linux/Documentation/cgroups/cpuacct.txt
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   1CPU Accounting Controller
   2-------------------------
   3
   4The CPU accounting controller is used to group tasks using cgroups and
   5account the CPU usage of these groups of tasks.
   6
   7The CPU accounting controller supports multi-hierarchy groups. An accounting
   8group accumulates the CPU usage of all of its child groups and the tasks
   9directly present in its group.
  10
  11Accounting groups can be created by first mounting the cgroup filesystem.
  12
  13# mkdir /cgroups
  14# mount -t cgroup -ocpuacct none /cgroups
  15
  16With the above step, the initial or the parent accounting group
  17becomes visible at /cgroups. At bootup, this group includes all the
  18tasks in the system. /cgroups/tasks lists the tasks in this cgroup.
  19/cgroups/cpuacct.usage gives the CPU time (in nanoseconds) obtained by
  20this group which is essentially the CPU time obtained by all the tasks
  21in the system.
  22
  23New accounting groups can be created under the parent group /cgroups.
  24
  25# cd /cgroups
  26# mkdir g1
  27# echo $$ > g1
  28
  29The above steps create a new group g1 and move the current shell
  30process (bash) into it. CPU time consumed by this bash and its children
  31can be obtained from g1/cpuacct.usage and the same is accumulated in
  32/cgroups/cpuacct.usage also.
  33
  34cpuacct.stat file lists a few statistics which further divide the
  35CPU time obtained by the cgroup into user and system times. Currently
  36the following statistics are supported:
  37
  38user: Time spent by tasks of the cgroup in user mode.
  39system: Time spent by tasks of the cgroup in kernel mode.
  40
  41user and system are in USER_HZ unit.
  42
  43cpuacct controller uses percpu_counter interface to collect user and
  44system 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  49- It is possible to see slightly outdated values for user and system times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.
  51
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