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

[Jie Chen <chen@jlab.org>]

Ingo Molnar wrote:
> * 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).

My test code are not doing much work but measuring overhead of various 
synchronization mechanisms such as barrier and lock. I am trying to see 
the scalability of different implementations/algorithms on multi-core 
machines.

> 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.
> 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!)

The qmt.tar.gz code contains a simple test program call pthread_sync 
under the src directory. You can change the number of threads by setting 
QMT_NUM_THREADS environment variable. You can build the qmt by doing 
configure --enable-public-release. I do not have Intel quad core 
machines, I am not sure whether the behavior will show up on Intel 
platform. Our cluster is dual quad-core opteron which has its own 
hardware problem :-).
http://hardware.slashdot.org/article.pl?sid=07/12/04/237248&from=rss

> 	Ingo

Hi, Ingo:

My test code qmt can be found at ftp://ftp.jlab.org/pub/hpc/qmt.tar.gz. 
There is a minor performance issue in qmt pointed out by Eric, which I 
have not put into the tar ball yet. If I can be any help, please let me 
know. Thank you very much.



--
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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@jlab.org
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