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2.7 Performance Models

The most widely recognized aspect of a machine's internal organization that relates to performance is the clock cycle time, which controls the rate of internal operations in the CPU (Section 2.1). A shorter clock cycle time, or equivalently a larger number of cycles per second, implies more operations can be performed per unit time.

For a given architecture, it is often possible to rank systems according to their clock rates. For example, the HP 9000/725 and 9000/735 workstations have basically the same architecture, meaning they have the same instruction set and, in general, appear to be the same system as far as compiler writers are concerned. The 725 has a 66MHz clock, while the 735 has a 99MHz clock, and indeed the 735 has a higher performance on most programs.

There are several reasons why simply comparing clock cycle times is an inadequate measure of performance. One reason is that processors don't operate ``in a vacuum'', but rely on memories and buses to supply information. The size and access times of the memories and the bandwidth of the bus all play a major role in performance. It is very easy to imagine a program that requires a large amount of memory running faster on an HP 725 that has a larger cache and more main memory than a 735. We will return to the topic of memory organization and processor- memory interconnection in later sections on vector processors and parallel processors since these two aspects of systems organization are even more crucial for high performance in those systems.