linux-kernel - Re: Performance loss 2.6.22->22.6.23->2.6.24-rc7 on CPU intensive benchmark on 8 Core Xeon

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Date:	Fri, 18 Jan 2008 11:55:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Colin Fowler <elethiomel@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Performance loss 2.6.22->22.6.23->2.6.24-rc7 on CPU intensive
	benchmark on 8 Core Xeon


* Colin Fowler <elethiomel@...il.com> wrote:

> > there are a handful of 'scheduler feature bits' in
> > /proc/sys/kernel/sched_features:
> >
> > enum {
> >         SCHED_FEAT_NEW_FAIR_SLEEPERS    = 1,
> >         SCHED_FEAT_WAKEUP_PREEMPT       = 2,
> >         SCHED_FEAT_START_DEBIT          = 4,
> >         SCHED_FEAT_TREE_AVG             = 8,
> >         SCHED_FEAT_APPROX_AVG           = 16,
> > };
> >
> 
> Toggling SCHED_FEAT_NEW_FAIR_SLEEPERS to 0 or 
> SCHED_FEAT_WAKEUP_PREEMPT to 0 gives me results more inline with my 
> 2.6.22 results. Toggling them both to 0 gives me slightly better 
> results than 2.6.22!

ok, but it would be nice to avoid having to turn these off. Could you 
try whether tuning the /proc/sys/kernel/*granularity* values (in 
particular wakeup_granularity) has any positive effect on your workload?

also, could you run your workload as SCHED_BATCH [via schedtool -B ], 
does that improve the results as well on a default-tuned kernel?

	Ingo
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