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I'm taking performance measurements of several bits of code. How can I disable SpeedStep on my Core i7 CPU so that the CPU clock speed is constant? I'm not trying to overclock here, and since SpeedStep only throttles up when it's safe (given the CPU temperature) to do so, I expect turning off SpeedStep will throttle it down to the minimum. That's fine. I just need it constant.

Setting power options to High Performance does not disable SpeedStep.

I am using a Dell Studio 1558 if that's relevant.

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+1 I would love to know if this shows any differences in measurements. –  Tom Wijsman Mar 9 '11 at 16:42
    
What programming language are you using? If it's a compiled language, you may want to get the x86 assembly and just count the number of instructions/clock cycles to compare function efficiency. That, and you won't be relying on timers or other system applications interrupting the run-time of your program... –  Breakthrough Mar 9 '11 at 18:31

3 Answers 3

up vote 9 down vote accepted

This will vary from board to board, but there should be an explicit BIOS option for "Intel SpeedStep" or "EIST" that you can disable.

EDIT: Also, I believe you have it backwards. SpeedStep is responsible for downthrottling, not upthrottling; you'll run at the base clockspeed if you turn it off. Turbo Boost is what upthrottles (but I believe SpeedStep does have to be on for that to work).

DOUBLEEDIT: You shouldn't need to, but you can also try disabling C1E support.

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As far as I understand, there should be a BIOS option that might or might not effect the clock throttling.

If the BIOS option doesn't exist and no BIOS firmware allows you to change this, it seems like a no-go...

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If you're on Vista or Windows 7 try setting the power plan to "High Performance." The resource monitor (Computer Management in Vista, stand-alone app in 7) will confirm that the CPU speed is constant.

In addition to SpeedStep though you should probably also disable the Core i7's turbo mode, which probably needs to be done in the BIOS.

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