Does SpeedStep change the bus speed, or just the multiplier?

Question

I want to estimate how a certain piece of code will perform on different Intel Westmere processors. The various processors all support the same QPI and memory speed, and they all have the same cache sizes; the only difference is the CPU core clock speed (2.66GHz vs. 2.93GHz etc.)

I already have one such processor running with a very fast CPU core clock. I would like to estimate the performance of the others without having to purchase them first.

Using cpufreq-set on Linux (acpi-cpufreq driver), I can set my core clock speed to a variety of slower values. My question is simply this: Does setting the core clock speed in this way change anything other than the core clock speed? That is, does it precisely simulate a slower CPU, or does it also slow down the front side bus or change the cache size or whatever?

Please provide a reference for your answer. Thank you!

Answer 1

According to the Wikipedia entry, SpeedStep only effects the processor frequency, and in some cases the processor voltage.

from Wikipedia:

Running a processor at high clock speeds allows for better performance. However, when the same processor is run at a lower frequency (speed), it generates less heat and consumes less power. In many cases, the core voltage can also be reduced, further reducing power consumption and heat generation. This can conserve battery power in notebooks, extend processor life, and reduce noise generated by variable-speed fans. By using SpeedStep, users can select the balance of power conservation and performance that best suits them, or even change the clock speed dynamically as the processor burden changes.

Answer 2

The CPU Multiplier is changed. Each CPU comes with a maximum limit for the multiplier (unless it is an unlocked version of the CPU, AMD Black Edition, or Intel K CPU).

These lower limits are what the processor down clocks to with SpeedStep, these multipliers are also hard coded for each CPU also with a voltage limit to save power, evidently speaking; the CPU Clock doesn't save you power, the voltage does.