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468 Part 5 The PMS level Section 3 Computers for multiprocessing and parallel processing

the evaluation of five currents. Storage interference has previously been defined. The parallel processing overhead represents as a percentage the excess of total number of storage cycles required for execution, excluding storage interference cycles, when more than one processor is used, relative to the number of cycles required by a one-processor execution.

Fig. 11. Storage and executive interference.

Fig. 12. Storage utilization and cost/performance factors.

Actual counts during execution show that in general some sixty-seven percent of store access are instruction fetches in this program and some thirty-three percent are data fetches. Thus incorporation of a substantial instruction buffer in each processor clearly reduces all interference by an order of magnitude, since of the four ways in which a storage interference can occur, only one-a data fetch conflicting with a data fetch-remains in the inner loop. Moreover, these measurements refer to a processor in which arithmetic speeds, as in Table 2, are of the order of magnitude of a memory cycle time, which implies a somewhat powerful processor. Thus in every sense the interference figures are worst case results which, with the performance curves to which they relate, support the view that storage interference is not a serious obstacle to parallel processing.

The four contours drawn on these curves represent lines of constant storage module-to-processor ratio. They slope slightly upward due to the statistical Marbles and Boxes [Rosenfeld, 1965] effect previously referred to.

Figure 12 presents two sets of data, based on the five-equation line loop. The upper family of curves relates to storage utilization. The reservations made at the end of Sec. 7.3.2, with reference to the significance of utilization figures, also apply. The second family of curves represents a first attempt at estimating the relative quality of processing, that is, some function of a cost/performance

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