linux-kernel - Re: fair group scheduler not so fair?

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Message-ID: <20080523101951.GL3780@linux.vnet.ibm.com>
Date:	Fri, 23 May 2008 15:49:51 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"Li, Tong N" <tong.n.li@...el.com>,
	Chris Friesen <cfriesen@...tel.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, pj@....com
Subject: Re: fair group scheduler not so fair?

On Fri, May 23, 2008 at 11:39:21AM +0200, Peter Zijlstra wrote:
> On Fri, 2008-05-23 at 15:12 +0530, Srivatsa Vaddagiri wrote:
> > On Thu, May 22, 2008 at 01:18:33PM -0700, Li, Tong N wrote:
> > > Peter,
> > > 
> > > I didn't look at your patches, but I thought you were flattening group
> > > weights down to task-level so that the scheduler only looks at per-task
> > > weights.
> > 
> > Wouldnt that require task weight readjustment upon every fork/exit?
> 
> If you were to do that - yes that would get you into some very serious
> trouble.
> 
> The route I've chosen is to basically recompute it every time I need the
> weight. So every time I use a weight, I do:

and which are those points when "you need the weight"?

Basically here's what I had in mind:

	Group A shares = 1024
	# of tasks in group A = 1 (T0)

So T0 weight can be 1024.

	T0 now forks 1000 children. Ideally now,
		T0.weight = T1.weight = .... = T999.weight = 1024/1000

If we don't change each task's weight like this, then group A will
cumulatively get more share than it deserves.

Are you saying you will change each task's weight lazily? If so how?


-- 
Regards,
vatsa
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