A formula is derived for predicting multiprocessing efficiency on Cray supercomputers equipped with the Cray Time‐Sharing System (CTSS). The model is applicable to an intensive time‐sharing environment. The actual efficiency estimate depends on three factors: the code size, task length, and job mix. The implementation of multitasking in a three‐dimensional plasma magnetohydrodynamics (MHD) code, TEMCO, is discussed. TEMCO solves the primitive one‐fluid compressible MHD equations and includes resistive and Hall effects in Ohm’s law. Virtually all segments of the main time‐integration loop are multitasked. The multiprocessing efficiency model is applied to TEMCO. Excellent agreement is obtained between the actual multiprocessing efficiency and the theoretical prediction.

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