Hi everyone. I have been benchmarking a Pentium IV chip for some time now, but I have sometimes gotten very illogical and inconsistant results when I did long series of tests, and I have just now discovered what went wrong. The Pentium IV has a safety device that slows the chip down if it gets to hot. It has been mentioned quite a lot on the net recently, but most sources concluded that it was unlikely to happen. It does happen however, on the machine I benchmark, after about 10 minutes of running at full speed. Take a look a the included .eps file. This is simply a run of 200 identical tests of my fastest SSE2 double precision code. After about 10 minutes of running at full speed, the machine start slowing down to about 3/4 the normal speed. I have repeated the test several times, and I get the same result every time. I have tried inserting a 4 seconds pause in between each test, at it solves the problem, so I see no other logical explanation that the thermal safety device in the chip. If anyone wants to repeat my test the can grab my codegenerator at www.cs.utk.edu/~soender/atlas/src/ps_atlas.tgz Untar it on top of your ATLAS tree, go to ATLAS/tune/blas/gemm and execute runhot.py (requires python) and look at the file runhotdata. First column is seconds, second column is mflops. If you have gnuplot installed execute 'gnuplot gnuplotrunhot' and look at runhot.eps Cheers, Peter.