linux-kernel - Re: [patch 14/15] sched: return unused runtime on voluntary sleep

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Date:	Tue, 10 May 2011 16:28:03 +0900
From:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
To:	Paul Turner <pjt@...gle.com>
CC:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [patch 14/15] sched: return unused runtime on voluntary sleep

(2011/05/03 18:29), 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@...gle.com>
> 
> ---

Reviewed-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>

Thanks,
H.Seto

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