Demystifying multithreading and multi-core

Kevin D. Kissell, MIPS Technologies
(09/26/2007 4:46 PM EDT), EE Times

Is multithreading better than multi-core? Is multi-core better than multithreading? The fact is that the best vehicle for a given application might have one, the other or both. Or neither. They are independent (but complementary) design decisions. As multithreaded processors and multi-core chips become the norm, architects and designers of digital systems need to understand their respective attributes, advantages and disadvantages.

Both multithreading and multi-core approaches exploit the concurrency in a computational workload. The cost, in silicon, energy and complexity, of making a CPU run a single instruction stream ever faster goes up nonlinearly and eventually hits a wall imposed by the physical limitations of circuit technology. That wall keeps moving out a little farther every year, but cost- and power-sensitive designs are constrained to follow the bleeding edge from a safe distance. Fortunately, virtually all computer applications have some degree of concurrency: At least some of the time, two or more independent tasks need to be performed simultaneously. Taking advantage of concurrency to improve computing performance and efficiency isn't always trivial, but it's certainly easier than violating the laws of physics.

Multi-processor, or multi-core, systems exploit concurrency to spread work around a system. As many software tasks can run at the same time as there are processors in the system. This tractability can be used to improve absolute performance, cost or power/performance. Clearly, once one has built the fastest single processor possible in a given technology, the only way to get even more computer power is to use more than one of these processors. More subtly, if a load that would saturate a 1GHz processor could be spread evenly across 4 processors, those processors could be run at roughly 250MHz each. If each 250MHz processor is less than a quarter the size of the 1GHz processor, or consumes less than a quarter the power (either of which may be the case because of the nonlinear cost of higher operating frequencies), the multi-core system might be more economical.

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