# Using OpenMP¶

The Clawpack Fortran Classic 3d code, AMRClaw 2d and 3d code, and GeoClaw codes include OpenMP directives for making use of multicore shared memory machines.

Note: Versions of gfortran before 4.6 are known to have OpenMP bugs. You should use a recent version or a different compiler if you want to use OpenMP.

To invoke OpenMP you need to compile the entire code with appropriate compiler flags (see Fortran Compilers). For example, with gfortran and the bash shell you could do:

export FFLAGS='-O2 -fopenmp'  # or hardwire FFLAGS in the Makefile
make new


in an application directory, which should recompile all of the library routines as well.

Then you may want to specify how many threads OpenMP should split the work between, e.g.

export OMP_NUM_THREADS=2


If you do not set this environment variable some default for your system will be used.

You may also need to increase the stack size if the code bombs for no apparent reason (and no useful error message):

export OMP_STACKSIZE=16M


and also:

ulimit -s unlimited


On a Mac this isn’t allowed and the best you can do is

ulimit -s hard


To stop using OpenMP you could do:

export FFLAGS=-O2   # or hardwire FFLAGS in the Makefile
make new


## Using OpenMP with AMR¶

The code in AMRClaw and GeoClaw is parallelized by splitting the list of patches that must be advanced in time between threads, and then each grid patch is handled by a single thread. For this reason good performance will be seen only when there are a sufficiently large number of patches at each level relative to the number of threads. For this reason it is recommended that the parameter max1d be set to 60 in the modules

• $CLAW/amrclaw/src/2d/amr_module.f90 •$CLAW/amrclaw/src/3d/amr_module.f90

when OpenMP is used. This limits the size of any patch to have at most max1d grid cells in each direction. If OpenMP is not used, a larger value of max1d might give somewhat better performance since there is less overhead associated with passing boundary values in ghost cells and other per-patch work. However, this is generally negligible and max1d=60 is the default value set in the code. If you do change this value, remember to recompile everything via:

make new


## Fixed grid output in GeoClaw¶

The original fixed grid output routines are not thread safe and so OpenMP should not be used if you want to produce output on fixed grids.

The newer fgmax routines that keep track of maxima on fixed grids should be thread safe, see Fixed grid monitoring.