CPU frequency scaling vs overclocking/underclocking

[unutbu]

Question 1: Would it be fair to say that

• if you have a 1.8GHz CPU, frequency scaling can alter the clock speed from about 1.2GHz to 1.8GHz.
  
• overclocking increases the clock speed above 1.8GHz (how high typically?)
  
• underclock decreases the clock speed below 1.2GHz

Question 2: I recently read an interesting (and short!) argument why underclocking may actually consume more power than letting the CPU run in its normal "ondemand" setting:

http://www.lesswatts.org/projects/ap...ce-to-idle.php

What do you guys think?

[markbuntu]

Way back at the beginning of time, a guy namde Linus Torvalds was fooling around trying to use all the available commands on a 386 prosessor chip. One of these was the halt command which no other operating system at the time took advantage of. When this command was used it stopped the clocking of the cpu while maintaining all the registers. The result was a processor that actually ran faster and better because performance was not degraded due to excess heat caused by the cpu continuosly running when it had nothing to do.

When you overclock your cpu you do it by increasing the voltage supplied to the chip. This, while speeding up the processor can also introduce more errors in internal chip processing due to increased thermal instability. Internal error correction in the chip will increase causing even greater power consumption. This can be only partially overcome by increasing external cooling but because silicon is not a very good heat conductor localized overheating cannot be fully compensated for.

Manufacturers set the voltage of their chips for a reasonable compromise between performance and longevity vs heat degradation for a wide range of uses and environments.

[sdennie]

Question 1: Would it be fair to say that

• if you have a 1.8GHz CPU, frequency scaling can alter the clock speed from about 1.2GHz to 1.8GHz.
  
• overclocking increases the clock speed above 1.8GHz (how high typically?)
  
• underclock decreases the clock speed below 1.2GHz

The number of frequencies available is chip dependent. For example, an intel T9300 can use 2.5Ghz, 2Ghz, 1.6Ghz, 1.2Ghz and 800Mhz. It's not very common for laptops to have the ability to properly under/overclock the CPU via BIOS so, I'm not sure how it would interact with the frequency max/min. Presumably if you were able to overclock the maximum frequency while retaining the same minimum frequency, you would be able to actually save more power (because of the second part of your post) but, I have no way to verify that.

Question 2: I recently read an interesting (and short!) argument why underclocking may actually consume more power than letting the CPU run in its normal "ondemand" setting:

http://www.lesswatts.org/projects/ap...ce-to-idle.php

What do you guys think?

This is not really about underclocking but about using a frequency governor that keeps the CPU at the lowest frequency. Another article about this can be found here: http://mjg59.livejournal.com/88608.html. Using "powersave" instead of "ondemand" can sometimes make the machine cooler but, it doesn't generally save extra power over "ondemand" under normal workloads.

[mcduck]

When you overclock your cpu you do it by increasing the voltage supplied to the chip. This, while speeding up the processor can also introduce more errors in internal chip processing due to increased thermal instability. Internal error correction in the chip will increase causing even greater power consumption. This can be only partially overcome by increasing external cooling but because silicon is not a very good heat conductor localized overheating cannot be fully compensated for.

Manufacturers set the voltage of their chips for a reasonable compromise between performance and longevity vs heat degradation for a wide range of uses and environments.

Increasing the voltage will not make your CPU run faster. And it definitely doesn't make it less stable. Quite the opposite, the whole point in increasing the CPU voltage is to make the CPU _more_ stable, and decrease errors.

In the end, a CPU is still an analog device. While it works with ones and zeroes, the voltage used to represent these values doesn't change immediately from one to other. Now, when you increase the CPU frequency the dealy it takes for the voltage can cause problems, the voltage might not be able to fully change from one value to other, giving the result of something in between. And these would cause errors. When the CPU voltage is increased the difference between the voltages used to represent 0 and 1 becomes bigger, which helps to get rid of the errors.

So, the COU speed is increased simply by changing it's bus frequency & multiplier, usually through BIOS. Depending on how much you overclocked, that might make your system less stable. This is compensated by increasing the CPU voltage, which then results in more heat generated. And then we introduce the better cooling. So neither the cooling, nor the higher voltage, will make your COU run faster. They are just used to make the overclocked system stable.

Manufacturers set the default voltage as something that is enough to provide a stable system with the default clock speeds. Ussually all the CPU models in same family will use the same voltage, regardless of their clock speed.

[meindian523]

Thanks to all for the replies and a big grrr at hijackers
I now summarise this thread as:

Overclocking is running a city sedan(CPU) in fourth gear(Max stable frequency) whether the road(demand for processing power) is a dirt track(low) or the freeway(high).While you get max speed,it shortens the life of the CPU.

Underclocking is running the city sedan in first gear whether the road is a dirt track or the freeway.You get the lowest speed and probably are bumped by other cars when on the freeway(you have a lower frequency processor,you lose bragging rights)But your car outdoes the life of those who don't underclock(presumably?)It also saves power.

CPU Frequency Scaling is an automatic transmission car,it will scale the gear depending the need.This is the best option provided the manufacturer of the car provided for it.

Now,how do I know whether CPU Frequency Scaling is enabled for my CPU( or is it a property of the motherboard?)

[unutbu]

To see what cpu scaling governor you are currently using, run:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

To see the available cpu scaling governors:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors

To see what else you can see,

ls /sys/devices/system/cpu/cpu0/cpufreq

To change the scaling governor to "performance" (though I don't recommend this in general. See http://mjg59.livejournal.com/88608.html):

echo performance | sudo tee /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

To watch your cpu clock speed:

watch -n 1 cat /sys/devices/system/cpu/*/cpufreq/scaling_cur_freq

If you have two CPUs, change cpu0 to cpu1 in the commands above to look at your second cpu.

[meindian523]

Failed at the first step,got as far as cpu0,but there's no cpufreq in the path.FYI,my processor is Pentium D

[unutbu]

Try the advice in this article:
http://polishlinux.org/linux/debian/...ling-in-linux/

[meindian523]

Oh,coo!,I thought CPU frequency scaling wasn't supported on Pentium D.Are you sure this is not a property of the motherboard?I'm not on Ubuntu right now,so I will login later to report on my attempts to save power,provided this is not a property of the motherboard,of course.

[meindian523]

I got this:

easwarh@l1nuxr0cks:/lib/modules/2.6.24-19-generic/kernel/arch/x86/kernel/cpu/cpufreq$ sudo modprobe speedstep-centrino
FATAL: Error inserting speedstep_centrino (/lib/modules/2.6.24-19-generic/kernel/arch/x86/kernel/cpu/cpufreq/speedstep-centrino.ko): No such device

while trying to insert speedstep-centrino into the kernel
What do I do now?