Performance Factors

Faster is not one thing

You have already opened up the processor and met the parts inside it: the ALU, the Control Unit, the registers and the cache, and how an instruction runs through fetch, decode and execute. That module told you what those parts are. This one is about what makes one processor faster than another, which is a different question and is examined differently. Your specification names three factors: clock speed, cache size and the number of cores. Learning those three words takes about a minute, and on its own it is worth very little. Here is why, and it is the single most important thing in this module. An exam question asks you to EXPLAIN the effect of a factor, and an explanation is a link, not a label. "A higher clock speed makes the computer faster" is a restatement of the question, not an answer. "A higher clock speed means more cycles per second, so more instructions can be executed each second" is an answer, because it says what the factor does and then what follows from it. So as you work through this module, do not collect the three names. Collect the three chains: more of X means Y, so Z. And collect the qualifications too, because each factor has a limit and knowing where it stops is what separates a good answer from a full one.

Words for speed and size

Six terms. Notice that each definition already contains the beginning of a chain, because that is how they are examined.

What clock speed counts

A processor is described as running at several gigahertz. What is being counted?

• The number of cycles the CPU performs each second, and in each cycle it can carry out instructions
• The exact number of programs the computer can run at once
• How much data the CPU can store while it works
• How quickly the computer can transfer data to other devices

Which part stops the waiting

This is the CPU diagram you met when learning the parts. Now a performance question about it: tap the block whose SIZE decides how often the CPU has to wait for data from main memory.

Match each factor to its effect

Three factors, three limits

Each factor improves performance in its own way, and each one runs out. An answer that gives the effect and the limit is doing what the higher marks ask for.

Which two follow from more cores

A computer is upgraded from one core to four. Select the TWO things that genuinely follow.

• More sets of instructions can be processed at the same time
• Tasks that can be split into parts may finish sooner
• Every program will now run four times as fast
• Each individual core now runs at a higher clock speed

When more does not mean faster

Every one of these factors has a point past which more of it stops helping, and saying so is how a good answer becomes a full one. More cores only help software written to use them. A core can only work on something if there is something separate for it to work on. A task that must be done in order, where each step needs the result of the last, cannot be shared out however many cores are available, so a program written for a single core gains little from three more. A bigger cache only helps up to a point. Cache is useful because it holds what is used often; data that is not reused gains nothing from being held there, so beyond a certain size the extra space is doing less and less. And clock speed is only comparable between similar processors. Two processors of different designs may do different amounts of work per cycle, so the one with the higher number is not automatically the faster machine, which is why a comparison of raw clock speeds between very different processors tells you less than it appears to. Higher clock speeds also generate more heat, which is a practical limit on how far they can be pushed. Notice what all three have in common: the factor describes a capability, and whether that capability is used depends on the work being given to it. That sentence, adapted to whichever factor a question names, is worth a great deal.

The chains in words

A higher clock speed means more _____ are performed each second, so more instructions are executed each second. A larger _____ means more of the frequently used data is held close to the CPU, so it has to fetch from main memory less _____. More _____ means more sets of instructions can be processed at the same time, although this only helps if the software is written to be _____ across them.

Explaining an effect, worked

A question says: "Explain how increasing the cache size could improve the performance of a computer." Three marks. Here is a weak answer and a full one. Weak: "A bigger cache makes the computer faster because it has more cache." That is circular. It repeats the question and adds nothing, and it would score very little. Full: "Cache holds data and instructions that the CPU uses often. A larger cache can hold more of them, so the CPU finds what it needs in the cache more of the time. That means it has to fetch from main memory less often, and because main memory is much slower than cache, the CPU spends less time waiting, so more instructions are completed in the same period." Look at the structure, because it works for all three factors. First say what the thing does at all (cache holds frequently used data). Then say what more of it means (more of that data is held). Then say what follows (fewer slow fetches, less waiting, more work done). Three sentences, each one earning its place. And if there is a fourth mark available, add the limit: data that is not reused gains nothing from being cached, so beyond a point a larger cache adds less. The test to apply to your own answer: does the word "so" or "because" appear, and does what follows it say something new? If not, you have named a factor and stopped.

The answer with no because

Four answers to "explain the effect of this factor on performance". Select the ONE that names a factor without explaining anything.

• More cores means more instructions can be processed at the same time, so tasks that can be split finish sooner.
• A faster clock speed improves performance because the processor is faster.
• A larger cache holds more frequently used data, so the CPU fetches from main memory less often.
• A higher clock speed means more cycles per second, so more instructions are executed each second.

Performance drill

Answer from memory. Every question asks for the effect, not the name.

Advising on a new machine

Someone asks which processor to choose. Work through the advice, and notice how often the answer is "it depends what for".

• They use software that renders video, which can be split into many independent parts. What matters most?
• Their other main program runs as a single sequence of steps, each needing the last result. What follows?
• Two processors are offered. One has a higher clock speed, the other a larger cache. What do you say?
• They ask you to write one sentence explaining your recommendation. What shape should it take?

Explain what makes a CPU faster

Explain how clock speed, cache size and the number of cores each affect the performance of a CPU. Explain the effect of each rather than naming it.

• Explain what clock speed measures and what a higher one means for instructions executed
• Explain what cache is for, and what a larger cache means for fetches from main memory
• Explain what a core is, and what more of them means for processing instructions
• For each factor, make sure your sentence contains a "so" or "because" that says something new
• Explain why more cores does not always mean faster
• Explain why clock speeds are only reliably compared between similar processors
• Finish by explaining why the best processor depends on the work being given to it