Hyperthreading performance

Hyperthreading performance

Posted Oct 4, 2007 20:10 UTC (Thu) by anton (guest, #25547)
In reply to: 64 processors? by giraffedata
Parent article: What every programmer should know about memory, Part 1

But I think the usual reason for hyperthreading slowdown is just the overhead of switching threads.

In SMT (and that includes hyperthreading), there is no thread switching overhead. The execution core just executes instructions from different contexts at the same time (but in different resources).

I don't know why the Pentium 4 variant of SMT performs as badly as it does; cache thrashing may contribute, but I don't think that this is the main reason. The main reasons are probably some obscure microarchitectural details, maybe the replay system, maybe something else.

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