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

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Date:	Wed, 5 Dec 2007 16:40:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jie Chen <chen@...b.org>
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

* Jie Chen <chen@...b.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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