Coarray Fortran

Coarray Fortran (CAF), formerly known as F--, started as an extension of Fortran 95/2003 for parallel processing created by Robert Numrich and John Reid in the 1990s. The Fortran 2008 standard (ISO/IEC 1539-1:2010) now includes coarrays (spelled without hyphen), as decided at the May 2005 meeting of the ISO Fortran Committee; the syntax in the Fortran 2008 standard is slightly different from the original CAF proposal. Coarray Fortran (CAF), formerly known as F--, started as an extension of Fortran 95/2003 for parallel processing created by Robert Numrich and John Reid in the 1990s. The Fortran 2008 standard (ISO/IEC 1539-1:2010) now includes coarrays (spelled without hyphen), as decided at the May 2005 meeting of the ISO Fortran Committee; the syntax in the Fortran 2008 standard is slightly different from the original CAF proposal. A CAF program is interpreted as if it were replicated a number of times and all copies were executed asynchronously. Each copy has its own set of data objects and is termed an image. The array syntax of Fortran is extended with additional trailing subscripts in square brackets to provide a concise representation of references to data that is spread across images. The CAF extension was implemented in some Fortran compilers such as those from Cray (since release 3.1). Since the inclusion of coarrays in the Fortran 2008 standard, the number of implementations is growing. The first open-source compiler which implemented coarrays as specified in the Fortran 2008 standard for Linux architectures is G95. Currently, GNU Fortran provides wide coverage of Fortran's coarray features in single- and multi-image configuration (the latter based on the OpenCoarrays library). Another implementation of coarrays and related parallel extensions from Fortran 2008 is available in the OpenUH compiler (a branch of Open64) developed at the University of Houston. CAF is often implemented on top of a Message Passing Interface (MPI) library for portability. Some implementations, such as the ones available in the GNU Fortran and OpenUH compilers, may run on top of other low-level layers (for example, GASNet) designed for supporting partitioned global address space languages. A simple example is given below. CAF is used in CGPACK, an open source package for simulating polycrystalline materials developed at the University of Bristol. The program above scales poorly because the loop that distributes information executes sequentially. Writing scalable programs often requires a sophisticated understanding of parallel algorithms, a detailed knowledge of the underlying network characteristics, and special tuning for application characteristics such as the size of data transfers. For most application developers, letting the compiler or runtime library decide the best algorithm proves more robust and high-performing. Fortran 2018 will offer collective communication subroutines that empower compiler and runtime library teams to encapsulate efficient parallel algorithms for collective communication and distributed computation in a set of collective subroutines. These subroutines and other new parallel programming features are summarized in a technical specification that the Fortran standards committee has voted to incorporate into Fortran 2018. These enable the user to write a more efficient version of the above algorithm