Date: Wed, 5 Dec 2007 16:40:14 +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: <20071205154014.GA6491@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>
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* Jie Chen <chen@jlab.org> wrote:

> I just ran the same test on two 2.6.24-rc4 kernels: one with 
> CONFIG_FAIR_GROUP_SCHED on and the other with CONFIG_FAIR_GROUP_SCHED 
> off. The odd behavior I described in my previous e-mails were still 
> there for both kernels. Let me know If I can be any more help. Thank 
> you.

ok, i had a look at your data, and i think this is the result of the 
scheduler balancing out to idle CPUs more agressively than before. Doing 
that is almost always a good idea though - but indeed it can result in 
"bad" numbers if all you do is to measure the ping-pong "performance" 
between two threads. (with no real work done by any of them).

the moment you saturate the system a bit more, the numbers should 
improve even with such a ping-pong test.

do you have testcode (or a modification of your testcase sourcecode) 
that simulates a real-life situation where 2.6.24-rc4 performs not as 
well as you'd like it to see? (or if qmt.tar.gz already contains that 
then please point me towards that portion of the test and how i should 
run it - thanks!)

	Ingo
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