[Sanddancer]

Switching tasks is expensive [1]. Twice as many cores running at half the speed can be considerably faster in the real world because you're not constantly stopping to flush the cache, save the kilobytes of register a modern CPU has, etc. Honestly, I'm surprised that x86 has kept with just two virtual threads for this long. Architectures like Sparc and Power have 4+ threads per core because so many modern jobs are built around hurrying up and waiting.

[1] http://www.cs.rochester.edu/u/cli/research/switch.pdf

[gpderetta]

A core at twice the frequency is better than two at half the frequency every time. The problem is usually the trade-off is not that clear (either the slower cores consume significantly less or they are faster than just half as slow).

Regarding HT, 2 threads is really a sweet spot for a 4-wide CPU. More than that and the competition for cache resources, execution units and register file become significant.

POWER8 is special because a factor of x2 is because each power 'core; is pretty much two distinct smaller cores that can gang together to speed up one thread (it also helps on per core software licensing), while the other x2 factor is for very specialized loads (this is also true, or used to be, for SPARC).

IIRC XeonPhi which is also a specialized cpu has 4xHT.