octave 64-bit and multi-threading

[Grothausmann, Roman Dr.]

Dear mailing list members,


Using instructions from
https://www.gnu.org/software/octave/doc/interpreter/Compiling-Octave-with-64_002dbit-Indexing.html
I managed to compile octave-4.0.0 with --enable-64 using a 64-bit atlas library 
under debian.
The test
a = zeros (1024*1024*1024*3, 1, 'int8');
works.
I was also able to load 10GB units with fread and do some computations with it 
successfully.

According to
http://stackoverflow.com/questions/11889118/get-gnu-octave-to-work-with-a-multicore-processor-multithreading
octave should be multi-threaded when using a self-compiled atlas lib.
However the test

tic
bigMatrixA = rand(3000000,80);
bigMatrixB = rand(80,30);
bigMatrixC = bigMatrixA * bigMatrixB;
toc
disp("done");

takes 4.8s with the debian default octave (3.8.2 32-bit)
and 7.1s with my self-compiled octave (4.0.0 64-bit)

Is it slower because it is now 64-bit?
Are there better test for multi-threading in octave?

Thanks for any help or hints
Roman


PS:
it says on the instruction page: "Suggestions on how to compile ATLAS would be 
most welcome."
I did it as debian/README.Debian of the source-package of debian suggests, so 
basically:

apt-get source atlas
apt-get build-dep atlas

for i in {0..23}; do echo "performance" > 
/sys/devices/system/cpu/cpu$i/cpufreq/scaling_governor; cat 
/sys/devices/system/cpu/cpu$i/cpufreq/scaling_governor; done

cd atlas-3.8.4/

adding in debian/rules to COMMON_CONFIG_PARAMS:
-Fa if '-fdefault-integer-8 -fPIC'     \

fakeroot debian/rules custom
cd

apt-get  autoremove  libopenblas-base
apt-get  autoremove  libatlas3-base
apt-get  autoremove  libblas3 #reinstalls libatlas3-base

dpkg -i libatlas3-base_3.8.4-9+deb7u1+custom10_amd64.deb #replaces 
libatlas3-base
dpkg -i libatlas-dev_3.8.4-9+deb7u1+custom10_all.deb 
libatlas-base-dev_3.8.4-9+deb7u1+custom10_amd64.deb


--
Dr. Roman Grothausmann

Tomographie und Digitale Bildverarbeitung
Tomography and Digital Image Analysis

Institut für Funktionelle und Angewandte Anatomie, OE 4120
Medizinische Hochschule Hannover
Carl-Neuberg-Str. 1
D-30625 Hannover

Tel. +49 511 532-2900