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Linux/Documentation/cgroups/cpuacct.txt

  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.
 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 49 - It is possible to see slightly outdated values for user and system times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.

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