Re: Octave versus matlab performances on linux platform 64 bits

[Jaroslav Hajek]

On Thu, Oct 30, 2008 at 7:01 AM, Luca Tagliacozzo
<address@hidden> wrote:
> Hello, I was wondering what  I am doing wrong:
>
> I was planning to pass some matlab programs to octave but as I will have to
> do real simulation I was planning to write them in C++.
>
> Before starting the project I decided to make a simple test.
>
> I took the DaCoda example of the matpow.m file, did the esteatical changes
> such that it can be run on matlab,
>
> and I run it on matlab via the following script: test.m
>
>
> a=matpow(rand(1000),10);
>
>
> That is it mutliplies log_2(10) times  a 1000x1000 matrix
>
> By running
> time matlab -nodisplay <test.m
>
>
> I get     0m4.076s as a user time
>
> we see that it take 4.076s to execute on my already fully loaded machine.
>
> Now always with the matpow.m file i did the following test:
>
> time octave test.m: user    0m21.189s
> where we see that Octave took 21.189 seconds  (like 7 times matlab)
>
> Till here nothing strange since I was exectuing the matpow.m file.
> Now I compile the matpow.cc and I crosscheck that octave sees it:
>
>
> octave:1> help matpow
> Return b = a^n for square matrix a, and non-negative, integral
> n./home/luca/Desktop/testing_perf/matpow.oct
> ......
>
> where we see that indeed it does see it in
> /home/luca/Desktop/testing_perf/matpow.oct
>
> So now I run again the test: and we see that the the time is now 20.161s
> very slight variation with respect to the non compiled case.
>
>
> I thought it could be a matter of overload taken by octave to startup so I
> decided to increase the matrix size to 3000x3000 where we would expect that
> this should run for around 10 minutes so that the starting overhead should
> be negligible
>
>
>
>
> By running the .m file with matlab I get: 1m3.564s
>
> By running the .oct file with octave I get: a user time 8m59.474s
>
> By running the .m file with octave I get: 8m58.702s
>
>
>
> So I do not get any increase of performance by compiling the matlab
> matpow.cc .
>
>
> What am I missing?


What Octave version do you use? How is it configured?
The speed of large matrix multiplication is basically all about what
BLAS library you use. If Octave doesn't find a suitable BLAS library,
it will compile a simplistic one for you, but depending on what
Fortran compiler you use, the compiled thing's speed may range from "a
little slow" to "really slow". Also, chances are that your Matlab's
BLAS is exploiting multiple cores, if you have a multi-core machine.

-- 
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz