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I've heard the term hyper-threading thrown around a bit recently, what exactly is hyper-threading and why is it important?

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Hyper-threading is where your processor pretends to have 2 physical processor cores, yet only has 1 and some extra junk.

The point of hyperthreading is that many times when you are executing code in the processor, there are parts of the processor that is idle. By including an extra set of CPU registers, the processor can act like it has two cores and thus use all parts of the processor in parallel. When the 2 cores both need to use one component of the processor, then one core ends up waiting of course. This is why it can not replace dual-core and such processors.

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+1 should add that hyper-threading is specific to Intel's implementation of SMT e.g. the SPARC processor has a different form of SMT implemented but with similar goals. –  sybreon Mar 22 '10 at 2:20
    
@Earlz are you suggesting that the processor is running two threads by dividing the cores in two? or is it looks like parallelism made in reality hyperthreading is causing processor to switch from one thread to another? –  Doopy Doo Oct 13 '12 at 11:17
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@DoopyDoo neither. Basically the processor has two "execution cores", which is the pipeline for instructions and such and it also has two sets of registers and other essential things.. The difference between hyper-threading and regular dual-core though is that some things are NOT duplicated. For instance, there may only be one ALU. So, this would mean while a dual-core processor can add two separate sets of numbers together at the same time, for a hyper-threading processor will have to make one of the virtual-cores wait until it's turn with the ALU.. Of course, this is a simplified example –  Earlz Oct 13 '12 at 18:05
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Hyper-Threading is where two threads are able to run on one single-threaded core. When a thread on the core in question is stalling or in a halt state, hyper-threading enables the core to work on a second thread instead.

Hyper-threading makes the OS think that the processor has double the number of cores, and often yields a performance improvement, but only in the region of 15-30% overall - though in some circumstances, there may actually be a performance hit (=<20%).

Currently, most Atom chips and all i7 (and Xeon-equivalent chips) have hyper-threading, as did some older P4s. In the case of the Atoms, it's a desperate attempt to improve performance without increasing power consumption much; in the case of i7s, it differentiates them from the i5 range of chips.

Complex processing work won't benefit much from HT, but certain (simple, highly multi-threaded) tasks, such as video encoding, benefit from HT. In reality, there is not a lot in it...

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95% correct. If a normal core = A + B then a hyper-threading core is more like A + 2 x B. They CAN execute two threads simultaneously as long as both threads don't need A. –  Vincent Vancalbergh Jul 5 '13 at 15:08
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