[sdiy] CPUs, was... Simulating a Moog

[Scott Gravenhorst]

Great example.  One reason I would choose processor B is that it's slower
clock rate probably means that it generates less heat and uses less power
because digital circuits draw the bulk of their current while gates are
switching state.

Rainer Buchty <buchty at cs.tum.edu> wrote:
>> Ultimately, fastness being measured in mHZ, cycles per second, can be
>> very deceptive. This tells you how fast cycles are, but not how many
>> operations per cycle. MIPS - million of instructions per second- is I
>> think I better guide to gauging performance, but I hear quoted less
>> often.
>
>Because MIPS by definition says nothing as well. A bit more than
>clock speed, but not very much (MIPS=clock/#instr). Plus, they are
>deceptive as well.
>
>A small example from the last computer architecture examn "my" students
>had to endure: Processor A runs an application in 2ms. It has a CPI of 7/5
>and the application has 3,500,000 instructions. Processor B runs the same
>(in function) application also in 2ms. It has a CPI of 3/2 and the
>application requires 1,500,000 instructions.
>
>Now do the math: "A" has 1750 MIPS, "B" only 750 MIPS. So "A" is better?
>
>Well, let's look at the frequency which comes down to 2450MHz for "A" and
>1125MHz for "B".
>
>Now which processor do you chose?
>
>For this specific task *I* would go with processor B... Although the
>common user's (and unfortunately most of the students') reflex is to pick
>"A" because it "can process so much more instructions in the same time"
>compared to "B"... Or in other words, because it has a highly inefficient
>instruction set resulting in 2.3x higher instruction count and needs to be
>clocked at more than twice the rate to achieve the same results as "B".
>
>And that's why you don't use MIPS, raw clock rate or any other hardware
>parameter anymore for comparison of architectures but standardized
>benchmarks (SPEC, EEMBC, etc.), so you can compare performance according
>to applications.
>
>Rainer
>
>(Sorry for the CS lesson :)
>

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