CPU Performance

1 State what is meant by the clock speed of a processor. 2

One mark for each of the following, to a maximum of 2.

• The number of cycles or instructions performed each second
• Measured in hertz, commonly gigahertz

Model answer. Clock speed is the number of fetch decode execute cycles a processor completes each second. It is measured in hertz, and modern processors are usually quoted in gigahertz, where one gigahertz is one billion cycles per second.

2 Explain why a computer with a quad core processor may not run a particular program four times faster than a computer with a single core processor of the same clock speed. 4

One mark for each of the following, to a maximum of 4.

• The program must be written to divide work across multiple cores
• A single threaded program can only run on one core, leaving the others idle
• Some tasks are sequential and cannot be split because later steps depend on earlier results
• Coordinating cores and sharing cache or memory adds overhead

Model answer. A quad core processor can only deliver four times the throughput if the work can actually be divided between the four cores. This requires the program to be written as multiple threads. If the program is single threaded then it runs on one core and the other three sit idle, so there is no gain at all. Even in multi threaded programs, some parts of a task are inherently sequential because later steps depend on results produced earlier, and those parts cannot be run in parallel. In addition, the cores must communicate with each other and share cache and memory bandwidth, and this coordination adds overhead that reduces the theoretical speed increase.

3 A video editing company must choose between Processor X at 4.1 GHz with two cores and 4 MB cache, and Processor Y at 3.0 GHz with eight cores and 16 MB cache. Recommend a processor and justify your choice. 6

One mark for each of the following, to a maximum of 6.

• Recommends Processor Y
• Video editing and rendering can be divided into independent tasks across cores
• Eight cores process far more instructions simultaneously than two
• The larger 16 MB cache raises the hit rate so the CPU waits on RAM less often
• Acknowledges that Processor X has the higher clock speed and would be better for single threaded work
• Concludes that the parallel nature of video work outweighs the clock speed advantage

Model answer. Processor Y is the better choice for this company. Video editing and rendering split naturally into independent tasks, such as processing separate frames or segments, so the work can be shared across all eight cores and run simultaneously. Eight cores at 3.0 GHz will therefore complete far more instructions per second on this workload than two cores at 4.1 GHz. Processor Y also has 16 MB of cache rather than 4 MB, which raises the cache hit rate and means the CPU spends less time waiting for data from RAM, and video work moves large amounts of data. Processor X does have the higher clock speed, and it would be the better choice for single threaded software where only one core can be used. However, because video rendering parallelises well, the four times increase in core count and the four times increase in cache outweigh the roughly 37 per cent clock speed advantage, so Processor Y should be chosen.

4 Explain how increasing cache size can improve processor performance, and give one reason why simply making the cache as large as possible is not the best solution. 4

One mark for each of the following, to a maximum of 4.

• More frequently or recently used data and instructions can be stored close to the CPU
• More requests are satisfied by cache rather than the slower RAM
• The CPU spends less time waiting so completes more instructions per second
• Cache is expensive per byte, takes physical space on the chip, and larger caches take longer to search, so returns diminish

Model answer. Increasing cache size means more of the instructions and data the processor uses frequently can be held in memory that sits inside or very close to the CPU. This raises the cache hit rate, so a greater proportion of requests are satisfied without going out to the much slower RAM, and the CPU therefore spends less time idle and completes more instructions per second. However, making the cache as large as possible is not the best solution because very fast memory is expensive per byte and takes up physical space on the processor die, and a larger cache also takes longer to search, so the performance gains get smaller and smaller as size increases.

5 Describe one similarity and one difference between a dual core processor and two separate single core processors installed in the same computer. 3

One mark for each of the following, to a maximum of 3.

• Both allow two instructions to be processed at the same time
• A dual core processor has both cores on a single chip
• Cores on one chip communicate faster and can share cache, reducing latency

Model answer. A similarity is that both arrangements allow two instructions to be fetched, decoded and executed at the same time, so both can genuinely process work in parallel. A key difference is that a dual core processor has both cores built onto a single chip, which means they are physically much closer together and can share cache and communicate over very short internal connections. Two separate processors must communicate across the motherboard, which is slower, so the dual core arrangement generally has lower latency when cores need to work together.