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Old 2008-07-16, 06:15   #3
sylvester
 
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Dec 2007
RSM, CA, USA

528 Posts
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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.
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