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

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Message-ID: <51A45B23.4050403@linux.vnet.ibm.com>
Date:	Tue, 28 May 2013 15:22:11 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Mike Galbraith <bitbucket@...ine.de>
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 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 :)

Regards,
Michael Wang

> 
> -Mike
> 

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