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Hyper-Threading and Dual-Core, What's the Difference?

Most Intel processors feature what they call "hyperthreading," basically another virtual core for each physical core.

Is there a significant difference between these virtual cores and physical cores? For example, if I bought an 8-core machine and a hyperthreaded quad-core both at the same clock with similar specifications, would the 8 physical core machine outperform the quad-core-with-hyperthreading machine?

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marked as duplicate by techie007, Sathya Nov 17 '11 at 4:32

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up vote 6 down vote accepted

Yes, there is a difference.

Obviously, physical cores would only outperform the virtual cores if programs actually use them. :)

Virtual cores simply take advantage of CPU stalls to process another thread, while the CPU is waiting. When nothing is stalling, though, they don't run more instructions at a time than a single core.

Physical cores, on the other hand, actually run multiple threads in parallel; they are very similar to multiple physical processors, except that caches and such components might be shared at some levels.

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Well, the difference is so significant, you could end up with no performance difference between a dual core with hyperthreading and a regular dual core. Consider hyperthreading as a 'recycling' feature, it could use clocks that otherwise unusable due to memory access. But on simplistic process, it could give you no improvement at all.

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+1, and I believe you meant to write "Well, the difference is so insignificant..."? –  Randolf Richardson Nov 17 '11 at 3:40
    
I thought it was rhetoricly accurate to the way it was sold :-) –  Psycogeek Nov 17 '11 at 5:55
    
Sorry, I mean the difference between a physical dual core with a hyperthreaded core –  Martheen Cahya Paulo Nov 18 '11 at 5:48
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