| Bharata B Rao | 934352f | 2008-11-10 20:41:13 +0530 | [diff] [blame] | 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 | |
| 13 | # mkdir /cgroups |
| 14 | # mount -t cgroup -ocpuacct none /cgroups |
| 15 | |
| 16 | With the above step, the initial or the parent accounting group |
| 17 | becomes visible at /cgroups. At bootup, this group includes all the |
| 18 | tasks in the system. /cgroups/tasks lists the tasks in this cgroup. |
| 19 | /cgroups/cpuacct.usage gives the CPU time (in nanoseconds) obtained by |
| 20 | this group which is essentially the CPU time obtained by all the tasks |
| 21 | in the system. |
| 22 | |
| 23 | New accounting groups can be created under the parent group /cgroups. |
| 24 | |
| 25 | # cd /cgroups |
| 26 | # mkdir g1 |
| 27 | # echo $$ > g1 |
| 28 | |
| 29 | The above steps create a new group g1 and move the current shell |
| 30 | process (bash) into it. CPU time consumed by this bash and its children |
| 31 | can be obtained from g1/cpuacct.usage and the same is accumulated in |
| 32 | /cgroups/cpuacct.usage also. |
| Bharata B Rao | ef12fef | 2009-03-31 10:02:22 +0530 | [diff] [blame] | 33 | |
| 34 | cpuacct.stat file lists a few statistics which further divide the |
| 35 | CPU time obtained by the cgroup into user and system times. Currently |
| 36 | the following statistics are supported: |
| 37 | |
| 38 | user: Time spent by tasks of the cgroup in user mode. |
| 39 | system: Time spent by tasks of the cgroup in kernel mode. |
| 40 | |
| 41 | user and system are in USER_HZ unit. |
| 42 | |
| 43 | cpuacct controller uses percpu_counter interface to collect user and |
| 44 | system times. This has two side effects: |
| 45 | |
| 46 | - It is theoretically possible to see wrong values for user and system times. |
| 47 | This is because percpu_counter_read() on 32bit systems isn't safe |
| 48 | against concurrent writes. |
| 49 | - It is possible to see slightly outdated values for user and system times |
| 50 | due to the batch processing nature of percpu_counter. |