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The following conditions exist.

Consider three diff erent processors P1, P2, and P3 executing the same instruction set. P1 has a 3 GHz clock rate and a CPI of 1.5. P2 has a 2.5 GHz clock rate and a CPI of 1.0. P3 has a 4.0 GHz clock rate and has a CPI of 2.2.

And the question is

Which processor has the highest performance when executing the same program?

I have learned to compare cpu execution when comparing computer performance.

However, since cpu execution time = CPI * instruction set * 1/clock rate, the size of the instruction set cannot be known only with the conditions in the above problem, I thought that the performance between processors could not be compared.

I looked for other issues similar to this one, and the issue is Which processor has the highest performance expressed in instructions per second? We compared the performance between processors under the condition of instructions per second as shown.

So what I want to know is whether it is possible to compare performance between processors without any special conditions. (Isn't the given problem wrong?) If possible, I wonder how they can be compared.

1 Answers1

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However, since cpu execution time = CPI * instruction set * 1/clock rate, the size of the instruction set cannot be known only with the conditions in the above problem, I thought that the performance between processors could not be compared.

For that formula; I'd assume that instruction set reflects how many instructions are needed to do the same work. E.g. if one instruction set can do a multiply in one instruction it might have instruction set = 1, and if another instruction set needs 20 instructions to do one multiply then it might have instruction set = 20 because you need 20 times as many instructions to get the same work done.

For your homework (3 processors that all execute the same instruction set), instruction set is irrelevant - they all take the same number of instructions to get any amount of work done. With this in mind you can just do performance = clock rate / CPI. More specifically:

P1 = 3 GHz / 1.5 = 2,000,000,000
P2 = 2.5 GHz / 1.0 = 2,500,000,000
P3 = 4 GHz / 2.2 = 1,818,181,818

Of course the homework is extremely over-simplified - stalls (time CPU spends doing nothing while waiting for things like cache misses and instruction fetch after a branch misprediction, etc) tend to have a larger impact on performance than clock speed or "theoretical maximum CPI"; and "measured CPI in practice" depends which specific instructions are used (and can never be a single number like "1.5 CPI" for all programs). In other words, in practice, it's easy to have a situation where a CPU is fastest for one program but slowest for another program.

Brendan
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