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fftw shockingly slow

 
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lqcd_chris



Joined: 05 Jul 2007
Posts: 2

PostPosted: Thu Jul 05, 2007 3:51 pm    Post subject: fftw shockingly slow Reply with quote

Hello:

I have compiled some physics code using fftw with pg... v. 6.2-3 and with icc/ifort v. 9.1 to compare performance.

For intel, I used the -fast option, and found the fftw3.1.2 part runs in 1 minute, which is a little less than what I get using gcc.

Then, I compiled with the Portland Group compiler (both the fftw libraries and my program, of course), and the same fftw takes 10 minutes!

After trying a few different sets of options including just -fast -tp amd64, I found the best one is -O0 -tp amd64, for which the fftw part still takes 3.5 minutes!

I am quite sure there is something funny going on, but I have no idea what it is. Has anyone here seen something like this before?

Thanks,
Chris
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mkcolg



Joined: 30 Jun 2004
Posts: 6208
Location: The Portland Group Inc.

PostPosted: Fri Jul 06, 2007 8:31 am    Post subject: Reply with quote

Hi Chris,

Quote:
After trying a few different sets of options including just -fast -tp amd64, I found the best one is -O0 -tp amd64, for which the fftw part still takes 3.5 minutes!

I am quite sure there is something funny going on, but I have no idea what it is. Has anyone here seen something like this before?

This is very odd especially that unoptimized code would run nearly three times fast than optimized code. Is it possible to obtain the source, workload, and build instruction for your physics code? I'd be very interested in determining where the slow down is.

Thanks,
Mat
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lqcd_chris



Joined: 05 Jul 2007
Posts: 2

PostPosted: Fri Jul 20, 2007 11:53 am    Post subject: Reply with quote

Hi, Mat

Thanks for your quick reply. I have been away for a while, and just found it. I don't think sharing the code is a problem, but I have to talk to a few people first.
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