Re: scheduler vs hardware? (was Re: another i7 (linux) bug?)

[Mike Galbraith]

On Fri, 2010-01-22 at 09:51 +0100, Peter Zijlstra wrote:
> On Fri, 2010-01-22 at 08:19 +0100, Mike Galbraith wrote:
> > > if I run a cpu intensive process with the lowest priority (19
> > > from man nice) I obtain much better performance that with the
> > > highest priority available (-20 from man nice).
> > >
> > > For example the same file is processed by lame in 8.7 seconds
> > > at the lowest priority, and in 12 seconds at the highest
> > > priority.  Before posting a bug I wold like to understand if
> > > this is a problem related to the i7 mobile (my processor is a
> > > i7 Q720).
> > > 
> > > As far as I tested on the same laptop series (dell studio 15),
> > > with the same kernel this problem does not exists.
> > 
> > So you only see this on the i7.  That's odd.  Can you try 33-rc5?
> > 
> > Posting a reliable reproducer would be nice.  It'd also be nice to see
> > what all is running when you see this, and where.
> 
> Using a sample from: http://lame.sourceforge.net/quality.php
> 
> My laptop does:
> 
> 
> # time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

> real    0m3.273s
> user    0m3.217s
> sys     0m0.022s
> 
> 
> # time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null 

> real    0m1.121s
> user    0m1.102s
> sys     0m0.013s
> 
> 
> # time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

> real    0m1.112s
> user    0m1.093s
> sys     0m0.018s
> 
> 
> 
> On a Nehalem class server machine it does:
> 
> 
> # time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

> real    0m0.932s
> user    0m0.917s
> sys     0m0.005s
> 
> 
> # time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

> real    0m0.927s
> user    0m0.922s
> sys     0m0.003s
> 
> 
> # time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

> real    0m0.919s
> user    0m0.914s
> sys     0m0.005s

Weird.  Here there is zip squat difference, as expected with 1 thread.

time nice -n 19 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

real    0m0.912s
user    0m0.908s
sys     0m0.000s

time nice -n 0 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

real    0m0.912s
user    0m0.904s
sys     0m0.004s

time nice -n -20 lame -b 256 -V0 -h youcantdothat.wav - > /dev/null

real    0m0.912s
user    0m0.904s
sys     0m0.004s

(bah, who needs a nehalem;)

	-Mike

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