Myths and Realities: 2×3GHz6GHz
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 doesn't mean getting two times the performance. True, there are some kinds of problems that are inherently parallelizable and can approach linear throughput gains; a typical example is compilation, which can run close to twice as fast on a carefully managed dual-CPU dual-disk system.
Similarly, most multi-threaded applications won't run twice as fast on a dual-core box, although they should run faster than on a single-core CPU. The performance gain just isn't linear, that's 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 isn't 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 won't be well utilized. (Despite this, I will speculate that today's single-threaded applications as actually used in the field could 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 you're 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 isn't going to be maxing out the CPU anyway, so one of the cores will be mostly idle. (Again, the spyware can share the OS's core most of the time.)
—H.S.
Herb Sutter chairs the ISO C++ Standards committee and is an architect in Microsoft's Developer Division. His most recent books are Exceptional C++ Style and C++ Coding Standards (Addison-Wesley).
