Enlisting the Help of Universities for GIMPS

[S00113]

Originally Posted by jroysdon

Speaking of, I've got a question regarding this. If you have an older CPU w/o the newer power management / cpu power scaling features, running mprime under Linux should not cause the CPU to use more power, right?

Any CPU capable of running Prime95 will use significantly more power when working than not. One of the first power optimizing techniques was to switch off the FPU when it was not doing anything. 486SL and newer CPUs also switch off other circuits to conserve power when not in use.

Linux (and I think newer versions of Windows) will also issue a HLT instruction whenever the scheduler don't find a process ready to run. The HLT instruction will put the processor in low power C1 sleep state until an interrupt wakes it up again. When mprime is running, the scheduler will always find work to do. Since mprime is using the FPU all the time (unless you are trial factoring), the FPU can't be switched off either.

The servers are on 24/7 anyway for dns, email, webserving and the like, and as far as I can tell don't support any cpu power scaling.

For this purpose CPU power scaling would actually make the servers use more power, not less. The CPU gets most work done per watt at high frequency. While the CPU is sleeping, the power usage is low and almost independent of CPU frequency. The amount of work to be done by a typical server is independent of CPU frequency, so it's better to get the work done quickly at a high frequency and get back to sleep ASAP, than to spend more time active doing work.

I did some measurements three years ago with a cheap wattmeter on a dual 3GHz Xeon, and measured the same wattage idling at all clock frequencies. Running mprime at full clock speed used about 100 watts more than when idling. The difference is probably higher on newer CPUs and chipsets.

Another interesting experiment to try is to start some other CPU intensive process when mprime is running. Your power consuption will decrease because the FPU will be idling and can be switched off when the other process is running.