Re: [patch 15/16] sched: return unused runtime on voluntary sleep

From: Hidetoshi Seto
Date: Wed Jun 22 2011 - 05:40:19 EST

(2011/06/21 16:17), Paul Turner wrote:
> When a local cfs_rq blocks we return the majority of its remaining quota to the
> global bandwidth pool for use by other runqueues.
> We do this only when the quota is current and there is more than
> min_cfs_rq_quota [1ms by default] of runtime remaining on the rq.
> In the case where there are throttled runqueues and we have sufficient
> bandwidth to meter out a slice, a second timer is kicked off to handle this
> delivery, unthrottling where appropriate.
> Using a 'worst case' antagonist which executes on each cpu
> for 1ms before moving onto the next on a fairly large machine:
> no quota generations:
> 197.47 ms /cgroup/a/cpuacct.usage
> 199.46 ms /cgroup/a/cpuacct.usage
> 205.46 ms /cgroup/a/cpuacct.usage
> 198.46 ms /cgroup/a/cpuacct.usage
> 208.39 ms /cgroup/a/cpuacct.usage
> Since we are allowed to use "stale" quota our usage is effectively bounded by
> the rate of input into the global pool and performance is relatively stable.
> with quota generations [1s increments]:
> 119.58 ms /cgroup/a/cpuacct.usage
> 119.65 ms /cgroup/a/cpuacct.usage
> 119.64 ms /cgroup/a/cpuacct.usage
> 119.63 ms /cgroup/a/cpuacct.usage
> 119.60 ms /cgroup/a/cpuacct.usage
> The large deficit here is due to quota generations (/intentionally/) preventing
> us from now using previously stranded slack quota. The cost is that this quota
> becomes unavailable.
> with quota generations and quota return:
> 200.09 ms /cgroup/a/cpuacct.usage
> 200.09 ms /cgroup/a/cpuacct.usage
> 198.09 ms /cgroup/a/cpuacct.usage
> 200.09 ms /cgroup/a/cpuacct.usage
> 200.06 ms /cgroup/a/cpuacct.usage
> By returning unused quota we're able to both stably consume our desired quota
> and prevent unintentional overages due to the abuse of slack quota from
> previous quota periods (especially on a large machine).
> Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
> ---

(For all but the patch title:)
Reviewed-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>


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