Co-array Fortran

Coarray Fortran

Paradigm(s)	multi-paradigm: parallel, message passing, imperative (procedural, object-oriented), structured
Designed by	Robert Numrich and John Reid
Developer	PL22.3 Fortran Committee
Stable release	Fortran 2008 (ISO/IEC 1539-1:2010)
Typing discipline	strong, static
Major implementations	Cray, g95, gfortran, Rice (CAF 2.0), Intel
Influenced by	Fortran
OS	Cross-platform

Co-array Fortran (CAF), formerly known as F--, is an extension of Fortran 95/2003 for parallel processing created by Robert Numrich and John Reid in 1990s. The Fortran 2008 standard (ISO/IEC 1539-1:2010) now includes coarrays (spelt 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 has been available for a long time and 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 implementation is growing. The first open-source compiler which implemented coarrays as specified in the Fortran 2008 standard for Linux architectures is G95.

Example

program Hello_World
  implicit none
  integer :: i  ! Local variable
  character(len=20) :: name[*] ! scalar coarray, one "name" for each image.
  ! Note: "name" is the local variable while "name[<index>]" accesses the
  ! variable in a specific image; "name[this_image()]" is the same as "name".
 
  ! Interact with the user on Image 1; execution for all others pass by.
  if (this_image() == 1) then   
    write(*,'(a)',advance='no') 'Enter your name: '
    read(*,'(a)') name
 
    ! Distribute information to other images
    do i = 2, num_images()
      name[i] = name
    end do
  end if
 
  sync_all ! Barrier to make sure the data have arrived.
 
  ! I/O from all images, executing in any order, but each record written is intact. 
  write(*,'(3a,i0)') 'Hello ',trim(name),' from image ', this_image()
end program Hello_world

An alternate perspective

A group at Rice University is pursuing an alternate vision of coarray extensions for the Fortran language. Their perspective is that the Fortran 2008 standard committee's design choices were shaped more by the desire to introduce as few modifications to the language as possible than to assemble the best set of extensions to support parallel programming. They don't believe that the set of extensions agreed upon by the committee are the right ones. In their view, both Numrich and Reid's original design and the coarray extensions proposed for Fortran 2008, suffer from the following shortcomings:

• There is no support for processor subsets; for instance, coarrays must be allocated over all images.
• The coarray extensions lack any notion of global pointers, which are essential for creating and manipulating any kind of linked data structure.
• Reliance on named critical sections for mutual exclusion hinders scalable parallelism by associating mutual exclusion with code regions rather than data objects.
• Fortran 2008's sync images statement doesn't provide a safe synchronization space. As a result, synchronization operations in user's code that are pending when a library call is made can interfere with synchronization in the library call.
• There are no mechanisms to avoid or tolerate latency when manipulating data on remote images.
• There is no support for collective communication.

To address these shortcomings, the Rice University group is developing a clean-slate redesign of the Coarray Fortran programming model. Rice's new design for Coarray Fortran, which they call Coarray Fortran 2.0, is an expressive set of coarray-based extensions to Fortran designed to provide a productive parallel programming model. Compared to Fortran 2008, Rice's new coarray-based language extensions include some additional features:

• process subsets known as teams, which support coarrays, collective communication, and relative indexing of process images for pair-wise operations,
• topologies, which augment teams with a logical communication structure,
• dynamic allocation/deallocation of coarrays and other shared data,
• team-based coarray allocation and deallocation,
• global pointers in support of dynamic data structures,
• support for latency hiding and avoidance, and
  • asynchronous copies,
  • asynchronous collective operations, and
  • function shipping.
• enhanced support for synchronization for fine-grain control over program execution.
  • safe and scalable support for mutual exclusion, including locks and lock sets,
  • events, which provide a safe space for point-to-point synchronization,
  • cofence, which forces local completion of asynchronous operations,
  • finish, a barrier-like SPMD construct that forces completion of asynchronous operations across a team,

See also

• Unified Parallel C
• Fortress (programming language)
• Chapel programming language
• X10 (programming_language)
• Array programming
• Parallel Computing
• Partitioned global address space

References

• Co-Array Fortran homepage (domain not renewed 26/9/11)
• ISO Fortran Committee
• ANSI/INCITS Fortran Committee
• Coarrays in the next Fortran Standard
• Rationale for Co-Arrays in Fortran 2008
• Final Draft International Standard (FDIS) of the ISO Fortran 2008 standard
• Coarray Fortran 2.0 at Rice University