Performance
Comparing CPUs
Two CPUs can have very different specifications, so you cannot judge one on a single number. A fair comparison weighs up three things at once: how fast it runs (the clock speed), how much cache it has, and how many cores it contains. Looking at just one of them can give you the wrong answer.
A CPU with more cores is not automatically faster. Doubling the number of cores does not double the speed, because extra cores only help when the software can share the work between them. A chip with fewer cores can still finish the job sooner if each of its cores runs faster, or if a bigger cache keeps more of the data it needs close by.
Comparing two CPUs
A school is buying two mini PCs for its lunchtime coding club. Both run the same software, which can share its work across several cores.
| Characteristic | Mini PC X | Mini PC Y | What it means |
|---|---|---|---|
| Clock speed | 2.6 GHz | 3.6 GHz | Y completes more fetch–execute cycles each second. |
| Cache size | 6 MB | 2 MB | X can keep more frequently used data and instructions close by, so it waits less for RAM. |
| Number of cores | 6 | 2 | X can work on more instructions at the same time. |
No single row settles it. Mini PC Y has the higher clock speed, so each of its cores works through instructions more quickly. Mini PC X has three times as many cores and a larger cache, so it can process more at once and spend less time waiting for RAM. Because the coding club's software can use several cores, X is likely to be the faster of the two overall. For software that only used one core, Y's higher clock speed would probably make it the faster machine.
A proper comparison weighs up all three characteristics together, then says which CPU is likely to be faster for the job it has to do.
Practise this
OCR GCSE Computer Science · Performance