linux-kernel - Re: [RFC] scheduler: improve SMP fairness in CFS

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Message-ID: <46A68C37.30808@nortel.com>
Date:	Tue, 24 Jul 2007 17:33:11 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Chris Snook <csnook@...hat.com>
CC:	"Li, Tong N" <tong.n.li@...el.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] scheduler: improve SMP fairness in CFS

Chris Snook wrote:

> A fraction of *each* CPU, or a fraction of *total* CPU?  Per-cpu 
> granularity doesn't make anything more fair.

Well, our current solution uses per-cpu weights, because our vendor 
couldn't get the load balancer working accurately enough.  Having 
per-cpu weights and cpu affinity gives acceptable results for the case 
where we're currently using it.

If the load balancer is good enough, per-system weights would be fine. 
It would have to play nicely with affinity though, in the case where it 
makes sense to lock tasks to particular cpus.

> If I have two threads with the same priority, and two CPUs, the 
> scheduler will put one on each CPU, and they'll run happily without any 
> migration or balancing.

Sure.  Now add a third thread.  How often do you migrate?  Put another 
way, over what time quantum do we ensure fairness?

Chris
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