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I don't quite understand how hyperthreading saves power. Is it better than doing OOO execution because there's less hardware required? I could imagine that in

• reitzensteinm 19y ago
  It doesn't save power in absolute terms (which is why the lower power Atom chips disable it), but it increases performance per watt (and performance per transistor). Basically, on modern chips the leakage power is a huge proportion of the total power used, so you're paying for blocks of logic whether you're using them or not. You can shut off blocks (most modern processors do this), but only at a very granular level. Hyperthreading runs two threads simultaneously, and when one thread stalls, the other thread takes over, so you get a much higher % utilization of the chips resources. Switching power used goes up, leakage stays the same, and performance increases. Hyperthreading speeds up simple, in order chips like the Atom particularly well (because a single thread is stalling so much!). So don't expect equivilant performance gains from Nehalem hyperthreading.