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Why does my Intel CPU underclock itself automatically?

Background: The CPU Frequency Scaling Monitor from my Ubuntu installation is set to "on demand" per default. This results in quite rapid changes of my CPU clock speed from 800 Mhz to 2.5 GHz. (At least, that's what is displayed.)

So, is it in any way harmful to change the clock speed of the CPU in quick succession?

Hardware info: Lenovo T61 Laptop with a Intel C2D T9300 @ 2.50GHz.

Bonus question :-) Is the actual clock speed really changed?

Regards
Mike

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marked as duplicate by Simon Sheehan, Sathya Jan 9 '12 at 11:07

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

4 Answers 4

up vote 4 down vote accepted

It will not harm your computer, it's in fact a way to keep it cool and save power.

The actual clock speed of the processor hasn't changed, the FSB multiplier is changing. And this changes the number we usually view as "the clock speed".

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Also, windows does this just the same. You must've just never noticed. If it didn't your battery life would be greatly diminished. –  user606723 Jan 6 '12 at 16:33
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The actual clock speed of the processor hasn't changed, the FSB multiplier is changing. - I don't think this is true. The FSB multiplier is the ratio of the CPU-clock to the FSB-clock. If the CPU-clock hasn't changed but the FSB multiplier has gone down, that means the FSB-clock speed has gone up. How does that help keep things cool!? –  BlueRaja Jan 6 '12 at 17:59
    
@BlueRaja, Also, lowering the cpu clock does save some power, but not much. What really saves power is that fact that you can you lower the voltage of the processor as you lower the clock. The is part of speedstep/powernow as well. –  user606723 Jan 6 '12 at 19:02

No, thats not of any harm, your CPU is doing that intentionally quite often and fast. It is supposed to be that way, for high dynamic CPU behaviour(idle vs. load).

Bonus: Of course the CPU-speed is changed, what did you else except?

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I have ample experience of computers that change their CPU speed all the time. In theory, no harm is to be expected, quite the contrary. I practice, I worked on a T61 for two years, and it was quite cool most of the time because the computer was bored, while it was reasonably fast once it was supposed to do some work. It remained ultra-stable the whole time.

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What you have is called Intel SpeedStep technology. It's a feature of your processor - when idle, or on low usage, it automatically downclocks the processor to keep it cool, and to keep your battery happy. I've got a fresh new i5 that goes from 2.5GHz all the way down to 800MHz.

Enhanced Intel SpeedStep® Technology is an advanced means of enabling very high performance while also meeting the power-conservation needs of mobile systems. Conventional Intel SpeedStep Technology switches both voltage and frequency in tandem between high and low levels in response to processor load.

It's just an awesome feature included in almost all new Intel desktop and mobile processors.

Whenever you question your processor doing something, check it out on the Intel Ark, here's the page for your processor. Just scroll down and you can see SpeedStep is included.

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