What are the pros and cons for Pentium D Hyper threading?

Hyperthreading is a Simultaneous multithreading technology (SMT) that was adopted by Intel in its Pentium 4 series – Pentium D, HT and extreme edition processors.  The technology involved simulating or duplicating a processor to make the operating system believe that there are two processors, and hence run two threads to the core simultaneously. To facilitate this, the operating system is required to support Symmetric Multiprocessing (SMP). A non-SMP core will simply treat the core as one.  Unless you move to a SMP enabled kernel, you would be better off disabling Hyperthreading. Also enabling Hyperthreading, apparently increases power consumption. There have been cases where performance of certain applications, not optimised for hyperthreading, has suffered with the feature enabled.

Folding @ home (FAH) clients try to utilise unused processing power of the personal computers where the software is installed. Hyperthreading is essentially two virtual CPUs (in conjunction with SMP), and does not have the same resources as dual CPU. When any CPU intensive activity is initiated by the user, the resources allocated to FAH are reallocated to the second application, slowing down the folding effort. It is recommended to run one instance of a FAH client for each core with Hyperthreading enabled. As stated before, with non-SMP kernel, enabling hyperthreading, would make no difference.