Possible Duplicate:
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?
Yes, there is a difference.
Obviously, physical cores would only outperform the virtual cores if programs actually use them. :)
Virtual cores share execution units in each physical core. They advantage of unused CPU execution units (e.g., multipliers) to process other threads. When the threads compete for the same execution units, though, they don't run more instructions at a time than a single core.
Physical cores, on the other hand, have separate execution units to 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.
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.
