Today I put Prime95 on a P4 with hyperthreading. When I do the ctrl-alt-del to see the task manager, Prime95 is only getting 49-50% of the CPU, compared to 95% or more on other machines. I assume this is because of hyperthreading.
yes, and the operating systems inability to know how effective hyperthreading can be. the max for two busy threads on a HT processor is around 115-120% compared with the same CPU with hyperthreading disabled, in other words, the 2nd virtual CPU will only gain about 15-20% over the one.
I did benchmarks on this machine and compared them to an older P4. They are only 14% faster even though the clock speed is 25% faster. The benchmark is 12% lower than the benchmark on the benchmark page for the same speed of P4.
try disabling hyperthreading, its generally a setting in your BIOS setup. prime95 might run slightly faster, but the rest of your system will run somewhat slower.
but, faster CPUs don't scale linearly with clock speed, as the memory is generally not proportionally faster.
Should you run two instances on a hyperthreading CPU? Are there other issues with hyperthreading and prime95?
no, two at once would cause major problems. As there is only one floating point/SSE processor, and Prime95 is heavily optimized to keep the FPU efficiently pipelined, trying to hyperthread two Prime95's would be a serious net LOSS due to resource contention.
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