Re: Instructions for Installing BLAS and LAPACK library on Octave 5.2.0

On Tue, Sep 29, 2020 at 11:42 AM Nicholas Jankowski <[email protected]>
wrote:

> On Tue, Sep 29, 2020 at 11:38 AM Ilaya Bharathi <
> [email protected]> wrote:
>
>> Hi Nicholas,
>>
>> Do you have the variable OPENBLAS_NUM_THREADS by default with the zip
>> installation?
>> Also, are there any codes to test the implementation?
>>
>>
> for the latter, I do not know. not my area of expertise.
>
> in a default configuration, getenv("OPENBLAS_NUM_THREADS") returns empty,
> so it appears the variable is not set.  i do not know if this is expected.
>
>
>
Openblas can be compiled as single-threaded library, or a multithreaded
library with either pthread or omp interface.
A long time ago somebody told me that I should use omp interface in a
multithreaded program (like octave).
With OMP interface you need to use OMP_NUM_THREADS variable,
OPENBLAS_NUM_THREADS
is ignored.
I also noticed that at least on linux you need to setup this variable
before starting octave; setting it from withing octave
does not work. Here is some benchmarks on i7-2600K (Fedora linux):

[dima@i7 gcc_def]$ OMP_NUM_THREADS=1 LD_PRELOAD=/usr/lib64/libopenblaso.so
./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 10.6499 seconds.
octave:2>
[dima@i7 gcc_def]$ OMP_NUM_THREADS=2 LD_PRELOAD=/usr/lib64/libopenblaso.so
./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 5.75883 seconds.
octave:2>
[dima@i7 gcc_def]$ OMP_NUM_THREADS=4 LD_PRELOAD=/usr/lib64/libopenblaso.so
./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 3.8937 seconds.
octave:2>
[dima@i7 gcc_def]$ OMP_NUM_THREADS=6 LD_PRELOAD=/usr/lib64/libopenblaso.so
./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 4.884 seconds.
octave:2>
[dima@i7 gcc_def]$ OMP_NUM_THREADS=8 LD_PRELOAD=/usr/lib64/libopenblaso.so
./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 6.45016 seconds.
octave:2>
[dima@i7 gcc_def]$ LD_PRELOAD=/usr/lib64/libopenblaso.so ./run-octave -q -f
octave:1> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 7.12494 seconds.
octave:2> a=randn(4000); tic; inv(a)*a; toc
Elapsed time is 7.05646 seconds.

I could not figure out how openblas is compiled for the win version of
octave, it could be that it is just a single-threaded library.

Sincerely,
Dmitri.
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