linux-kernel - Re: Possible bug from kernel 2.6.22 and above, 2.6.24-rc4

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Message-ID: <475708A7.4030708@jlab.org>
Date:	Wed, 05 Dec 2007 15:23:03 -0500
From:	Jie Chen <chen@...b.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Simon Holm Th??gersen <odie@...aau.dk>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Possible bug from kernel 2.6.22 and above, 2.6.24-rc4

Ingo Molnar wrote:
> * Jie Chen <chen@...b.org> wrote:
> 
>> Since I am using affinity flag to bind each thread to a different 
>> core, the synchronization overhead should increases as the number of 
>> cores/threads increases. But what we observed in the new kernel is the 
>> opposite. The barrier overhead of two threads is 8.93 micro seconds vs 
>> 1.86 microseconds for 8 threads (the old kernel is 0.49 vs 1.86). This 
>> will confuse most of people who study the 
>> synchronization/communication scalability. I know my test code is not 
>> real-world computation which usually use up all cores. I hope I have 
>> explained myself clearly. Thank you very much.
> 
> btw., could you try to not use the affinity mask and let the scheduler 
> manage the spreading of tasks? It generally has a better knowledge about 
> how tasks interrelate.
> 
> 	Ingo
Hi, Ingo:

I just disabled the affinity mask and reran the test. There were no 
significant changes for two threads (barrier overhead is around 9 
microseconds). As for 8 threads, the barrier overhead actually drops a 
little, which is good. Let me know whether I can be any help. Thank you 
very much.

-- 
###############################################
Jie Chen
Scientific Computing Group
Thomas Jefferson National Accelerator Facility
12000, Jefferson Ave.
Newport News, VA 23606

(757)269-5046 (office) (757)269-6248 (fax)
chen@...b.org
###############################################

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