Fortran comparison Re: [SciPy-dev] scipy.weave versus simple C++.
Pearu Peterson
pearu at cens.ioc.ee
Sat Jan 12 17:30:32 CST 2002
On Sat, 12 Jan 2002, eric wrote:
> > Iterations took 6.12530398369 seconds.
> > Fortran iterations took 3.15447306633 seconds.
>
> Hey! That is larger than I would have expected. Fortran often has a slight
> advantage ( a few percent), but not a factor of 2. Several things could be
> in play here.
>
> (1) Prabhu and I have no clue how to set compiler switches in gcc for best
> results. (very possible)
> (2) Your using a better optimizing Fortran compiler (wonder what the Intel
> or KAI compiler would do on the C++ code). Which one are you using? If it
> is g77, then this is exhibit A for supporting (1).
> (3) If comparisons are made against Prabhu's C++ code instead of my
> extension module which (mis) uses blitz++ arrays for indexing, would only
> show a 33% improvement. Correct? This is starting to be more what I'd
> expect.
I used the same code (laplace.py) that was in your message to this list.
>
> Anyway, I'd be surprised if the difference is this great in the final
> analysis -- I'll bet less than 10% difference either way.
I used g77 in these tests. But indeed, also with heavy optimizations
switches turned on. Here are other results with different optimization
flags enabled:
1) g77 -02 (standard)
Iterations took 6.34938693047 seconds.
Error: 38047.7567709
Fortran iterations took 3.69506299496 seconds.
Error: 21778.5117188
2) g77 (no optimization)
Iterations took 6.44155204296 seconds.
Error: 38047.7567709
Fortran iterations took 3.82356095314 seconds.
Error: 21778.5117188
3) g77 -O3 -funroll-loops -march=i686 -malign-double (machine depend.)
Iterations took 6.34858202934 seconds.
Error: 38047.7567709
Fortran iterations took 3.29500699043 seconds.
Error: 21778.5117188
4) ifc -O3 -tpp6 -xi (this is Intel Fortran 90 compiler for Linux)
Iterations took 6.38630092144 seconds.
Error: 38047.7567709
Fortran iterations took 2.84432804585 seconds.
Error: 21779.0996094
Note that even without any optimization and with g77, the factor is still
quite high: 1.6. And with Intel compiler this is 2.2.
So, I would not be so pessimistic about Fortran capabilities. I guess that
these numbers also depend on the particular test, and the current one just
fits for Fortran. Anyway, I don't think that people would use Fortran if
the gain in speed would be only few percent. It must be higher than that.
Regards,
Pearu
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