Index Map

The index_map_type Fortran module provides core capabilities that support the use of distributed arrays in SPMD parallel programs through the index_map derived type, which describes the mapping of an array's index set to parallel processes. The mapping allows for overlap between processes, and provides collective array gather procedures and scatter-reduction procedures associated with that overlap.

There are two implementations of the module: one using MPI located in src/mpi, and one using Fortran coarrays located in src/caf. The interfaces are virtually the same, making it relatively simple for client code to switch between the two versions.

• The MPI version is stable and in production use (see Truchas).

• The coarray version is beta quality. It is fully tested, but its current performance is highly variable. Performance of the example programs is comparable to the MPI version when using the NAG Fortran compiler and its shared memory-only implementation of coarrays. However performance is very poor when using GFortran / OpenCoarrays, and worse still with the Intel Fortran compilers and their implementation of coarrays. Links to performance results can be found in this README. This is a work in progress.

Documentation

Documentation for the module is in the file doc/index_map_type.md.

The doc directory README has links to some additional information.

Examples

The example directory contains several example programs that illustrate the usage of the index_map_type module. These programs are automatically compiled as part of the build process. See its README file for details on the examples and how to run them.

Compiling

First clone the repository:

$ git clone https://github.com/nncarlson/index-map.git
$ cd index-map

The project uses CMake (version 3.19 or later) to compile the project. You may need to set your FC environment variable to the path of your Fortran compiler before running cmake to ensure that CMake finds the correct compiler.

MPI version

$ mkdir build
$ cd build
$ cmake .. -DCMAKE_Fortran_FLAGS="-O3 -DNDEBUG"
$ make

Set your desired compiler flags using CMAKE_Fortran_FLAGS. Define the preprocessor macro NDEBUG (as shown) to disable runtime assertion checks.

If CMake does not automatically find the MPI installation that matches your Fortran compiler, you can help it by setting the environment variable MPI_HOME to the root directory of the MPI installation. Note that the MPI Fortran interface (the mpi module) must have been compiled with the same Fortran compiler being used to compile this project.

Fortran coarray version

This CMake setup understands how to build the coarray version when using one of these Fortran compilers:

• NAG 7.1 or later with its built-in coarray support for shared-memory systems.
• Intel OneAPI with its built-in coarray support. Both the classic ifort and the new LLVM-based ifx compiler are supported. The companion Intel MPI package must be installed and Intel's setup script run to configure your environment. The Intel coarray implementation uses MPI under the hood.
• GFortran with OpenCoarrays. OpenCoarrays supplies the implementation of coarrays used by the gfortran compiler. Be sure the bin directory of the opencoarrays installation is in your path so that the compiler wrapper caf and runner cafrun can be found. Set FC=caf before running cmake. OpenCoarrays uses MPI under the hood, and at the time of this writing is compatible with MPICH version 4.0, but not 4.1 or later. Refer to the OpenCoarrays website for requirements.

$ mkdir build
$ cd build
$ cmake .. -DUSE_CAF=Yes -DCMAKE_Fortran_FLAGS="-O3 -DNDEBUG"
$ make

Installation

The CMake setup does not currently install a library. It will merely build the unit test programs and example programs. The current thinking is that a user will simply incorporate the source code for the index_map_type module directly into the source for their application. This could change in the future.

Testing

From the build directory simply run ctest:

$ ctest
Test project /home/user/index-map/build
    Start 1: localize
1/5 Test #1: localize .........................   Passed    0.07 sec
    Start 2: gather
2/5 Test #2: gather ...........................   Passed    0.07 sec
    Start 3: scatter
3/5 Test #3: scatter ..........................   Passed    0.08 sec
    Start 4: collate
4/5 Test #4: collate ..........................   Passed    0.07 sec
    Start 5: distribute
5/5 Test #5: distribute .......................   Passed    0.08 sec

100% tests passed, 0 tests failed out of 5

Total Test time (real) =   0.37 sec

Each test is actually a collection of sub-tests (about 100 in all); run ctest -V to get the verbose output. All tests should pass.

Note: These parallel tests use 4 processes. Without additional options / environment settings, MPI may refuse to run "over-subscribed" when testing on a system with fewer than 4 cores or hardware threads.

License

This project is distributed under the terms of the MIT license. See LICENSE.md for details.