A video editing company is choosing between two computers.
| Computer A | Computer B | |
|---|---|---|
| Clock speed | 3.2 GHz | 2.8 GHz |
| Number of cores | 4 | 8 |
| Cache size | 6 MB | 12 MB |
(a) State what a clock speed of 3.2 GHz means. [1]
(b) Explain why Computer B may perform better than Computer A when editing videos, even though its clock speed is lower. [3]
(c) Explain why doubling the number of cores does not always double the performance of a computer. [2]
- (a) 3.2 billion (3 200 000 000) fetch–decode–execute cycles / instructions processed per second; allow 'cycles per second' with correct number(1)
- (b) B has more cores (8 vs 4) so it can process more instructions at the same time / in parallel; allow: B's total processing is greater (8 × 2.8 = 22.4 billion vs 4 × 3.2 = 12.8 billion cycles per second) so the extra cores outweigh the lower clock speed(1)
- (b) Video editing software can split tasks between cores / is designed to use multiple cores(1)
- (b) B has a larger cache so more frequently used data/instructions can be stored close to the CPU, so fewer slow fetches from RAM(1)
- (c) Any one reason: some programs / software are not written to use multiple cores; or some tasks cannot be split into parts that run in parallel (instructions depend on the result of an earlier one so must be done in sequence)(1)
- (c) A second, different reason: cores have to communicate / coordinate / share out the work, which takes time (overhead); or one core may be idle waiting for another to finish; or other components (e.g. RAM / storage / shared cache) limit the speed (bottleneck). Do not credit the same idea twice (e.g. 'tasks cannot be split' and 'tasks depend on each other' are one idea)(1)
- Answer: (a) 3.2 billion (FDE) cycles per second. (b) More cores → more parallel processing that video software uses; bigger cache → fewer RAM fetches. (c) Software may not use all cores / tasks may be sequential; cores must coordinate (overhead) / other components bottleneck
- Examiner tip: In (b) you must link each spec to the video editing task – listing 'more cores, more cache' without saying why that helps scores little.