linux-kernel - Re: balance storm

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Date:	Mon, 26 May 2014 16:19:20 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Libo Chen <libo.chen@...wei.com>
Cc:	tglx@...utronix.de, mingo@...e.hu,
	LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Li Zefan <lizefan@...wei.com>, peterz@...radead.org
Subject: Re: balance storm

On Mon, 2014-05-26 at 20:16 +0800, Libo Chen wrote: 
> On 2014/5/26 13:11, Mike Galbraith wrote:

> > Your synthetic test is the absolute worst case scenario.  There has to
> > be work between wakeups for select_idle_sibling() to have any chance
> > whatsoever of turning in a win.  At 0 work, it becomes 100% overhead.
> 
> not synthetic, it is a real problem in our product. under no load, waste
> much cpu time.

What happens in your product if you apply the commit I pointed out?

> >> so I think 15% cpu usage and migration event are too high, how to fixed?
> > 
> > You can't for free, low latency wakeup can be worth one hell of a lot.
> > 
> > You could do a decayed hit/miss or such to shut the thing off when the
> > price is just too high.  Restricting migrations per unit time per task
> > also helps cut the cost, but hurts tasks that could have gotten to the
> > CPU quicker, and started your next bit of work.  Anything you do there
> > is going to be a rob Peter to pay Paul thing.
> > 
> 
> I had tried to change sched_migration_cost and sched_nr_migrate in /proc,
> but no use.  any other  suggestion?
> 
> I still think this is a problem to schedular.  it is better to directly solve
> this issue instead of a workaroud

I didn't say it wasn't a problem, it is.  I said whatever you do will be
a tradeoff.

-Mike

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