HPF Programming Issues

HPF is always a trade-off between parallelism and communication,

• more processors, more communications,
• try to load balance, assume owner-computes rule,
• try to ensure data locality,
• use array syntax or array intrinsics,
• avoid storage and sequence association (and assumed-size arrays).

It is fairly easy to write a correct HPF program but it is an awful lot harder to write an efficient one. The technique is more or less:

1. write a correct serial version of the intended HPF program, test and debug,
2. add distribution directives so as to generate the minimum number of communications in pursuit of the solution.

Data locality it the key - well chosen alignment and distribution strategies will enhance performance significantly.

Programmers should always add as many directives as possible: always add INDEPENDENT directives where correct, always align things explicitly and so on.

The most important factor is to obtain a good parallel algorithm, many serial algorithms are wholly inappropriate for HPF programs.

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Badly written programs will result in poor execution speeds:

• if there are complicated subscript expressions then the compiler may not be able to efficiently work out which elements are needed from other processors,
• vector subscripting is a very powerful technique which is not handled efficiently by the current compilers. Use with caution.
• sequential DO loops may well be executed sequentially, only a very sophisticated compiler will be able to detect parallelism in such structures. This may mean that processor 1 performs its iterations of a loop, when it has finised it informs processor 2 of this fact which means that processor 2 can perform its iterations, when finished it will inform processor 3 and so on. If there are hundreds of processors then this kind of loop will absolutely cripple performance.
• remapping objects can be performed across procedure boundaries; unnecessary remappings are very time consuming. Again, be careful.
• ill-chosen mappings will mean that excessive communication will occur during program execution. Writing an HPF program is much more than simply `getting the correct answers'!
• if the system is unevenly balanced then some processors will be constantly waiting for other processors to finish. This will also ruin performance.

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