Comments on: Multi-threading reviewed

Simultaneous multi-threading is a useful performance tool for processor designers, but it doesn't have the same benefit that adding a full core does.

[GRobLewis]

A supercomputer startup called Tera attempted to build systems in the 1990s with, IIRC, 128 threads per core. The challenge, as always, was writing compilers to efficiently parallelize applications. But on hand-coded tests like integer sort algorithms, the Tera could really fly. (The company eventually absorbed Cray and dropped its multithreaded architecture in favor of Cray's, I believe.)

[ghaff]

More recently SiCortex was doing something along those lines but they recently shut down.

[Pishkado]

A chip has N transistors. Thanks to Moore's "law," N has become in the last five years too large to use them all effectively to make a fast single-thread processor core. As a result, today's processors have C cores that can execute T threads each. The chip designer challenge is to pick the optimum values of C and T, taking into account the eventual market (and therefore target cost) of each chip, which in turn depends on ... but this is a comment, not a dissertation.

One limiting case is C=1, T=maximum possible (one core with the highest possible degree of multithreading). Not cost-effective because processor resources, even with multiple execution units, become a bottleneck.

The other limiting case is T=1, C=maximum possible (lots of single-threaded cores). Not cost-effective because, as Haff discussed, some degree of multithreading helps more than it hurts.

Therefore, except at the very low end, we end up with C>1 and T>1. The exact values vary with the chip, the market, the designers' preferences and will both increase over time (to the also-increasing sound of programmers tearing their hair out - but that's also another topic).