projects / linux-2.6-block.git / blame
| Commit | Line | Data |
|---|---|---|
| 934352f2 BR |
1 | CPU Accounting Controller |
| 2 | ------------------------- | |
| 3 | ||
| 4 | The CPU accounting controller is used to group tasks using cgroups and | |
| 5 | account the CPU usage of these groups of tasks. | |
| 6 | ||
| 7 | The CPU accounting controller supports multi-hierarchy groups. An accounting | |
| 8 | group accumulates the CPU usage of all of its child groups and the tasks | |
| 9 | directly present in its group. | |
| 10 | ||
| 11 | Accounting groups can be created by first mounting the cgroup filesystem. | |
| 12 | ||
| f6e07d38 JS |
13 | # mount -t cgroup -ocpuacct none /sys/fs/cgroup |
| 14 | ||
| 15 | With the above step, the initial or the parent accounting group becomes | |
| 16 | visible at /sys/fs/cgroup. At bootup, this group includes all the tasks in | |
| 17 | the system. /sys/fs/cgroup/tasks lists the tasks in this cgroup. | |
| 18 | /sys/fs/cgroup/cpuacct.usage gives the CPU time (in nanoseconds) obtained | |
| 19 | by this group which is essentially the CPU time obtained by all the tasks | |
| 934352f2 BR |
20 | in the system. |
| 21 | ||
| f6e07d38 | 22 | New accounting groups can be created under the parent group /sys/fs/cgroup. |
| 934352f2 | 23 | |
| f6e07d38 | 24 | # cd /sys/fs/cgroup |
| 934352f2 | 25 | # mkdir g1 |
| e47f9d84 | 26 | # echo $$ > g1/tasks |
| 934352f2 BR |
27 | |
| 28 | The above steps create a new group g1 and move the current shell | |
| 29 | process (bash) into it. CPU time consumed by this bash and its children | |
| 30 | can be obtained from g1/cpuacct.usage and the same is accumulated in | |
| f6e07d38 | 31 | /sys/fs/cgroup/cpuacct.usage also. |
| ef12fefa BR |
32 | |
| 33 | cpuacct.stat file lists a few statistics which further divide the | |
| 34 | CPU time obtained by the cgroup into user and system times. Currently | |
| 35 | the following statistics are supported: | |
| 36 | ||
| 37 | user: Time spent by tasks of the cgroup in user mode. | |
| 38 | system: Time spent by tasks of the cgroup in kernel mode. | |
| 39 | ||
| 40 | user and system are in USER_HZ unit. | |
| 41 | ||
| 42 | cpuacct controller uses percpu_counter interface to collect user and | |
| 43 | system times. This has two side effects: | |
| 44 | ||
| 45 | - It is theoretically possible to see wrong values for user and system times. | |
| 46 | This is because percpu_counter_read() on 32bit systems isn't safe | |
| 47 | against concurrent writes. | |
| 48 | - It is possible to see slightly outdated values for user and system times | |
| 49 | due to the batch processing nature of percpu_counter. |