Commit | Line | Data |
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1b6df3aa BS |
1 | Memory Controller |
2 | ||
3 | Salient features | |
4 | ||
5 | a. Enable control of both RSS (mapped) and Page Cache (unmapped) pages | |
6 | b. The infrastructure allows easy addition of other types of memory to control | |
7 | c. Provides *zero overhead* for non memory controller users | |
8 | d. Provides a double LRU: global memory pressure causes reclaim from the | |
9 | global LRU; a cgroup on hitting a limit, reclaims from the per | |
10 | cgroup LRU | |
11 | ||
dfc05c25 | 12 | NOTE: Swap Cache (unmapped) is not accounted now. |
1b6df3aa BS |
13 | |
14 | Benefits and Purpose of the memory controller | |
15 | ||
16 | The memory controller isolates the memory behaviour of a group of tasks | |
17 | from the rest of the system. The article on LWN [12] mentions some probable | |
18 | uses of the memory controller. The memory controller can be used to | |
19 | ||
20 | a. Isolate an application or a group of applications | |
21 | Memory hungry applications can be isolated and limited to a smaller | |
22 | amount of memory. | |
23 | b. Create a cgroup with limited amount of memory, this can be used | |
24 | as a good alternative to booting with mem=XXXX. | |
25 | c. Virtualization solutions can control the amount of memory they want | |
26 | to assign to a virtual machine instance. | |
27 | d. A CD/DVD burner could control the amount of memory used by the | |
28 | rest of the system to ensure that burning does not fail due to lack | |
29 | of available memory. | |
30 | e. There are several other use cases, find one or use the controller just | |
31 | for fun (to learn and hack on the VM subsystem). | |
32 | ||
33 | 1. History | |
34 | ||
35 | The memory controller has a long history. A request for comments for the memory | |
36 | controller was posted by Balbir Singh [1]. At the time the RFC was posted | |
37 | there were several implementations for memory control. The goal of the | |
38 | RFC was to build consensus and agreement for the minimal features required | |
39 | for memory control. The first RSS controller was posted by Balbir Singh[2] | |
40 | in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the | |
41 | RSS controller. At OLS, at the resource management BoF, everyone suggested | |
42 | that we handle both page cache and RSS together. Another request was raised | |
43 | to allow user space handling of OOM. The current memory controller is | |
44 | at version 6; it combines both mapped (RSS) and unmapped Page | |
45 | Cache Control [11]. | |
46 | ||
47 | 2. Memory Control | |
48 | ||
49 | Memory is a unique resource in the sense that it is present in a limited | |
50 | amount. If a task requires a lot of CPU processing, the task can spread | |
51 | its processing over a period of hours, days, months or years, but with | |
52 | memory, the same physical memory needs to be reused to accomplish the task. | |
53 | ||
54 | The memory controller implementation has been divided into phases. These | |
55 | are: | |
56 | ||
57 | 1. Memory controller | |
58 | 2. mlock(2) controller | |
59 | 3. Kernel user memory accounting and slab control | |
60 | 4. user mappings length controller | |
61 | ||
62 | The memory controller is the first controller developed. | |
63 | ||
64 | 2.1. Design | |
65 | ||
66 | The core of the design is a counter called the res_counter. The res_counter | |
67 | tracks the current memory usage and limit of the group of processes associated | |
68 | with the controller. Each cgroup has a memory controller specific data | |
69 | structure (mem_cgroup) associated with it. | |
70 | ||
71 | 2.2. Accounting | |
72 | ||
73 | +--------------------+ | |
74 | | mem_cgroup | | |
75 | | (res_counter) | | |
76 | +--------------------+ | |
77 | / ^ \ | |
78 | / | \ | |
79 | +---------------+ | +---------------+ | |
80 | | mm_struct | |.... | mm_struct | | |
81 | | | | | | | |
82 | +---------------+ | +---------------+ | |
83 | | | |
84 | + --------------+ | |
85 | | | |
86 | +---------------+ +------+--------+ | |
87 | | page +----------> page_cgroup| | |
88 | | | | | | |
89 | +---------------+ +---------------+ | |
90 | ||
91 | (Figure 1: Hierarchy of Accounting) | |
92 | ||
93 | ||
94 | Figure 1 shows the important aspects of the controller | |
95 | ||
96 | 1. Accounting happens per cgroup | |
97 | 2. Each mm_struct knows about which cgroup it belongs to | |
98 | 3. Each page has a pointer to the page_cgroup, which in turn knows the | |
99 | cgroup it belongs to | |
100 | ||
101 | The accounting is done as follows: mem_cgroup_charge() is invoked to setup | |
102 | the necessary data structures and check if the cgroup that is being charged | |
103 | is over its limit. If it is then reclaim is invoked on the cgroup. | |
104 | More details can be found in the reclaim section of this document. | |
105 | If everything goes well, a page meta-data-structure called page_cgroup is | |
106 | allocated and associated with the page. This routine also adds the page to | |
107 | the per cgroup LRU. | |
108 | ||
109 | 2.2.1 Accounting details | |
110 | ||
111 | All mapped pages (RSS) and unmapped user pages (Page Cache) are accounted. | |
112 | RSS pages are accounted at the time of page_add_*_rmap() unless they've already | |
113 | been accounted for earlier. A file page will be accounted for as Page Cache; | |
114 | it's mapped into the page tables of a process, duplicate accounting is carefully | |
115 | avoided. Page Cache pages are accounted at the time of add_to_page_cache(). | |
116 | The corresponding routines that remove a page from the page tables or removes | |
117 | a page from Page Cache is used to decrement the accounting counters of the | |
118 | cgroup. | |
119 | ||
120 | 2.3 Shared Page Accounting | |
121 | ||
122 | Shared pages are accounted on the basis of the first touch approach. The | |
123 | cgroup that first touches a page is accounted for the page. The principle | |
124 | behind this approach is that a cgroup that aggressively uses a shared | |
125 | page will eventually get charged for it (once it is uncharged from | |
126 | the cgroup that brought it in -- this will happen on memory pressure). | |
127 | ||
128 | 2.4 Reclaim | |
129 | ||
130 | Each cgroup maintains a per cgroup LRU that consists of an active | |
131 | and inactive list. When a cgroup goes over its limit, we first try | |
132 | to reclaim memory from the cgroup so as to make space for the new | |
133 | pages that the cgroup has touched. If the reclaim is unsuccessful, | |
134 | an OOM routine is invoked to select and kill the bulkiest task in the | |
135 | cgroup. | |
136 | ||
137 | The reclaim algorithm has not been modified for cgroups, except that | |
138 | pages that are selected for reclaiming come from the per cgroup LRU | |
139 | list. | |
140 | ||
141 | 2. Locking | |
142 | ||
143 | The memory controller uses the following hierarchy | |
144 | ||
145 | 1. zone->lru_lock is used for selecting pages to be isolated | |
dfc05c25 | 146 | 2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone) |
1b6df3aa BS |
147 | 3. lock_page_cgroup() is used to protect page->page_cgroup |
148 | ||
149 | 3. User Interface | |
150 | ||
151 | 0. Configuration | |
152 | ||
153 | a. Enable CONFIG_CGROUPS | |
154 | b. Enable CONFIG_RESOURCE_COUNTERS | |
155 | c. Enable CONFIG_CGROUP_MEM_CONT | |
156 | ||
157 | 1. Prepare the cgroups | |
158 | # mkdir -p /cgroups | |
159 | # mount -t cgroup none /cgroups -o memory | |
160 | ||
161 | 2. Make the new group and move bash into it | |
162 | # mkdir /cgroups/0 | |
163 | # echo $$ > /cgroups/0/tasks | |
164 | ||
165 | Since now we're in the 0 cgroup, | |
166 | We can alter the memory limit: | |
0eea1030 BS |
167 | # echo -n 4M > /cgroups/0/memory.limit_in_bytes |
168 | ||
169 | NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo, | |
170 | mega or gigabytes. | |
171 | ||
172 | # cat /cgroups/0/memory.limit_in_bytes | |
2324c5dd | 173 | 4194304 |
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174 | |
175 | NOTE: The interface has now changed to display the usage in bytes | |
176 | instead of pages | |
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177 | |
178 | We can check the usage: | |
0eea1030 | 179 | # cat /cgroups/0/memory.usage_in_bytes |
2324c5dd | 180 | 1216512 |
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181 | |
182 | A successful write to this file does not guarantee a successful set of | |
183 | this limit to the value written into the file. This can be due to a | |
184 | number of factors, such as rounding up to page boundaries or the total | |
185 | availability of memory on the system. The user is required to re-read | |
186 | this file after a write to guarantee the value committed by the kernel. | |
187 | ||
188 | # echo -n 1 > memory.limit_in_bytes | |
189 | # cat memory.limit_in_bytes | |
2324c5dd | 190 | 4096 |
1b6df3aa BS |
191 | |
192 | The memory.failcnt field gives the number of times that the cgroup limit was | |
193 | exceeded. | |
194 | ||
dfc05c25 KH |
195 | The memory.stat file gives accounting information. Now, the number of |
196 | caches, RSS and Active pages/Inactive pages are shown. | |
197 | ||
198 | The memory.force_empty gives an interface to drop *all* charges by force. | |
199 | ||
200 | # echo -n 1 > memory.force_empty | |
201 | ||
202 | will drop all charges in cgroup. Currently, this is maintained for test. | |
203 | ||
1b6df3aa BS |
204 | 4. Testing |
205 | ||
206 | Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11]. | |
207 | Apart from that v6 has been tested with several applications and regular | |
208 | daily use. The controller has also been tested on the PPC64, x86_64 and | |
209 | UML platforms. | |
210 | ||
211 | 4.1 Troubleshooting | |
212 | ||
213 | Sometimes a user might find that the application under a cgroup is | |
214 | terminated. There are several causes for this: | |
215 | ||
216 | 1. The cgroup limit is too low (just too low to do anything useful) | |
217 | 2. The user is using anonymous memory and swap is turned off or too low | |
218 | ||
219 | A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of | |
220 | some of the pages cached in the cgroup (page cache pages). | |
221 | ||
222 | 4.2 Task migration | |
223 | ||
224 | When a task migrates from one cgroup to another, it's charge is not | |
225 | carried forward. The pages allocated from the original cgroup still | |
226 | remain charged to it, the charge is dropped when the page is freed or | |
227 | reclaimed. | |
228 | ||
229 | 4.3 Removing a cgroup | |
230 | ||
231 | A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a | |
232 | cgroup might have some charge associated with it, even though all | |
dfc05c25 KH |
233 | tasks have migrated away from it. Such charges are automatically dropped at |
234 | rmdir() if there are no tasks. | |
1b6df3aa | 235 | |
1b6df3aa BS |
236 | 5. TODO |
237 | ||
238 | 1. Add support for accounting huge pages (as a separate controller) | |
dfc05c25 KH |
239 | 2. Make per-cgroup scanner reclaim not-shared pages first |
240 | 3. Teach controller to account for shared-pages | |
241 | 4. Start reclamation when the limit is lowered | |
242 | 5. Start reclamation in the background when the limit is | |
1b6df3aa | 243 | not yet hit but the usage is getting closer |
1b6df3aa BS |
244 | |
245 | Summary | |
246 | ||
247 | Overall, the memory controller has been a stable controller and has been | |
248 | commented and discussed quite extensively in the community. | |
249 | ||
250 | References | |
251 | ||
252 | 1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/ | |
253 | 2. Singh, Balbir. Memory Controller (RSS Control), | |
254 | http://lwn.net/Articles/222762/ | |
255 | 3. Emelianov, Pavel. Resource controllers based on process cgroups | |
256 | http://lkml.org/lkml/2007/3/6/198 | |
257 | 4. Emelianov, Pavel. RSS controller based on process cgroups (v2) | |
2324c5dd | 258 | http://lkml.org/lkml/2007/4/9/78 |
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259 | 5. Emelianov, Pavel. RSS controller based on process cgroups (v3) |
260 | http://lkml.org/lkml/2007/5/30/244 | |
261 | 6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/ | |
262 | 7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control | |
263 | subsystem (v3), http://lwn.net/Articles/235534/ | |
2324c5dd | 264 | 8. Singh, Balbir. RSS controller v2 test results (lmbench), |
1b6df3aa | 265 | http://lkml.org/lkml/2007/5/17/232 |
2324c5dd | 266 | 9. Singh, Balbir. RSS controller v2 AIM9 results |
1b6df3aa | 267 | http://lkml.org/lkml/2007/5/18/1 |
2324c5dd | 268 | 10. Singh, Balbir. Memory controller v6 test results, |
1b6df3aa | 269 | http://lkml.org/lkml/2007/8/19/36 |
2324c5dd LZ |
270 | 11. Singh, Balbir. Memory controller introduction (v6), |
271 | http://lkml.org/lkml/2007/8/17/69 | |
1b6df3aa BS |
272 | 12. Corbet, Jonathan, Controlling memory use in cgroups, |
273 | http://lwn.net/Articles/243795/ |