12
Mar
15

Under-volting my CPU

It’s not often I tinker with modern hardware, but I decided recently to try and reduce the temperature and power consumption of my main PC. It is the most modern PC I own, yet is still probably about 7 years old. I have a AMD Phenom II 955 x4 @ ~3.2 Ghz which normally will reach over 50 degrees C when running a single thread at full load in warmish weather. Not really great sounding, but it usually idles around 44 degrees C.

People who are familiar with overclocking will know that as you increase the clock rate of a chip you frequently need to increase the voltage as well to maintain stability. This can also work in the reverse, a lower speed can utilise a lower voltage and be stable. I didn’t particularly want to make my machine slower, but I did want it to run cooler, so I went about lowering the voltage.

It’s a fairly simple process, lower the voltage by one setting in the BIOS, test with Prime95 using the maximum heat FFT, see if it fails after a while (best to test overnight) and repeat if the machine is still stable. The goal is to find the minimum voltage that the machine will be stable at.

My processor is so far stable at ~1.18v which is about 0.15 under voltage. Doesn’t sound like much, but it has had a profound effect on the temperature the machine runs at under load. Under the same conditions (one core fully loaded) the temperature dropped by 4-5 degrees, and had a larger decrease for full load. Idle temperatures remained about the same.

I’m still running Windows XP on the machine, which whilst old still suites me. I’m thinking about what to upgrade to, favouring Debian linux or FreeBSD. I’ll have to find out whether these support Cool-n-quiet for my processor, and how well that works. By default Windows XP doesn’t use it, so this is something I decided to rectify.

Enabling Cool-n-quiet wasn’t particularly hard fortunately as long as you’ve installed the processor driver from AMD. Turn it on in the BIOS and then set the power management plan to portable/laptop or minimal, the default desktop setting disables it again. Using a program like CPU-Z you can see the clock speed and voltage going up and down. Interestingly Cool-n-quiet maintains the under-volting I configured as the maximum voltage.

My only real issue with Cool-n-quiet is an interesting one. I play older games and use software that is sometimes single threaded, the windows XP scheduler will move the single process from one CPU to another to balance the load. This has an interesting effect with Cool-n-quiet. Basically it reduces the clock rate of each core (independently it seems) based on how much usage each core has. Because the scheduler is moving the single thread around, each individual core is not as busy and is then under-clocked by the Cool-n-quiet system. I haven’t yet measured the impact this has on a single thread, but from what I’ve observed it does appear to lose performance.

Ultimately this leads to the single thread not getting the maximum performance even if it uses an entire core worth of processor. Fortunately you can set the affinity in the task scheduler which stops the thread being migrated around, this means cool-n-quiet does it’s job better by being able to slow down the other cores until they are needed and the single thread gets the maximum performance from the single core.

Unfortunately this is a bit of a pain in the butt to do each time. So I’m not sure if I’ll continue to use the Cool-n-quiet technology. It does work exceptionally well in the sense that my processor is another 4 degrees cooler with a single core fully loaded. I will have to make some performance measurements and decide if it’s worth it.

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