CPU Accounting Controller¶
The CPU accounting controller is used to group tasks using cgroups andaccount the CPU usage of these groups of tasks.
The CPU accounting controller supports multi-hierarchy groups. An accountinggroup accumulates the CPU usage of all of its child groups and the tasksdirectly present in its group.
Accounting groups can be created by first mounting the cgroup filesystem:
# mount -t cgroup -ocpuacct none /sys/fs/cgroup
With the above step, the initial or the parent accounting group becomesvisible at /sys/fs/cgroup. At bootup, this group includes all the tasks inthe system. /sys/fs/cgroup/tasks lists the tasks in this cgroup./sys/fs/cgroup/cpuacct.usage gives the CPU time (in nanoseconds) obtainedby this group which is essentially the CPU time obtained by all the tasksin the system.
New accounting groups can be created under the parent group /sys/fs/cgroup:
# cd /sys/fs/cgroup# mkdir g1# echo $$ > g1/tasks
The above steps create a new group g1 and move the current shellprocess (bash) into it. CPU time consumed by this bash and its childrencan be obtained from g1/cpuacct.usage and the same is accumulated in/sys/fs/cgroup/cpuacct.usage also.
cpuacct.stat file lists a few statistics which further divide theCPU time obtained by the cgroup into user and system times. Currentlythe following statistics are supported:
user: Time spent by tasks of the cgroup in user mode.system: Time spent by tasks of the cgroup in kernel mode.
user and system are in USER_HZ unit.
cpuacct controller uses percpu_counter interface to collect user andsystem times. This has two side effects:
It is theoretically possible to see wrong values for user and system times.This is because
percpu_counter_read()on 32bit systems isn’t safeagainst concurrent writes.It is possible to see slightly outdated values for user and system timesdue to the batch processing nature of percpu_counter.