clock rate

If the clock is a drummer, the clock rate is simply how fast the drummer beats — how many beats per second. It is the headline number stamped on processors, like 3.2 GHz, and for years it was the figure shops shouted about. It tells you how many clock cycles happen each second, and it is just the inverse of the clock cycle time: more beats per second means each beat is shorter.

Clock rate is measured in hertz: 1 GHz (gigahertz) is one billion cycles per second, so a 3 GHz chip ticks three billion times a second and each cycle lasts one third of a nanosecond. Raising the clock rate squeezes more cycles into each second, which can do more work per second — but only if the processor still completes useful work each cycle and does not have to wait. Past a point, pushing the clock higher burns far more power and heat for little gain, which is one reason clock rates largely stopped climbing in the mid-2000s.

Here is the crucial honesty, the megahertz myth: a higher clock rate does not by itself mean a faster computer. What you actually care about is the time to run your program, and that depends on three things together — how many instructions it takes, how many cycles each instruction needs on average, and how long a cycle is. A chip with a lower clock rate can easily beat a higher-clocked one if it does more per cycle. Clock rate is one factor among three, not the answer.