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