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Computer retailers describe the performance of their machines in terms of clock speed (usually in MHz or GHz). This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have higher performance. Most modern CPUs are capable of [[superscalar|executing multiple instructions per clock cycle]], which can dramaticallly speed-up a program. Other factors aid speed, such as the mix of [[functional unit]]s, [[computer bus|bus]] speeds, available memory, and the type and order of instructions in the programs being run.
Better metrics can be obtained by [[benchmark]]ing, which attempts to take all these factors into account by measuring the time the system takes to run through a series of provided programs. This can be used to obtain an average throughput for the system. Even still, benchmarking may not show that one of two systems is clearly better, since one system may, for example, be
Another important performance consideration is [[interrupt latency]], which is the guaranteed maximum response time of the system to an event such as the click of a mouse or the reception of data by a modem. This number is also affected by a very wide range of design choices. Computers that control machinery usually need low interrupt latencies, because the machine can't, won't or should not wait. For example, computer-controlled anti-lock brakes should not wait for the computer to finish what it's doing - they should brake.
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