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

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Date:	Tue, 28 May 2013 10:49:30 +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 15:22 +0800, Michael Wang wrote: 
> On 05/28/2013 02:29 PM, Mike Galbraith wrote:
> > 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.
> 
> That's right, I'm trying to rely on the frequency of a task switching
> it's wakee in the new idea, it's really cheap and somewhat reliable, I
> appreciate if you could pay an eye on the new patch and let me know you
> opinion :)

I'll feed it to my 40 core box, hopefully soon.

-Mike

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