Processor Performance

AQA 4.6.1 — Processor performance

Retrieval: the previous lesson traced a single Fetch–Decode–Execute cycle in detail. This lesson zooms back out: what actually determines how many of those cycles a CPU can get through, and how quickly real programs run as a result?

Key vocabulary

• Clock speed — how many cycles per second a CPU can perform, measured in Hertz (e.g. 3 GHz = 3 billion cycles per second).
• Cores — independent processing units within one CPU, each able to run instructions in parallel.
• Cache — small, very fast memory built into (or very close to) the CPU, holding data likely to be reused soon.
• Word length — the number of bits a CPU can process in one go.

Understand — four different levers, not one

Clock speed determines how many cycles happen per second — but a cycle only does useful work if the data it needs is actually ready in time. Cores let genuinely independent tasks (or parts of a program written to use them) run at the same time, rather than one CPU doing everything one instruction after another. Cache exists because fetching from main memory is comparatively slow — keeping recently- or soon-to-be-needed data in cache means the CPU spends far less time waiting. Word length affects how much data moves and gets processed per instruction. None of these four alone fully determines real-world speed — a program that can't use extra cores gains nothing from them, however many there are.

Visualise — comparing two CPU specifications

Neither CPU is simply "better" — which one performs better depends entirely on what the software running on it actually needs.

Calculate it — clock speed as a first estimate

A CPU with a clock speed of 3 GHz can, at most, perform roughly 3 billion cycles per second — a genuinely useful ballpark figure, even though real instructions can take more than one cycle each, and modern CPUs pipeline multiple instructions at once (both of which this simple estimate deliberately ignores for now).

Worked example: if a task genuinely requires 1 cycle per instruction and consists of 9 billion instructions, a 3 GHz single-core CPU needs roughly 9,000,000,000 ÷ 3,000,000,000 = 3 seconds. A CPU running at 4.5 GHz instead would need only 9,000,000,000 ÷ 4,500,000,000 = 2 seconds for the exact same task.

Compare — matching hardware to the workload

Using CPU A and CPU B from the table above: video editing software that's been written to split rendering work across many cores benefits far more from CPU B's 8 cores than from CPU A's higher clock speed — even though CPU A "sounds faster" on a single spec alone. Conversely, older software that can only ever use a single core would run faster on CPU A, since none of CPU B's extra 6 cores can be used at all for that specific program.

Common mistake

Assuming clock speed alone determines performance (this was flagged briefly in the previous lesson too) — a CPU with a higher GHz figure can genuinely be the slower choice for a specific task once cores, cache and what the software can actually make use of are taken into account.

Evaluate — a real upgrade decision

A user complains their laptop is slow specifically when editing 4K video, but feels perfectly fast for browsing and email. Video-editing software is normally written to use multiple cores heavily; browsing rarely uses more than one or two. Evaluate whether upgrading to a CPU with a higher clock speed, or one with more cores, is the better recommendation here, and justify your answer using the difference between the two workloads.

Challenge

A school is choosing a CPU for a shared computer lab running many lightweight, single-threaded exam-practice applications simultaneously across different user logins. Using the four factors above, recommend which specification matters most for this specific scenario, and justify why the other three matter comparatively less here.

Looking ahead: the next lesson (Types of Processor) asks a related but different question — not "how fast is this CPU", but "is a general-purpose CPU even the right kind of processor for this job at all?"