An HTT cpu can still only execute one instruction at a time; there are often instructions that cause the cpu to have idle time (stalled waiting for data) and hyperthreading allows for the cpu to spend that otherwise idle thread making progress on a separate task list. However, this still means that the two scheduled threads are contending for the same execution unit... The parent is suggesting that this contest may cause more of a performance degradation than the advantages that HTT provides, which would be easily resolved with some benchmarking :D

I'd be humored to hear your idea of what the difference is, given the misplaced use of scare-quotes.

A hyperthread core is a virtual core -- it is not actually a core at all but is a re-purposed, possibly stalled physical core. While it can improve some scenarios, in some cases (particularly core-saturating benchmarks) it can actually hurt performance.

This is hardly an out there or controversial statement. Further I didn't say to disable hyperthreading, I said to try setting parallelism to the physical cores. Again, nothing, whatsoever, controversial about that.

There is a huge difference between the two. Hyperthreaded cores only give you a speed up in specific situations where additional work can be squeezed into the pipeline.

http://en.wikipedia.org/wiki/Hyper-threading

The speedup is very work dependent and in practice for things like web pages and api servers you generally only get another 20-40% of performance from them rather than a full 100%.

Well it shouldn't matter that much anyway, I don't think modern kernels will put the same process threads on the same physical core.

That's the reason intel tells you to shut hyperthreading off if your operating system doesn't support it.