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

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Date:	Tue, 24 Jul 2007 17:09:28 -0400
From:	Chris Snook <csnook@...hat.com>
To:	"Li, Tong N" <tong.n.li@...el.com>
CC:	Chris Friesen <cfriesen@...tel.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] scheduler: improve SMP fairness in CFS

Li, Tong N wrote:
> 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).

I don't think Chris's scenario has much bearing on your patch.  What he wants is 
to have a task that will always be running, but can't monopolize either CPU. 
This is useful for certain realtime workloads, but as I've said before, realtime 
requires explicit resource allocation.  I don't think this is very relevant to 
SCHED_FAIR balancing.

	-- Chris
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