Co-array Fortran on the GPC

Version 12 of the Intel Fortran compiler supports almost all of Co-array Fortran, and is installed on the GPC. This page will briefly sketch how to compile and run Co-array Fortran programs.

Loading necessary modules

First, you need to load the module for version 12 of the Intel compilers, as well as Intel MPI.

   module load intel/intel-v12.0.0.084 intelmpi

(you may put this in your .bashrc file.)

Note: For multiple node usage, it makes sense to have to load the IntelMPI module, since Intel's implementation of Co-array Fortran uses MPI. However, the Intel MPI module is needed even for single-node usage, just in order to link successfully.

Example

Here is an example of a co-array fortran program: <source lang="fortran"> program Hello_World

 integer :: i ! Local variable
 integer :: num[*] ! scalar coarray
 if (this_image() == 1) then
   write(*,'(a)') 'Enter a number: '
   read(*,'(i)') num
   ! Distribute information to other images
   do i = 2, num_images()
     num[i] = num
   end do
 end if
 sync all ! Barrier to make sure the data has arrived
 ! I/O from all nodes
 write(*,'(a,i,a,i0)') 'Hello ',num,' from image ', this_image()

end program Hello_world </source> (Adapted from [1]).

Compiling, linking and running co-array fortran programs is different depending on whether you will run the program only on a single node (with 8 cores), or on several nodes.

Single node usage

Compilation

<source lang="bash"> ifort -O3 -xHost -coarray=shared -c [sourcefile] -o [objectfile] </source>

Linking

<source lang="bash"> ifort -coarray=shared [objectfile] -o [executable] </source>

Running

To run this co-array program on one node with 16 images (co-array version for what openmp calls a thread and mpi calls a process), you simply put <source lang="bash"> ./[executable] </source> in your job submission script. The reason that this gives 16 images is that HyperThreading is enabled on the GPC nodes, which makes it seem to the system as if there are 16 computing units on a node, even though physically there are only 8.

To control the number of images, you can change the FOR_COARRAY_NUM_IMAGES environment variable: <source lang="bash"> export FOR_COARRAY_NUM_IMAGES=2 ./[executable] </source> This can be useful for testing.

An example submission script would look as follows:

<source lang="bash">

1. !/bin/bash
2. MOAB/Torque submission script for SciNet GPC (OpenMP)
4. PBS -l nodes=1:ppn=8,walltime=1:00:00
5. PBS -N test

1. DIRECTORY TO RUN - $PBS_O_WORKDIR is directory job was submitted from

cd $PBS_O_WORKDIR

export FOR_COARRAY_NUM_IMAGES=16 ./[executable] </source>

Multiple nodes usage

Compilation

<source lang="bash"> ifort -O3 -xHost -coarray=distributed -c [sourcefile] -o [objectfile] </source>

Linking

<source lang="bash"> ifort -coarray=distributed [objectfile] -o [executable] </source>

Running

Because distributed co-array fortran is based on MPI, we need to launch the mpi processes on different nodes. The easiest way to do this is to set the number of images using FOR_COARRAY_NUM_IMAGES and then to use mpirun without an -np parameter: <source lang="bash"> export FOR_COARRAY_NUM_IMAGES=32 mpirun ./[executable] </source> Note that the total number of images is set explicitly, and should not be given to mpirun. You can still pass other parameters to mpirun, though.

An example submission script would look as follows:

<source lang="bash">

1. !/bin/bash
2. MOAB/Torque submission script for SciNet GPC (ethernet)
4. PBS -l nodes=4:ppn=8,walltime=1:00:00
5. PBS -N test

1. DIRECTORY TO RUN - $PBS_O_WORKDIR is directory job was submitted from

cd $PBS_O_WORKDIR

1. EXECUTION COMMAND; FOR_COARRAY_NUM_IMAGES = nodes*ppn

export FOR_COARRAY_NUM_IMAGES=32 mpirun -env I_MPI_FABRICS shm:tcp ./[executable] </source>

On Infiniband, in the last line you should replace tcp by dapl (see GPC MPI Versions), or omit the whole -env ... option.

Features known not to work

There are a few features that are known not to work in the current version of the Intel Fortran compiler (v12.0), such as character coarrays. See section 3.2.3.3 of the official release notes. These issues may get fixed in later releases.