Hi,

Could someone explain me in which cases it is useful to enable hyperthreading on a machine running FreeBSD 8.0 and in which other cases it is not a good idea? Is that possible that hyperthreading is disadvantageous unless the number of active (non-sleeping) threads is really high?


For example, if I have an i7 CPU with 4 physical cores and that I run some multi-threaded code that has only 4 threads, it will run almost always (twice) slower with hyperthreading enabled than when I disable it in the BIOS. If I understand correctly, hyperthreading has the advantage of being able to do CPU context switching faster than the OS, but it does this context switching systematically instead of only when requested, so it slows things down unless the number of running (non-sleeping) threads is greater or equal to let say the number of physical threads x 1.5-1.75.

Thanks!
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