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

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Date:	Tue, 24 Jul 2007 13:58:11 -0700
From:	"Li, Tong N" <tong.n.li@...el.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	Chris Friesen <cfriesen@...tel.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] scheduler: improve SMP fairness in CFS

On Tue, 2007-07-24 at 16:39 -0400, Chris Snook wrote:

> Divining the intentions of the administrator is an AI-complete problem and we're 
> not going to try to solve that in the kernel.  An intelligent administrator 
> could also allocate 50% of each CPU to a resource group containing all the 
> *other* processes.  Then, when the other processes are scheduled out, your 
> single task will run on whichever CPU is idle.  This will very quickly 
> equilibrate to the scheduling ping-pong you seem to want.  The scheduler 
> deliberately avoids this kind of migration by default because it hurts cache and 
> TLB performance, so if you want to override this very sane default behavior, 
> you're going to have to explicitly configure it yourself.
> 

Well, the admin wouldn't specifically ask for 50% of each CPU. He would
just allocate 50% of total CPU time---it's up to the scheduler to
fulfill that. If a task is entitled to one CPU, then it'll stay there
and have no migration. Migration occurs only if there's overload, in
which case I think you agree in your last email that the cache and TLB
impact is not an issue (at least in SMP).

  tong
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