Date: Wed, 5 Dec 2007 17:47:23 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Jie Chen <chen@jlab.org>
Cc: Simon Holm Th??gersen <odie@cs.aau.dk>, Eric Dumazet <dada1@cosmosbay.com>,
       linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: Possible bug from kernel 2.6.22 and above, 2.6.24-rc4
Message-ID: <20071205164723.GA25641@elte.hu>
References: <4744966C.900@jlab.org> <4744ADA9.7040905@cosmosbay.com> <4744E0DC.7050808@jlab.org> <1195698770.11808.4.camel@odie.local> <4744F042.4070002@jlab.org> <20071204131707.GA4232@elte.hu> <4756C3D9.9030107@jlab.org> <20071205154014.GA6491@elte.hu> <4756D058.1070500@jlab.org>
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* Jie Chen <chen@jlab.org> wrote:

>> the moment you saturate the system a bit more, the numbers should 
>> improve even with such a ping-pong test.
>
> You are right. If I manually do load balance (bind unrelated processes 
> on the other cores), my test code perform as well as it did in the 
> kernel 2.6.21.

so right now the results dont seem to be too bad to me - the higher 
overhead comes from two threads running on two different cores and 
incurring the overhead of cross-core communications. In a true 
spread-out workloads that synchronize occasionally you'd get the same 
kind of overhead so in fact this behavior is more informative of the 
real overhead i guess. In 2.6.21 the two threads would stick on the same 
core and produce artificially low latency - which would only be true in 
a real spread-out workload if all tasks ran on the same core. (which is 
hardly the thing you want on openmp)

In any case, if i misinterpreted your numbers or if you just disagree, 
or if have a workload/test that shows worse performance that it 
could/should, let me know.

	Ingo
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