Quote Originally Posted by Sn0wm@n View Post
then help me figure it out with proof of how it actually works...
Well, let's start with multiprocessing and multithreading then.

Multiprocessing - using 2 or more central processing units ; originally this meant servers which had more CPUs, now you have multiprocessing at socket level due to multicore CPUs. For this to be used efficiently you need programs which use multiple threads or to run multiple single threaded programs in parallel. SW that can take advantage of multiprocesseing configs has been around since the '50s.

Multithreading ( as in SMT or HT ) refers to a CPU's ability to process simultaneously instructions from multiple threads. In both single and multiprocessing configurations it was soon discovered that a single thread isn't able to fully utilize the resources of the HW. There is a limited amount of instruction level parallelism which can be extracted in any given thread. A multithreaded core thus executes instructions from multiple threads at the same time. Those threads can be either from the same program or from different single threaded programs (I'm talking here about SMT or pure multi-threading and not the derivates like SoeMT ). A multithreaded core appears to the SW as a multiprocessing system ( that's why you have a single real core and multiple logical cores like in Task Manager ).
In fact, a Nehalem CPU is basically a multiprocessing multithreaded system at socket level.

If you connect the bolded part in the 2 concepts you'll understand why HyperThreading helps any software designed for multiprocessor use and even desktop sessions where you are multitasking ( running programs in parallel ). That's why many Pentium 4 users felt their systems snappier or more responsive when multitasking. You didn't have to flush the pipeline and switch to the new thread, they were going in parallel.

*Obviously I'm greatly simplying everything here, but I hope you get the point.