283 | Kernel memory won't be accounted at all until limit on a group is set. This |
284 | allows for existing setups to continue working without disruption. The limit | |
285 | cannot be set if the cgroup have children, or if there are already tasks in the | |
286 | cgroup. Attempting to set the limit under those conditions will return -EBUSY. | |
287 | When use_hierarchy == 1 and a group is accounted, its children will | |
288 | automatically be accounted regardless of their limit value. | |
289 | ||
290 | After a group is first limited, it will be kept being accounted until it | |
291 | is removed. The memory limitation itself, can of course be removed by writing | |
292 | -1 to memory.kmem.limit_in_bytes. In this case, kmem will be accounted, but not | |
293 | limited. | |
294 | ||
e5671dfa | 295 | Kernel memory limits are not imposed for the root cgroup. Usage for the root |
d5bdae7d GC |
296 | cgroup may or may not be accounted. The memory used is accumulated into |
297 | memory.kmem.usage_in_bytes, or in a separate counter when it makes sense. | |
298 | (currently only for tcp). | |
299 | The main "kmem" counter is fed into the main counter, so kmem charges will | |
300 | also be visible from the user counter. | |
e5671dfa | 301 | |
e5671dfa GC |
302 | Currently no soft limit is implemented for kernel memory. It is future work |
303 | to trigger slab reclaim when those limits are reached. | |
304 | ||
305 | 2.7.1 Current Kernel Memory resources accounted | |
306 | ||
d5bdae7d GC |
307 | * stack pages: every process consumes some stack pages. By accounting into |
308 | kernel memory, we prevent new processes from being created when the kernel | |
309 | memory usage is too high. | |
310 | ||
92e79349 | 311 | * slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy |
f884ab15 | 312 | of each kmem_cache is created every time the cache is touched by the first time |
92e79349 GC |
313 | from inside the memcg. The creation is done lazily, so some objects can still be |
314 | skipped while the cache is being created. All objects in a slab page should | |
315 | belong to the same memcg. This only fails to hold when a task is migrated to a | |
316 | different memcg during the page allocation by the cache. | |
317 | ||
e1aab161 GC |
318 | * sockets memory pressure: some sockets protocols have memory pressure |
319 | thresholds. The Memory Controller allows them to be controlled individually | |
320 | per cgroup, instead of globally. | |
e5671dfa | 321 | |
d1a4c0b3 GC |
322 | * tcp memory pressure: sockets memory pressure for the tcp protocol. |
323 | ||
29d293b6 | 324 | 2.7.2 Common use cases |
d5bdae7d GC |
325 | |
326 | Because the "kmem" counter is fed to the main user counter, kernel memory can | |
327 | never be limited completely independently of user memory. Say "U" is the user | |
328 | limit, and "K" the kernel limit. There are three possible ways limits can be | |
329 | set: | |
330 | ||
331 | U != 0, K = unlimited: | |
332 | This is the standard memcg limitation mechanism already present before kmem | |
333 | accounting. Kernel memory is completely ignored. | |
334 | ||
335 | U != 0, K < U: | |
336 | Kernel memory is a subset of the user memory. This setup is useful in | |
337 | deployments where the total amount of memory per-cgroup is overcommited. | |
338 | Overcommiting kernel memory limits is definitely not recommended, since the | |
339 | box can still run out of non-reclaimable memory. | |
340 | In this case, the admin could set up K so that the sum of all groups is | |
341 | never greater than the total memory, and freely set U at the cost of his | |
342 | QoS. | |
19717542 VD |
343 | WARNING: In the current implementation, memory reclaim will NOT be |
344 | triggered for a cgroup when it hits K while staying below U, which makes | |
345 | this setup impractical. | |
d5bdae7d GC |
346 | |
347 | U != 0, K >= U: | |
348 | Since kmem charges will also be fed to the user counter and reclaim will be | |
349 | triggered for the cgroup for both kinds of memory. This setup gives the | |
350 | admin a unified view of memory, and it is also useful for people who just | |
351 | want to track kernel memory usage. | |
352 | ||
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353 | 3. User Interface |
354 | ||
29d293b6 | 355 | 3.0. Configuration |
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356 | |
357 | a. Enable CONFIG_CGROUPS | |
5b1efc02 JW |
358 | b. Enable CONFIG_MEMCG |
359 | c. Enable CONFIG_MEMCG_SWAP (to use swap extension) | |
d5bdae7d | 360 | d. Enable CONFIG_MEMCG_KMEM (to use kmem extension) |
1b6df3aa | 361 | |
29d293b6 | 362 | 3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?) |
f6e07d38 JS |
363 | # mount -t tmpfs none /sys/fs/cgroup |
364 | # mkdir /sys/fs/cgroup/memory | |
365 | # mount -t cgroup none /sys/fs/cgroup/memory -o memory | |
1b6df3aa | 366 | |
29d293b6 | 367 | 3.2. Make the new group and move bash into it |
f6e07d38 JS |
368 | # mkdir /sys/fs/cgroup/memory/0 |
369 | # echo $$ > /sys/fs/cgroup/memory/0/tasks | |
1b6df3aa | 370 | |
dc10e281 | 371 | Since now we're in the 0 cgroup, we can alter the memory limit: |
f6e07d38 | 372 | # echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes |
0eea1030 BS |
373 | |
374 | NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo, | |
dc10e281 KH |
375 | mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.) |
376 | ||
c5b947b2 | 377 | NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited). |
4b3bde4c | 378 | NOTE: We cannot set limits on the root cgroup any more. |
0eea1030 | 379 | |
f6e07d38 | 380 | # cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes |
2324c5dd | 381 | 4194304 |
0eea1030 | 382 | |
1b6df3aa | 383 | We can check the usage: |
f6e07d38 | 384 | # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes |
2324c5dd | 385 | 1216512 |
0eea1030 | 386 | |
1939c557 | 387 | A successful write to this file does not guarantee a successful setting of |
dc10e281 | 388 | this limit to the value written into the file. This can be due to a |
0eea1030 | 389 | number of factors, such as rounding up to page boundaries or the total |
dc10e281 | 390 | availability of memory on the system. The user is required to re-read |
0eea1030 BS |
391 | this file after a write to guarantee the value committed by the kernel. |
392 | ||
fb78922c | 393 | # echo 1 > memory.limit_in_bytes |
0eea1030 | 394 | # cat memory.limit_in_bytes |
2324c5dd | 395 | 4096 |
1b6df3aa BS |
396 | |
397 | The memory.failcnt field gives the number of times that the cgroup limit was | |
398 | exceeded. | |
399 | ||
dfc05c25 KH |
400 | The memory.stat file gives accounting information. Now, the number of |
401 | caches, RSS and Active pages/Inactive pages are shown. | |
402 | ||
1b6df3aa BS |
403 | 4. Testing |
404 | ||
dc10e281 KH |
405 | For testing features and implementation, see memcg_test.txt. |
406 | ||
407 | Performance test is also important. To see pure memory controller's overhead, | |
408 | testing on tmpfs will give you good numbers of small overheads. | |
409 | Example: do kernel make on tmpfs. | |
410 | ||
411 | Page-fault scalability is also important. At measuring parallel | |
412 | page fault test, multi-process test may be better than multi-thread | |
413 | test because it has noise of shared objects/status. | |
414 | ||
415 | But the above two are testing extreme situations. | |
416 | Trying usual test under memory controller is always helpful. | |
1b6df3aa BS |
417 | |
418 | 4.1 Troubleshooting | |
419 | ||
420 | Sometimes a user might find that the application under a cgroup is | |
1939c557 | 421 | terminated by the OOM killer. There are several causes for this: |
1b6df3aa BS |
422 | |
423 | 1. The cgroup limit is too low (just too low to do anything useful) | |
424 | 2. The user is using anonymous memory and swap is turned off or too low | |
425 | ||
426 | A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of | |
427 | some of the pages cached in the cgroup (page cache pages). | |
428 | ||
1939c557 | 429 | To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and |
dc10e281 KH |
430 | seeing what happens will be helpful. |
431 | ||
1b6df3aa BS |
432 | 4.2 Task migration |
433 | ||
a33f3224 | 434 | When a task migrates from one cgroup to another, its charge is not |
7dc74be0 | 435 | carried forward by default. The pages allocated from the original cgroup still |
1b6df3aa BS |
436 | remain charged to it, the charge is dropped when the page is freed or |
437 | reclaimed. | |
438 | ||
dc10e281 KH |
439 | You can move charges of a task along with task migration. |
440 | See 8. "Move charges at task migration" | |
7dc74be0 | 441 | |
1b6df3aa BS |
442 | 4.3 Removing a cgroup |
443 | ||
444 | A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a | |
445 | cgroup might have some charge associated with it, even though all | |
dc10e281 KH |
446 | tasks have migrated away from it. (because we charge against pages, not |
447 | against tasks.) | |
448 | ||
cc926f78 KH |
449 | We move the stats to root (if use_hierarchy==0) or parent (if |
450 | use_hierarchy==1), and no change on the charge except uncharging | |
451 | from the child. | |
1b6df3aa | 452 | |
8c7c6e34 KH |
453 | Charges recorded in swap information is not updated at removal of cgroup. |
454 | Recorded information is discarded and a cgroup which uses swap (swapcache) | |
455 | will be charged as a new owner of it. | |
456 | ||
cc926f78 | 457 | About use_hierarchy, see Section 6. |
8c7c6e34 | 458 | |
c1e862c1 KH |
459 | 5. Misc. interfaces. |
460 | ||
461 | 5.1 force_empty | |
462 | memory.force_empty interface is provided to make cgroup's memory usage empty. | |
c1e862c1 KH |
463 | When writing anything to this |
464 | ||
465 | # echo 0 > memory.force_empty | |
466 | ||
f61c42a7 | 467 | the cgroup will be reclaimed and as many pages reclaimed as possible. |
c1e862c1 | 468 | |
1939c557 | 469 | The typical use case for this interface is before calling rmdir(). |
c1e862c1 KH |
470 | Because rmdir() moves all pages to parent, some out-of-use page caches can be |
471 | moved to the parent. If you want to avoid that, force_empty will be useful. | |
472 | ||
d5bdae7d GC |
473 | Also, note that when memory.kmem.limit_in_bytes is set the charges due to |
474 | kernel pages will still be seen. This is not considered a failure and the | |
475 | write will still return success. In this case, it is expected that | |
476 | memory.kmem.usage_in_bytes == memory.usage_in_bytes. | |
477 | ||
cc926f78 KH |
478 | About use_hierarchy, see Section 6. |
479 | ||
7f016ee8 | 480 | 5.2 stat file |
c863d835 | 481 | |
185efc0f | 482 | memory.stat file includes following statistics |
c863d835 | 483 | |
dc10e281 | 484 | # per-memory cgroup local status |
c863d835 | 485 | cache - # of bytes of page cache memory. |
b070e65c DR |
486 | rss - # of bytes of anonymous and swap cache memory (includes |
487 | transparent hugepages). | |
488 | rss_huge - # of bytes of anonymous transparent hugepages. | |
dc10e281 | 489 | mapped_file - # of bytes of mapped file (includes tmpfs/shmem) |
0527b690 YH |
490 | pgpgin - # of charging events to the memory cgroup. The charging |
491 | event happens each time a page is accounted as either mapped | |
492 | anon page(RSS) or cache page(Page Cache) to the cgroup. | |
493 | pgpgout - # of uncharging events to the memory cgroup. The uncharging | |
494 | event happens each time a page is unaccounted from the cgroup. | |
dc10e281 | 495 | swap - # of bytes of swap usage |
9cb2dc1c SZ |
496 | writeback - # of bytes of file/anon cache that are queued for syncing to |
497 | disk. | |
a15e4190 | 498 | inactive_anon - # of bytes of anonymous and swap cache memory on inactive |
dc10e281 KH |
499 | LRU list. |
500 | active_anon - # of bytes of anonymous and swap cache memory on active | |
a15e4190 | 501 | LRU list. |
dc10e281 KH |
502 | inactive_file - # of bytes of file-backed memory on inactive LRU list. |
503 | active_file - # of bytes of file-backed memory on active LRU list. | |
c863d835 BR |
504 | unevictable - # of bytes of memory that cannot be reclaimed (mlocked etc). |
505 | ||
dc10e281 KH |
506 | # status considering hierarchy (see memory.use_hierarchy settings) |
507 | ||
508 | hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy | |
509 | under which the memory cgroup is | |
510 | hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to | |
511 | hierarchy under which memory cgroup is. | |
512 | ||
eb6332a5 JW |
513 | total_<counter> - # hierarchical version of <counter>, which in |
514 | addition to the cgroup's own value includes the | |
515 | sum of all hierarchical children's values of | |
516 | <counter>, i.e. total_cache | |
dc10e281 KH |
517 | |
518 | # The following additional stats are dependent on CONFIG_DEBUG_VM. | |
c863d835 | 519 | |
c863d835 BR |
520 | recent_rotated_anon - VM internal parameter. (see mm/vmscan.c) |
521 | recent_rotated_file - VM internal parameter. (see mm/vmscan.c) | |
522 | recent_scanned_anon - VM internal parameter. (see mm/vmscan.c) | |
523 | recent_scanned_file - VM internal parameter. (see mm/vmscan.c) | |
524 | ||
525 | Memo: | |
dc10e281 KH |
526 | recent_rotated means recent frequency of LRU rotation. |
527 | recent_scanned means recent # of scans to LRU. | |
7f016ee8 KM |
528 | showing for better debug please see the code for meanings. |
529 | ||
c863d835 BR |
530 | Note: |
531 | Only anonymous and swap cache memory is listed as part of 'rss' stat. | |
532 | This should not be confused with the true 'resident set size' or the | |
dc10e281 KH |
533 | amount of physical memory used by the cgroup. |
534 | 'rss + file_mapped" will give you resident set size of cgroup. | |
535 | (Note: file and shmem may be shared among other cgroups. In that case, | |
536 | file_mapped is accounted only when the memory cgroup is owner of page | |
537 | cache.) | |
7f016ee8 | 538 | |
a7885eb8 | 539 | 5.3 swappiness |
a7885eb8 | 540 | |
688eb988 MH |
541 | Overrides /proc/sys/vm/swappiness for the particular group. The tunable |
542 | in the root cgroup corresponds to the global swappiness setting. | |
543 | ||
544 | Please note that unlike during the global reclaim, limit reclaim | |
545 | enforces that 0 swappiness really prevents from any swapping even if | |
546 | there is a swap storage available. This might lead to memcg OOM killer | |
547 | if there are no file pages to reclaim. | |
a7885eb8 | 548 | |
dc10e281 KH |
549 | 5.4 failcnt |
550 | ||
551 | A memory cgroup provides memory.failcnt and memory.memsw.failcnt files. | |
552 | This failcnt(== failure count) shows the number of times that a usage counter | |
553 | hit its limit. When a memory cgroup hits a limit, failcnt increases and | |
554 | memory under it will be reclaimed. | |
555 | ||
556 | You can reset failcnt by writing 0 to failcnt file. | |
557 | # echo 0 > .../memory.failcnt | |
a7885eb8 | 558 | |
a111c966 DN |
559 | 5.5 usage_in_bytes |
560 | ||
561 | For efficiency, as other kernel components, memory cgroup uses some optimization | |
562 | to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the | |
1939c557 | 563 | method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz |
a111c966 DN |
564 | value for efficient access. (Of course, when necessary, it's synchronized.) |
565 | If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP) | |
566 | value in memory.stat(see 5.2). | |
567 | ||
50c35e5b YH |
568 | 5.6 numa_stat |
569 | ||
570 | This is similar to numa_maps but operates on a per-memcg basis. This is | |
571 | useful for providing visibility into the numa locality information within | |
572 | an memcg since the pages are allowed to be allocated from any physical | |
1939c557 MK |
573 | node. One of the use cases is evaluating application performance by |
574 | combining this information with the application's CPU allocation. | |
50c35e5b | 575 | |
071aee13 YH |
576 | Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable" |
577 | per-node page counts including "hierarchical_<counter>" which sums up all | |
578 | hierarchical children's values in addition to the memcg's own value. | |
579 | ||
8173d5a4 | 580 | The output format of memory.numa_stat is: |
50c35e5b YH |
581 | |
582 | total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ... | |
583 | file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ... | |
584 | anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ... | |
585 | unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ... | |
071aee13 | 586 | hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ... |
50c35e5b | 587 | |
071aee13 | 588 | The "total" count is sum of file + anon + unevictable. |
50c35e5b | 589 | |
52bc0d82 | 590 | 6. Hierarchy support |
c1e862c1 | 591 | |
52bc0d82 BS |
592 | The memory controller supports a deep hierarchy and hierarchical accounting. |
593 | The hierarchy is created by creating the appropriate cgroups in the | |
594 | cgroup filesystem. Consider for example, the following cgroup filesystem | |
595 | hierarchy | |
596 | ||
67de0162 | 597 | root |
52bc0d82 | 598 | / | \ |
67de0162 JS |
599 | / | \ |
600 | a b c | |
601 | | \ | |
602 | | \ | |
603 | d e | |
52bc0d82 BS |
604 | |
605 | In the diagram above, with hierarchical accounting enabled, all memory | |
606 | usage of e, is accounted to its ancestors up until the root (i.e, c and root), | |
dc10e281 | 607 | that has memory.use_hierarchy enabled. If one of the ancestors goes over its |
52bc0d82 BS |
608 | limit, the reclaim algorithm reclaims from the tasks in the ancestor and the |
609 | children of the ancestor. | |
610 | ||
611 | 6.1 Enabling hierarchical accounting and reclaim | |
612 | ||
dc10e281 | 613 | A memory cgroup by default disables the hierarchy feature. Support |
52bc0d82 BS |
614 | can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup |
615 | ||
616 | # echo 1 > memory.use_hierarchy | |
617 | ||
618 | The feature can be disabled by | |
619 | ||
620 | # echo 0 > memory.use_hierarchy | |
621 | ||
689bca3b GT |
622 | NOTE1: Enabling/disabling will fail if either the cgroup already has other |
623 | cgroups created below it, or if the parent cgroup has use_hierarchy | |
624 | enabled. | |
52bc0d82 | 625 | |
daaf1e68 | 626 | NOTE2: When panic_on_oom is set to "2", the whole system will panic in |
dc10e281 | 627 | case of an OOM event in any cgroup. |
52bc0d82 | 628 | |
a6df6361 BS |
629 | 7. Soft limits |
630 | ||
631 | Soft limits allow for greater sharing of memory. The idea behind soft limits | |
632 | is to allow control groups to use as much of the memory as needed, provided | |
633 | ||
634 | a. There is no memory contention | |
635 | b. They do not exceed their hard limit | |
636 | ||
dc10e281 | 637 | When the system detects memory contention or low memory, control groups |
a6df6361 BS |
638 | are pushed back to their soft limits. If the soft limit of each control |
639 | group is very high, they are pushed back as much as possible to make | |
640 | sure that one control group does not starve the others of memory. | |
641 | ||
1939c557 | 642 | Please note that soft limits is a best-effort feature; it comes with |
a6df6361 BS |
643 | no guarantees, but it does its best to make sure that when memory is |
644 | heavily contended for, memory is allocated based on the soft limit | |
1939c557 | 645 | hints/setup. Currently soft limit based reclaim is set up such that |
a6df6361 BS |
646 | it gets invoked from balance_pgdat (kswapd). |
647 | ||
648 | 7.1 Interface | |
649 | ||
650 | Soft limits can be setup by using the following commands (in this example we | |
dc10e281 | 651 | assume a soft limit of 256 MiB) |
a6df6361 BS |
652 | |
653 | # echo 256M > memory.soft_limit_in_bytes | |
654 | ||
655 | If we want to change this to 1G, we can at any time use | |
656 | ||
657 | # echo 1G > memory.soft_limit_in_bytes | |
658 | ||
659 | NOTE1: Soft limits take effect over a long period of time, since they involve | |
660 | reclaiming memory for balancing between memory cgroups | |
661 | NOTE2: It is recommended to set the soft limit always below the hard limit, | |
662 | otherwise the hard limit will take precedence. | |
663 | ||
7dc74be0 DN |
664 | 8. Move charges at task migration |
665 | ||
666 | Users can move charges associated with a task along with task migration, that | |
667 | is, uncharge task's pages from the old cgroup and charge them to the new cgroup. | |
02491447 DN |
668 | This feature is not supported in !CONFIG_MMU environments because of lack of |
669 | page tables. | |
7dc74be0 DN |
670 | |
671 | 8.1 Interface | |
672 | ||
8173d5a4 | 673 | This feature is disabled by default. It can be enabled (and disabled again) by |
7dc74be0 DN |
674 | writing to memory.move_charge_at_immigrate of the destination cgroup. |
675 | ||
676 | If you want to enable it: | |
677 | ||
678 | # echo (some positive value) > memory.move_charge_at_immigrate | |
679 | ||
680 | Note: Each bits of move_charge_at_immigrate has its own meaning about what type | |
681 | of charges should be moved. See 8.2 for details. | |
1939c557 MK |
682 | Note: Charges are moved only when you move mm->owner, in other words, |
683 | a leader of a thread group. | |
7dc74be0 DN |
684 | Note: If we cannot find enough space for the task in the destination cgroup, we |
685 | try to make space by reclaiming memory. Task migration may fail if we | |
686 | cannot make enough space. | |
dc10e281 | 687 | Note: It can take several seconds if you move charges much. |
7dc74be0 DN |
688 | |
689 | And if you want disable it again: | |
690 | ||
691 | # echo 0 > memory.move_charge_at_immigrate | |
692 | ||
1939c557 | 693 | 8.2 Type of charges which can be moved |
7dc74be0 | 694 | |
1939c557 MK |
695 | Each bit in move_charge_at_immigrate has its own meaning about what type of |
696 | charges should be moved. But in any case, it must be noted that an account of | |
697 | a page or a swap can be moved only when it is charged to the task's current | |
698 | (old) memory cgroup. | |
7dc74be0 DN |
699 | |
700 | bit | what type of charges would be moved ? | |
701 | -----+------------------------------------------------------------------------ | |
1939c557 MK |
702 | 0 | A charge of an anonymous page (or swap of it) used by the target task. |
703 | | You must enable Swap Extension (see 2.4) to enable move of swap charges. | |
87946a72 | 704 | -----+------------------------------------------------------------------------ |
1939c557 | 705 | 1 | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory) |
dc10e281 | 706 | | and swaps of tmpfs file) mmapped by the target task. Unlike the case of |
1939c557 | 707 | | anonymous pages, file pages (and swaps) in the range mmapped by the task |
87946a72 DN |
708 | | will be moved even if the task hasn't done page fault, i.e. they might |
709 | | not be the task's "RSS", but other task's "RSS" that maps the same file. | |
1939c557 MK |
710 | | And mapcount of the page is ignored (the page can be moved even if |
711 | | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to | |
87946a72 | 712 | | enable move of swap charges. |
7dc74be0 DN |
713 | |
714 | 8.3 TODO | |
715 | ||
7dc74be0 DN |
716 | - All of moving charge operations are done under cgroup_mutex. It's not good |
717 | behavior to hold the mutex too long, so we may need some trick. | |
718 | ||
2e72b634 KS |
719 | 9. Memory thresholds |
720 | ||
1939c557 | 721 | Memory cgroup implements memory thresholds using the cgroups notification |
2e72b634 KS |
722 | API (see cgroups.txt). It allows to register multiple memory and memsw |
723 | thresholds and gets notifications when it crosses. | |
724 | ||
1939c557 | 725 | To register a threshold, an application must: |
dc10e281 KH |
726 | - create an eventfd using eventfd(2); |
727 | - open memory.usage_in_bytes or memory.memsw.usage_in_bytes; | |
728 | - write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to | |
729 | cgroup.event_control. | |
2e72b634 KS |
730 | |
731 | Application will be notified through eventfd when memory usage crosses | |
732 | threshold in any direction. | |
733 | ||
734 | It's applicable for root and non-root cgroup. | |
735 | ||
9490ff27 KH |
736 | 10. OOM Control |
737 | ||
3c11ecf4 KH |
738 | memory.oom_control file is for OOM notification and other controls. |
739 | ||
1939c557 | 740 | Memory cgroup implements OOM notifier using the cgroup notification |
dc10e281 KH |
741 | API (See cgroups.txt). It allows to register multiple OOM notification |
742 | delivery and gets notification when OOM happens. | |
9490ff27 | 743 | |
1939c557 | 744 | To register a notifier, an application must: |
9490ff27 KH |
745 | - create an eventfd using eventfd(2) |
746 | - open memory.oom_control file | |
dc10e281 KH |
747 | - write string like "<event_fd> <fd of memory.oom_control>" to |
748 | cgroup.event_control | |
9490ff27 | 749 | |
1939c557 MK |
750 | The application will be notified through eventfd when OOM happens. |
751 | OOM notification doesn't work for the root cgroup. | |
9490ff27 | 752 | |
1939c557 | 753 | You can disable the OOM-killer by writing "1" to memory.oom_control file, as: |
dc10e281 | 754 | |
3c11ecf4 KH |
755 | #echo 1 > memory.oom_control |
756 | ||
dc10e281 KH |
757 | If OOM-killer is disabled, tasks under cgroup will hang/sleep |
758 | in memory cgroup's OOM-waitqueue when they request accountable memory. | |
3c11ecf4 | 759 | |
dc10e281 | 760 | For running them, you have to relax the memory cgroup's OOM status by |
3c11ecf4 KH |
761 | * enlarge limit or reduce usage. |
762 | To reduce usage, | |
763 | * kill some tasks. | |
764 | * move some tasks to other group with account migration. | |
765 | * remove some files (on tmpfs?) | |
766 | ||
767 | Then, stopped tasks will work again. | |
768 | ||
769 | At reading, current status of OOM is shown. | |
770 | oom_kill_disable 0 or 1 (if 1, oom-killer is disabled) | |
dc10e281 | 771 | under_oom 0 or 1 (if 1, the memory cgroup is under OOM, tasks may |
3c11ecf4 | 772 | be stopped.) |
9490ff27 | 773 | |
70ddf637 AV |
774 | 11. Memory Pressure |
775 | ||
776 | The pressure level notifications can be used to monitor the memory | |
777 | allocation cost; based on the pressure, applications can implement | |
778 | different strategies of managing their memory resources. The pressure | |
779 | levels are defined as following: | |
780 | ||
781 | The "low" level means that the system is reclaiming memory for new | |
782 | allocations. Monitoring this reclaiming activity might be useful for | |
783 | maintaining cache level. Upon notification, the program (typically | |
784 | "Activity Manager") might analyze vmstat and act in advance (i.e. | |
785 | prematurely shutdown unimportant services). | |
786 | ||
787 | The "medium" level means that the system is experiencing medium memory | |
788 | pressure, the system might be making swap, paging out active file caches, | |
789 | etc. Upon this event applications may decide to further analyze | |
790 | vmstat/zoneinfo/memcg or internal memory usage statistics and free any | |
791 | resources that can be easily reconstructed or re-read from a disk. | |
792 | ||
793 | The "critical" level means that the system is actively thrashing, it is | |
794 | about to out of memory (OOM) or even the in-kernel OOM killer is on its | |
795 | way to trigger. Applications should do whatever they can to help the | |
796 | system. It might be too late to consult with vmstat or any other | |
797 | statistics, so it's advisable to take an immediate action. | |
798 | ||
799 | The events are propagated upward until the event is handled, i.e. the | |
800 | events are not pass-through. Here is what this means: for example you have | |
801 | three cgroups: A->B->C. Now you set up an event listener on cgroups A, B | |
802 | and C, and suppose group C experiences some pressure. In this situation, | |
803 | only group C will receive the notification, i.e. groups A and B will not | |
804 | receive it. This is done to avoid excessive "broadcasting" of messages, | |
805 | which disturbs the system and which is especially bad if we are low on | |
806 | memory or thrashing. So, organize the cgroups wisely, or propagate the | |
807 | events manually (or, ask us to implement the pass-through events, | |
808 | explaining why would you need them.) | |
809 | ||
810 | The file memory.pressure_level is only used to setup an eventfd. To | |
811 | register a notification, an application must: | |
812 | ||
813 | - create an eventfd using eventfd(2); | |
814 | - open memory.pressure_level; | |
815 | - write string like "<event_fd> <fd of memory.pressure_level> <level>" | |
816 | to cgroup.event_control. | |
817 | ||
818 | Application will be notified through eventfd when memory pressure is at | |
819 | the specific level (or higher). Read/write operations to | |
820 | memory.pressure_level are no implemented. | |
821 | ||
822 | Test: | |
823 | ||
824 | Here is a small script example that makes a new cgroup, sets up a | |
825 | memory limit, sets up a notification in the cgroup and then makes child | |
826 | cgroup experience a critical pressure: | |
827 | ||
828 | # cd /sys/fs/cgroup/memory/ | |
829 | # mkdir foo | |
830 | # cd foo | |
831 | # cgroup_event_listener memory.pressure_level low & | |
832 | # echo 8000000 > memory.limit_in_bytes | |
833 | # echo 8000000 > memory.memsw.limit_in_bytes | |
834 | # echo $$ > tasks | |
835 | # dd if=/dev/zero | read x | |
836 | ||
837 | (Expect a bunch of notifications, and eventually, the oom-killer will | |
838 | trigger.) | |
839 | ||
840 | 12. TODO | |
1b6df3aa | 841 | |
f968ef1c LZ |
842 | 1. Make per-cgroup scanner reclaim not-shared pages first |
843 | 2. Teach controller to account for shared-pages | |
844 | 3. Start reclamation in the background when the limit is | |
1b6df3aa | 845 | not yet hit but the usage is getting closer |
1b6df3aa BS |
846 | |
847 | Summary | |
848 | ||
849 | Overall, the memory controller has been a stable controller and has been | |
850 | commented and discussed quite extensively in the community. | |
851 | ||
852 | References | |
853 | ||
854 | 1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/ | |
855 | 2. Singh, Balbir. Memory Controller (RSS Control), | |
856 | http://lwn.net/Articles/222762/ | |
857 | 3. Emelianov, Pavel. Resource controllers based on process cgroups | |
858 | http://lkml.org/lkml/2007/3/6/198 | |
859 | 4. Emelianov, Pavel. RSS controller based on process cgroups (v2) | |
2324c5dd | 860 | http://lkml.org/lkml/2007/4/9/78 |
1b6df3aa BS |
861 | 5. Emelianov, Pavel. RSS controller based on process cgroups (v3) |
862 | http://lkml.org/lkml/2007/5/30/244 | |
863 | 6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/ | |
864 | 7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control | |
865 | subsystem (v3), http://lwn.net/Articles/235534/ | |
2324c5dd | 866 | 8. Singh, Balbir. RSS controller v2 test results (lmbench), |
1b6df3aa | 867 | http://lkml.org/lkml/2007/5/17/232 |
2324c5dd | 868 | 9. Singh, Balbir. RSS controller v2 AIM9 results |
1b6df3aa | 869 | http://lkml.org/lkml/2007/5/18/1 |
2324c5dd | 870 | 10. Singh, Balbir. Memory controller v6 test results, |
1b6df3aa | 871 | http://lkml.org/lkml/2007/8/19/36 |
2324c5dd LZ |
872 | 11. Singh, Balbir. Memory controller introduction (v6), |
873 | http://lkml.org/lkml/2007/8/17/69 | |
1b6df3aa BS |
874 | 12. Corbet, Jonathan, Controlling memory use in cgroups, |
875 | http://lwn.net/Articles/243795/ |