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Next: Hints Up: High Performance Fortran Journal Previous: Constraints

Parallel I/O

High Performance Fortran is primarily designed to obtain high performance on massively parallel computers. Such massively parallel machines also need massively parallel I/O.

There are difficulties in getting high performance I/O:

• Efficient programs must avoid sequential bottlenecks from processors to file systems
• Fortran specifies that a file appears in element storage order; this conflicts with striped files (for example, an array distributed by rows may be written to a file striped by columns).

In particular Fortran file organization has limits:

• Files have a sequential organization. (Even direct access files have records in sequential order, though they can be accessed out of order)
• Fortran files are record oriented
• Storage and sequence association are in force (when writing and then reading a file, for instance)
• No specification of the physical organization is possible
• No compatibility with other languages/machines is guaranteed

With these in mind there are two major approaches that have been suggested:

1. Define hints (annotations) that do not change file semantics, in the spirit of data distribution. (This gives some information to the compiler.)
2. Introduce parallel read/write operations that are not necessarily compatible with sequential ones.

• Hints
  • Mapping Files
  • Node Directive
  • FILEMAP Directive
  • File mapping
• Explicit Parallel I/O Statements
  • OPEN statement
  • Parallel Data Transfer
  • Restrictions
  • Extensions