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Old 2008-07-17, 03:25   #6
Jul 2008

7 Posts

Originally Posted by sylvester View Post
I don't know about Pentium M, but some model of Mobile Pentium 4 have thermal throttling. They up the clock speed but then as soon as they hit temperature limit they start inserting HALT cycles. This causes reduced computational throughput without saving much energy. I had that happen on one of my laptops when one of two fans had been blocked.

On Linux the information about thermal throttlins is somewhere in /proc/acpi and was readable by just cat-ing the appropriate file in one of the subdirectories.

I've fixed the fan since, and I never experienced that aftererwards.

The other possibility is that only at 1.33 GHz your CPU speed matches your memory timings. At 1.73 GHz the CPU may require additional wait states to get the data from the memory.

This whole issue of running CPU asynchronously from the memory banks causes all kinds of strange performance issues. Especially with George's program where he carefully optimized the cache prefetches.
I'll have to check it out. There's a good possibility it is thermal throttling, since I noticed at 1.73 the fan speed seems to vary a bit, while a the other clock speeds, it's constant. Also, it would make more sense (to me) for memory timings to match the highest clock speed of the processor, but I could be wrong (there may be reasons why it would be matched at 1.33GHz instead of 1.73, but that's beyond my scope of knowledge). Thanks for your help! :)
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