linux-kernel - Re: [PATCH] x86,sched: Fix sched_smt_power

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Message-ID: <4F0C5138.5010109@linux.intel.com>
Date:	Tue, 10 Jan 2012 06:54:48 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Youquan Song <youquan.song@...el.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, stable@...r.kernel.org,
	len.brown@...el.com, anhua.xu@...el.com, chaohong.guo@...el.com,
	Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH] x86,sched: Fix sched_smt_power_savings totally broken

On 1/10/2012 6:41 AM, Peter Zijlstra wrote:
> On Tue, 2012-01-10 at 06:32 -0800, Arjan van de Ven wrote:
>>
>> a very good default would be to keep all tasks on one package until half
>> the cores in the package are busy, and then start spreading out. 
> 
> Does that still make sense when there's strong NUMA preference? By
> forcing stuff on a single package you increase the number of remote
> memory fetches (which generally generate more stalls), also the memory
> controllers need to stay awake anyway.

the memory controllers need to stake regardless of what you do;
it's more a memory bandwidth kind of thing.

if you have an enormous numa factor (>= 10 or so), then you really need
a completely different policy I suspect.
Thankfully those are rare.
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