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Old 2008-07-16, 03:22   #1
Jul 2008

7 Posts
Default Mprime is faster on lower CPU clock speed

I'm running mprime on several of my machines, including the laptop I'm typing this on (I need to hurry, so I can kill gdm and get back to text mode!). It's a 3 year old Toshiba Satellite M55-S331 (1.73 GHz Pentium M) with 1.5 GB of DDR RAM. I'm running Ubuntu 8.04 (Hardy Heron) on it, and mprime 25.5 as a background startup daemon with the output ported to a log file.

I'm using cpufreq-set and cpufreq-info as frontends for CPU frequency scaling (this processor comes with 4 steps, 800 MHz, 1.07 GHz, 1.33 GHz, and 1.73GHz). What struck me as curious one day as I was running without gdm, was that on the conservative governor (which increases the cpu speed smoothly as needed...ondemand will leave it at 800MHz for just mprime, I'm assuming because it's priority is so low, but conservative will raise it) it would sometimes only crank the clock speed up to 1.33 GHz. I checked the range of frequencies, and it was correct, and even tried the performance governor (which is supposed to max the clock speed out), and it would only go to 1.33.

So I changed the minimum clock speed to 1.73 to force it to run at 1.73 GHz (and you can tell, because the fan cranks up to high speed and it puts out quite a bit more heat), and then I noticed something very interesting...the iteration times (I'm assuming it's doing LL testing...I'm an mprime newbie, so go easy on me) are consistently about 25% higher at 1.73 GHz than at 1.33 GHz (without GNOME or gdm running). And it is without fail...on M19643627, each iteration takes about .134 seconds at best at 1.73 GHz...all I have to do is change the clock speed to 1.33 GHz, and the fan speed comes down (as well as the heat level), and the times drop to .114 sec per can this be?

Is it affecting a clock multiplier somewhere on the FPU (does the FPU use the same clock as the rest of the CPU?) or the system bus, resutling in a higher clock speed somewhere else at the lower CPU clock speed? Or is the readout of the clock speed wrong (but like I said, fan speed and heat production seem to be proportional to the indicated clock speed, and power consumption increases to max at 1.73 GHz, too...courtesy of a Kill-A-Watt meter). What am I missing here?

Thanks for all your help!
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