linux-kernel - Re: High wake up latencies with FAIR_USER

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Date:	Mon, 28 Jan 2008 17:57:49 +0100
From:	"Guillaume Chazarain" <guichaz@...oo.fr>
To:	vatsa@...ux.vnet.ibm.com
Cc:	"Ingo Molnar" <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, dhaval@...ux.vnet.ibm.com
Subject: Re: High wake up latencies with FAIR_USER_SCHED

Hi Srivatsa,

On Jan 28, 2008 3:31 AM, Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> wrote:
> Given that sysctl_sched_wakeup_granularity is set to 10ms by default,
> this doesn't sound abnormal.

Indeed, by lowering sched_wakeup_granularity I get much better
latencies, but lowering sched_latency seems to be more effective.

> NEW_FAIR_SLEEPERS feature gives credit for sleeping only to tasks and
> not group-level entities. With the patch attached, I could see that wakeup
> latencies with FAIR_USER_SCHED are restored to the same level as
> !FAIR_USER_SCHED.

Thanks for the patch, it works perfectly.

> However I am not sure whether that is the way to go. We want to let one group of
> tasks running as much as possible until the fairness/wakeup-latency threshold is
> exceeded. If someone does want better wakeup latencies between groups too, they
> can always tune sysctl_sched_wakeup_granularity.

Having an inconsistency here between FAIR_USER_SCHED and
!FAIR_USER_SCHED sounds strange, but Ingo took the patch, so I'm happy
:-)

Thanks for the replies.

-- 
Guillaume
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