linux-kernel - Re: [PATCH v2] sched: wake-affine throttle

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Message-ID: <1369722552.5857.72.camel@marge.simpson.net>
Date:	Tue, 28 May 2013 08:29:12 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Alex Shi <alex.shi@...el.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH v2] sched: wake-affine throttle

On Tue, 2013-05-28 at 13:02 +0800, Michael Wang wrote: 
> On 05/22/2013 10:55 PM, Mike Galbraith wrote:
> > On Wed, 2013-05-22 at 17:25 +0800, Michael Wang wrote:
> > 
> >> I've not test the hackbench with wakeup-buddy before, will do it this
> >> time, I suppose the 15% illegal income will suffered, anyway, it's
> >> illegal :)
> > 
> > On a 4 socket 40 core (+SMT) box, hackbench wasn't too happy.
> 
> I've done more test and now I got the reason of regression...
> 
> The writer and reader in hackbench is N:N, prev writer will write all
> the fd then switch to next writer and repeat the same work, so it's
> impossible to setup the buddy relationship by just record the last one,
> and we have to record all the waker/wakee in history, but that means
> unacceptable memory overhead...

Yeah, that's why I was thinking we'd need a dinky/fast as hell FIFO of
tokens or such to bind waker/wakee more or less reliably.  Making such a
scheme cheap enough could be hard.

> So this buddy idea seems to be bad...
> 
> I think a better way may should be allowing pull in most time, but
> filter the very bad cases carefully.

Any way that is cheap, and fairly accurately recognizes when we're being
stupid will help.  First and foremost, it has to be dirt cheap :)

> For workload like pgbench, we actually just need to avoid pull if that
> will damage the 'mother' thread, which is busy and be relied by many
> 'child'.

Yeah, 'mom' is the key player.  If we can cheaply recognize mom, that
should get us a generic improvement.  Not as good as being able to
recognize the size of her+brood as size changes, but better anyway.

-Mike

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