README.openmp

                                OpenMP                    revised 7.2016, 1.2020

OpenMP is a shared-memory parallel-programming API.  As the grid dimensions get larger,
the need for parallel computing becomes more important.  With OpenMP, wgrib2 will use
multiple threads, typically one thread per core.  For the typical PC, wgrib2 will run 
on the number of cores that are on the CPU chip which is typically 2, 3, 4, 6 or 8.
(Yes, I've owned machines with 2, 3, 4, 6 and 8 cores.) The multi-core speedup is only 
significant when the grids have several million grid points. (Update: 2019 brings
12 and 16 core PCs.)

You can control the number of cores used by the environment variable OMP_NUM_THREADS
 and the -ncpu option.  The latter overrides the former.

Status: working, requires OpenMP v3.1
  Tested: AIX: xlc
          Ubuntu 12.04+
          Redhat 4: gcc did not officially support OpenMP, had problems
          Redhat 5+: gcc, open64
	  Clang 9: tested AOCC version
Hints:
     complex-packing reading is parallelized when bitmaps are not used
     complex-packing reading is partly parallelized if bitmaps are used 
     complex-packing writing is partly parallelized.
     simple-packing is parallelized
     jpeg2000, png, AEC are not parallelized because they depends on external libraries
     geolocation is parallelized except when using Proj4.
     Running multiple copies of wgrib2 can be done along with OpenMP.
     Nodes with 24+ cores should have environment variable OMP_NUM_THREADS set to
      a number less than the number of cores. Little perforance gain for
      a large number of cores.
     NUMA should be considered.
     wgrib2ms/wgrib2mv is faster than using wgrib2 with OpenMP.  
        really fast: wgrib2ms/wgrib2mv with AEC compression