linux-kernel - Re: balance storm

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Message-ID: <53848B38.5090408@huawei.com>
Date:	Tue, 27 May 2014 20:55:20 +0800
From:	Libo Chen <libo.chen@...wei.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <umgwanakikbuti@...il.com>, <tglx@...utronix.de>,
	<mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: balance storm

On 2014/5/27 17:48, Peter Zijlstra wrote:
> So: 
> 
> 1) what kind of weird ass workload is that? Why are you waking up so
> often to do no work?

it's just a testcase, I agree it doesn`t exist in real world.

> 
> 2) turning on/off share_pkg_resource is a horrid hack whichever way
> aruond you turn it.
> 
> So I suppose this is due to the select_idle_sibling() nonsense again,
> where we assumes L3 is a fair compromise between cheap enough and
> effective enough.
> 
> Of course, Intel keeps growing the cpu count covered by L3 to ridiculous
> sizes, 8 cores isn't nowhere near their top silly, which shifts the
> balance, and there's always going to be pathological cases (like the
> proposed workload) where its just always going to suck eggs.
> 
> Also, when running 50 such things on a 16 cpu machine, you get roughly 3
> per cpu, since their runtime is stupid low, I would expect it to pretty
> much always hit an idle cpu, which in turn should inhibit the migration.
> 
> Then again, maybe the timer slack is causing you grief, resulting in all
> 3 being woken at the same time, instead of having them staggered.
> 
> In any case, I'm not sure what the 'regression' report is against, as
> there's only a single kernel version mentioned: 3.4, and that's almost a
upstream has the same problem, I have mentioned before.

> dinosaur.


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