linux-kernel - Re: [RFC PATCH v3 1/3] sched: schedule balance map foundation

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Message-ID: <5126F83E.9060206@intel.com>
Date:	Fri, 22 Feb 2013 12:46:54 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH v3 1/3] sched: schedule balance map foundation

On 02/22/2013 12:19 PM, Michael Wang wrote:
> 
>> > Why not seek other way to change O(n^2) to O(n)?
>> > 
>> > Access 2G memory is unbelievable performance cost.
> Not access 2G memory, but (2G / 16K) memory, the sbm size is O(N).
> 
> And please notice that on 16k cpus system, topology will be deep if NUMA
> enabled (O(log N) as Peter said), and that's really a good stage for
> this idea to perform on, we could save lot's of recursed 'for' cycles.
> 

CPU execute part is very very fast compare to the memory access, the
'for' cycles cost is most on the memory access for many domain/groups
data, not instruction execution.

In a hot patch, several KB memory access will cause clear cpu cache
pollution then make kernel slowly.

-- 
Thanks Alex
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