So I just performed a little experiment testing effect of Intel's "hyperthreading" (HT) technology on CFD solver speed. HT, or as some have called it, "hype threading", increases multitasking performance by trying to more efficiently allocate processor resources. One HT core is seen as two cores by the computer.
So I would expect that CFD solver speed would not benefit from HT, as opposed to disabling HT. My computer contains an i7 2600, a four-core processor. I decomposed the HT off case into four domains (one for each core), and of course I decomposed the HT On case into eight domains (one for each virtual core). The usage of all of the processors at 100% and that the right number of cores was perceived by the OS was verified by checking Ubuntu's System Monitor application. HT was enabled and disabled in BIOS.
In OpenFOAM, running the same exact case, the run times for HT on and off were:
HT On (8 virtual cores): 0.0692875 simulation seconds in 6213 real-life seconds.
HT Off (4 cores): 0.0692822 simulation seconds in 5154 real-life seconds.
So, as expected, HT does not give performance gain, but in fact imparts performance loss. We should expect similar performance because it is the same hardware, and computational efficiency usually decreases on a per-core basis with more cores (time is wasted in inter-core communication, etc.). Also, we see that the processor performs as expected (as four normal cores) with HT disabled.
Of course, I am not saying anything about HT's usefulness for multitasking, which is what it was designed for.