[Users] OpenMP is making it slower?

[Scott Hawley]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


I'm doing a Cactus run which I've compiled with OpenMp support on my linux machine, and looking at the performance in the routine in which I explicitly added a "parallel do" directive. 
When I change the environment variable OMP_NUM_THREADS to anything larger than 1, it actually takes LONGER than the 1-thread run.   I have *24* processors on my machine.
It runs fine on my mac, but not my linux machine.


Does anyone have a suggestion?

I notice my version of gcc doesn't have --enable-openmp (see below)... but then the worst it should do is not have any effect when OMP_NUM_THREADS is set to anything.

Thanks!
Scott



> mpicc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-8) 

libgomp is version 4.4.5-8  ... but there is no lib64gomp.  Odd.  I'm using Debian squeeze which supposedly contains lib64gomp.



- --
Scott H. Hawley, Ph.D. 	 		Asst. Prof. of Physics                            
Chemistry & Physics Dept       		Office: Hitch 100D             
Belmont University                 	Tel:  +1-615-460-6206
Nashville, TN 37212 USA           	Fax: +1-615-460-5458
PGP Key at http://sks-keyservers.net



-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.17 (Darwin)
Comment: GPGTools - http://gpgtools.org

iQEcBAEBAgAGBQJN0pn+AAoJENp9UKaXQr6GlwQIAKEYSnYmyiSOEni7yDj36NTq
GXZg5wijj6flhsK/r/PKMIFslKa/k1BaONJKG8wY9tgV03QIpv5VuTH2Pw0RF3GF
c7E8i5FoxT3p8yvQSS/oTQND32pTiBiIu+S0Yzv8lydNjRkoE2Dt6CJtZVpcEBn2
9993/kb8YlnD6jvLF9ycrqeyppKbWclSwZND3LA7kPEGxZmiLGPWVNBwOFHC5VUs
BF+u42mWQQHOGsOiQMGGbgnS/btnlxdYQlvj8rFNpGcOQeWf2FEzlE7biXtMoFbX
y0W24EHuoz5ggSq9T7hGcG1rFiLOfdpsJWmiY1I4A/kjHLpRHcXl4H50lt2ifbQ=
=LZoK
-----END PGP SIGNATURE-----