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78 Cray T3E User’s Guide

0:c= 10121416

1:c= 2468

The output of the C program is similar.

7.5 High Performance Fortran (HPF)

The Cray T3E system at CSC has a High Performance Fortran (HPF) com-

piler. HPF is a developing standard agreed by several computer and

software vendors. The Portland Group HPF (PGHPF) version 2.3 on T3E

also supports the CRAFT programming model used on Cray T3D sys-

tems.

Both HPF and CRAFT are implicit Fortran programming models where

the user writes a Fortran 90 program and speciﬁes how arrays are to be

distributed among the processors. The compiler then analyzes the data

dependencies in the code and generates a parallel executable version of

the code. In these data-parallel programs individual processes execute

the same operations on their respective parts of the distributed data

structures, and the parallelism is mainly on the loop level. Since both the

HPF and CRAFT directives are actually Fortran 90 comments, the parallel

programs can be compiled using the standard Fortran 90 compilers for

serial execution.

These implicit programming models provide a much faster and simpler

way to parallelize programs than the explicit message passing libraries.

On the other hand, in many cases hand-tuned message passing codes

can outperform the HPF compilers. Typically the HPF version is at most

two times slower. Moreover, the HPF language currently has little sup-

port for problems with irregular data structures, but this will change

in future revisions of the language. HPF is well suited for prototyping

at the development phase, while later critical parts of the code can be

parallelized with the message passing libraries or the shared memory

library on the T3E.

With the CRAFT programming model, the user can specify also the dis-

tribution of work in more detail. It also supports parallel I/O, private

data and task parallelism in MIMD style.

On Cray T3D systems the CRAFT programming model was the only im-

plicit programming model available. On T3E systems the CRAFT model

is incorporated into the HPF compiler with some syntactical changes.

The data distribution directives of the original CRAFT model have been

changed to conform with the HPF standard in the following way:

• The directive !DIR$ has been changed to !HPF$.

• The directive DOSHARED has been changed to INDEPENDENT.

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Cypress CSC-1200T Guía de usuario Pagina 78