Strange thread behaviour on 8-way x86 machine

[Sasha Pachev <sasha@mysql.com>]

Hi,

I have observed a rather strange behaviour doing a multi-threaded CPU 
benchmark on an 8-way machine running 2.4.2 SMP kernel. Even when the 
priority is reniced to the highest possible value, I am still unable to reach 
more than 50% CPU utilization. My benchmark just creates a bunch of threads 
with pthread_create(), and then runs a simple integer computation in each 
thread. On a dual with 2.4.3 kernel, and a 4-way with 2.4.2 kernel, I am able 
to reach full CPU utilization. 

At first glance, it appears to be like some overzealous fairness problem in 
the scheduler that is exhibited only when you have more than 4 CPUs. Before I 
start scrutinizing the source trying to understand the inner workings of the 
scheduler, I would like to get some feedback from people that know something 
about the subject. Any ideas/suggestions would be appreciated.

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  /  |/  /_ __/ __/ __ \/ /   Sasha Pachev <sasha@mysql.com>
 / /|_/ / // /\ \/ /_/ / /__  MySQL AB, http://www.mysql.com/
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