Re: [PATCH v7 06/11] sched: document the cpu cgroup.
From: Tejun Heo
Date: Thu Jun 06 2013 - 19:28:15 EST
On Wed, May 29, 2013 at 03:03:17PM +0400, Glauber Costa wrote:
> The CPU cgroup is so far, undocumented. Although data exists in the
> Documentation directory about its functioning, it is usually spread,
> and/or presented in the context of something else. This file
> consolidates all cgroup-related information about it.
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxx>
Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>
Some minor points below.
> +The CPU controller exposes the following files to the user:
> + - cpu.shares: The weight of each group living in the same hierarchy, that
> + translates into the amount of CPU it is expected to get. Upon cgroup creation,
> + each group gets assigned a default of 1024. The percentage of CPU assigned to
> + the cgroup is the value of shares divided by the sum of all shares in all
> + cgroups in the same level.
> + - cpu.cfs_period_us: The duration in microseconds of each scheduler period, for
> + bandwidth decisions. This defaults to 100000us or 100ms. Larger periods will
> + improve throughput at the expense of latency, since the scheduler will be able
> + to sustain a cpu-bound workload for longer. The opposite of true for smaller
> + periods. Note that this only affects non-RT tasks that are scheduled by the
> + CFS scheduler.
> +- cpu.cfs_quota_us: The maximum time in microseconds during each cfs_period_us
> + in for the current group will be allowed to run. For instance, if it is set to
in for? doesn't parse for me.
> + half of cpu_period_us, the cgroup will only be able to peak run for 50 % of
to run at maximum?
> + the time. One should note that this represents aggregate time over all CPUs
> + in the system. Therefore, in order to allow full usage of two CPUs, for
> + instance, one should set this value to twice the value of cfs_period_us.
> +- cpu.stat: statistics about the bandwidth controls. No data will be presented
> + if cpu.cfs_quota_us is not set. The file presents three
Unnecessary line break?
> + numbers:
> + nr_periods: how many full periods have been elapsed.
> + nr_throttled: number of times we exausted the full allowed bandwidth
> + throttled_time: total time the tasks were not run due to being overquota
> + - cpu.rt_runtime_us and cpu.rt_period_us: Those files are the RT-tasks
> + analogous to the CFS files cfs_quota_us and cfs_period_us. One important
> + difference, though, is that while the cfs quotas are upper bounds that
> + won't necessarily be met, the rt runtimes form a stricter guarantee.
runtimes are strict guarantees?
> + Therefore, no overlap is allowed. Implications of that are that given a
maybe overcommit is a better term?
> + hierarchy with multiple children, the sum of all rt_runtime_us may not exceed
> + the runtime of the parent. Also, a rt_runtime_us of 0, means that no rt tasks
> + can ever be run in this cgroup. For more information about rt tasks runtime
> + assignments, see scheduler/sched-rt-group.txt
> + - cpuacct.usage: The aggregate CPU time, in nanoseconds, consumed by all tasks
> + in this group.
> + - cpuacct.usage_percpu: The CPU time, in nanoseconds, consumed by all tasks in
> + this group, separated by CPU. The format is an space-separated array of time
> + values, one for each present CPU.
> + - cpuacct.stat: aggregate user and system time consumed by tasks in this group.
> + The format is
> + user: x
> + system: y
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