Myths and Realities: 2 x 3GHz < 6 GHz
So a dual-core CPU that combines two 3GHz cores practically offers 6GHz of processing power. Right?
Wrong. Even having two threads running on two physical processors doesnt mean getting two times the performance. Similarly, most multi-threaded applications wont run twice as fast on a dual-core box. They should run faster than on a single-core CPU; the performance gain just isnt linear, thats all.
Why not? First, there is coordination overhead between the cores to ensure cache coherency (a consistent view of cache, and of main memory) and to perform other handshaking. Today, a two- or four-processor machine isnt really two or four times as fast as a single CPU even for multi-threaded applications. The problem remains essentially the same even when the CPUs in question sit on the same die.
Second, unless the two cores are running different processes, or different threads of a single process that are well-written to run independently and almost never wait for each other, they wont be well utilized. (Despite this, I will speculate that todays single-threaded applications as actually used in the field could actually see a performance boost for most users by going to a dual-core chip, not because the extra core is actually doing anything useful, but because it is running the adware and spyware that infest many users systems and are otherwise slowing down the single CPU that user has today. I leave it up to you to decide whether adding a CPU to run your spyware is the best solution to that problem.)
If youre running a single-threaded application, then the application can only make use of one core. There should be some speedup as the operating system and the application can run on separate cores, but typically the OS isnt going to be maxing out the CPU anyway so one of the cores will be mostly idle. (Again, the spyware can share the OSs core most of the time.)