DRUFAN on the Cray. An Analysis of cpu Usage, ParallelProcessing Possibilities and Comparison with Other JRC Codes

THE THERMOHYDRAULIC REACTOR SAFETY CODE DRUFAN HAS BEEN CONVERTED TO RUN ON A CRAY COMPUTER. IT WAS FOUND THAT THE CODE RUNS ABOUT THREE TIMES FASTER ON THIS COMPUTER THAN ON THE ORIGINAL AMDAHL COMPUTER AND THAT THIS SPEEDUP IS DUE TO THE INCREASED SCALAR SPEED OF THE CRAY WITH RESPECT TO THE AMDAHL. THE EFFECT OF VECTORISATION IS VERY SLIGHT. DRUFAN SPENDS MOST OF ITS TIME CAL- CULATING THERMODYNAMIC PROPERTIES, MOSTLY IN ORDER TO CALCULATE THE THERMAL INTERACTION BETWEEN THE FLUID AND THE SOLID STRUCTURE. THIS PROCEDURE CANNOT BE VECTORISED WITHIN THE FRAMEWORK OF THE PRESENT DRUFAN VERSION BUT COULD, THEORETICALLY, BE MULTITASKED. ATTEMPTS TO IMPLEMENT MULTITASKING FAILED HOWEVER DUE TO THE COMPLICATED NATURE OF THE FORTRAN IN DRUFAN. OTHER WORKES HAVE CONVERTED CODES TO RUN ON THE CRAY. FOR LARGE CODES (THOSE WITH TENS OF THOUSANDS OF FORTRAN STATEMENTS) SIMILAR RESULTS TO THOSE FROM DRUFAN WERE FOUND. GAINS FROM VECTORISATION WERE SLIGHT. THE ONLY CODE THAT SPEEDED UP SIGNIFICANTLY AFTER VECTORISATION WAS MUCH SMALLER AND HAD BEEN COMPLETELY RESTRUCTURED. AN UNDERSTANDING OF WHICH PARTS OF A CODE TAKE UP MOST CPU TIME CAN LEAD TO SPEEDUPS USING CONVENTIONAL (I.E. NON-PARALLEL) TECHNIQUES ON BOTH THE CRAY AND THE AMDAHLNA-NOT AVAILABL