linux-kernel - Re: volano ~30% regression with 2.6.33-rc1 & -rc2

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Message-ID: <1262611608.6408.126.camel@laptop>
Date:	Mon, 04 Jan 2010 14:26:48 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Lin Ming <ming.m.lin@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Subject: Re: volano ~30% regression with 2.6.33-rc1 & -rc2

On Mon, 2010-01-04 at 14:15 +0100, Mike Galbraith wrote:
> On Mon, 2010-01-04 at 14:02 +0100, Peter Zijlstra wrote:
> > On Mon, 2010-01-04 at 13:57 +0100, Mike Galbraith wrote:
> > > On Mon, 2010-01-04 at 04:40 -0800, Arjan van de Ven wrote:
> > > > On Mon, 04 Jan 2010 16:15:58 +0800
> > > > Lin Ming <ming.m.lin@...el.com> wrote:
> > > > 
> > > > > Mike & Peter,
> > > > > 
> > > > > Compared with 2.6.32, volano has ~30% regression with 2.6.33-rc1 &
> > > > > -rc2. Testing machine: Tigerton Xeon, 16cpus(4P/4Core), 16G memory
> > > > 
> > > > did this show up only on this cpu?
> > > > (since this is a multi-core-without-shared-cache cpu, it could be that
> > > > we get the topology wrong and think cores share cache where they don't)
> > > 
> > > My fault for using PREFER_SIBLING I guess.  However, I do wonder why in
> > > the heck we set that at the CPU domain level.  Siblings lie northward.
> > 
> > Ah, PREFER_SIBLING means prefer sibling domain, not sibling thread. Its
> > set at the CPU (really socket) level so make tasks spread over sockets
> > first, so that there is no competition for the socket wide resources.
> 
> WRT the regression, would you prefer only the sched_fair.c hunk, and
> maybe plunking the topology hunk in sched_devel, or both lines in one
> patch, since ramp-up gain remains unrealized half of the time on Nehalem
> and ilk.

Both bits seem sane I guess, you change SD_SIBLING_INIT(), right?
Threads really do share package resources so it makes sense to set it.

I guess its back to poking at nehalem to see what makes it tick..

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