A straightforward example of hyperthreading side effects is well known of chess engines users.
Rybka 3, for instance, incurs a 20% penalty when hyperthreading is enabled (on my i7 950, affinities not set).
So one must set affinities from the task manager, mapping each engine instance to an even-numbered logical processor. Unfortunately it does not work under all chess GUIs and you often trigger (with Vista and 7) an access violation when trying to do so. Curiously chess engine programmers seem not to worry much about this issue.
As expected, chess engines run faster when HT is on and affinities are set, than when HT is off. Probably because it lets free logical processors for the OS. |
How good is hyperthreading? new - Agner Fog - 2009-09-29
How good is hyperthreading? new - Agner Fog - 2009-11-03