linux-kernel - Re: [patch] Re: PostgreSQL pgbench performance regression in 2.6.23+

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Date:	Sat, 07 Jun 2008 18:16:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Ingo Molnar <mingo@...e.hu>, Greg Smith <gsmith@...gsmith.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: [patch] Re: PostgreSQL pgbench performance regression in
	2.6.23+

On Sat, 2008-06-07 at 16:16 +0200, Mike Galbraith wrote:
> On Sat, 2008-06-07 at 15:07 +0200, Peter Zijlstra wrote:
> > On Sat, 2008-06-07 at 14:50 +0200, Mike Galbraith wrote:
> > > Since I tested mysql+oltp and made the dang pdf of the results, I may
> > > as well actually attach the thing <does that before continuing...>.
> > > 
> > > BTW, I have a question wrt avg_overlap.  When a wakeup cause the current
> > > task to begin sharing CPU with a freshly awakened task, the current task
> > > is tagged.. but the wakee isn't.  How come?  If one is sharing, so is
> > > the other.
> > 
> > avg_overlap is about measuring how long we'll run after waking someone
> > else. The other measure, how long our waker shares the cpu with us,
> > hasn't proven to be relevant so far.
> 
> Yeah wrt relevance, I've been playing with making it mean this and that,
> with approx 0 success ;-)  If it's a measure of how long we run after
> waking though, don't we need to make sure it's not a cross CPU wakeup?

The idea was to dynamically detect sync wakeups, who's defining property
is that the waker will sleep after waking the wakee. And who's effect is
pulling tasks together on wakeups - so that we might have the most
benefit of cache sharing.

So if we were to exclude cross cpu wakeups from this measurement we'd
handicap the whole scheme, because then we'd never measure that its
actually a sync wakeup and wants to run on the same cpu.

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