openmp seems do not work for ddscat

openmp seems do not work for ddscat

Jun (guest) 03 Jan 2018 13:47

I am testing DDSCAT7.3.0 on a linux machine. The compilation tools are: gcc5.4.0/gfortran, mpich-2.1.4. And the Openmp is enabled in Makefile by setting : "DOMP=-Dopenmp" and "OPENMP=-fopenmp". The compilation process is successful.

The testing problem is the scattering problem of a sphere: the geometry parameters of the target is
'ELLIPSOID'
80 80 80

Then I set OMP_NUM_THREADS=12 and run the executable ddscat.

Here is the problem: it seems the program is not running in openmp mode. The command "top d1" shows only about 200%, instead of 1200%, of CPU is used. No matter what value is given to OMP_NUM_THREADS, the usage of CPU is always about 200%.

Did I miss anything in compiling, or running the program ?

Thank you very much!

Re: openmp seems do not work for ddscat

draine 03 Jan 2018 18:41

OpenMP does appear to be working. However, only some easily-parallelized parts of DDSCAT have been set up to use OpenMP directives; part of the computation is still done in serial mode. As a result, the OpenMP speedup is typically only factors of 2 or 3. This seems consistent with your report of ~200% cpu usage.

One probably doesn't gain much from using more than 4 OpenMP threads. In my own calculations I typically set OMP_NUM_THREADS=4 .

In general, users are strongly encouraged to use the current release: DDSCAT 7.3.2