Factors affecting CPU performance: clock speed, number of cores, cache

The factors affecting the performance of the CPU: clock speed, number of cores, cache

1.1.1 Structure and function of the processor

Structure and function of the processor — Note 3 of 5

Candidates need to understand how the performance of the CPU can be affected by many factors. Candidates need to understand how and why the performance is affected by the clock speed, the number of cores and the size and speed of the cache.

What you need to know

The performance of a processor depends mainly on its clock speed, its number of cores and the size and speed of its cache. At A Level you need to explain HOW each factor affects performance and recognise the limits of each.

What to know

Cache is organised in levels: L1 is smallest and fastest (per core), L2 is larger and slower, L3 is larger again and usually shared between cores.

Key terms

Exam tipCommon mistake

Clock speed: The number of clock cycles the processor completes per second, measured in hertz (Hz). A higher clock speed means more instructions can be executed per second.
Limits of clock speed: higher speeds generate more heat and use more power, and the processor may have to wait for memory anyway.
Core: An independent processing unit that can fetch, decode and execute its own instructions. More cores mean more instructions can be processed at the same time.
Limits of cores: performance only improves if the software is written to split work across cores. Some tasks cannot be split because each step depends on the previous one, and coordinating cores adds overhead.
Common mistake: Saying doubling the number of cores doubles performance. It rarely does, because not all tasks can run in parallel.
Cache: A small amount of very fast memory on or near the processor that holds frequently used data and instructions so the processor does not have to wait for slower RAM.
A larger cache increases the chance of a cache HIT (the data is found in cache), reducing the time spent waiting for RAM.
Exam tip: Always explain the mechanism. 'A larger cache means more frequently used instructions can be stored close to the processor, so fewer slower accesses to RAM are needed.'