Sun Baked said:
It is
Simultaneous Multi-Threading (.pdf) which is one of the software tricks the Power5 uses to make a single CPU core act like two (one physical and one logical -- see page 4), so it can run two threads at once.
SMT isn't really a "software trick".
The real idea behind SMT is that modern CPUs have many independent execution units and some of these units sit idle at any given time.
I'm making up these numbers to prove a point, but they are close. A G5 (PPC970) has something like 3 interger units, 2 floating point units, an Altivec unit and a load store unit. I'm not 100% on those numbers but you get the idea.
Now, no processor can use every section of the processor at once. If we accept (for the sake of argument) that a G5 has 7 units, you'd find that in reality it probably is only capable of a maximum of 3, maybe 4 operations per cpu cycle. Generally it breaks down something like 2 int ops, 1 fp, 1 load/store, maybe 1 Altivec. That means that roughly half the cpu is idle at any given time.
The idea of SMT is to use those idle sections of the CPU. If you think of the active units as the real cpu, you can imagine that all the inactive units could be grouped into another 'logical' processor... an imaginary processor. SMT (called HyperThreading by Intel) deals with one physical processor as if it were two 'logical' processors. HT enabled machines look like they are Multiprocessor machines.
SMT is good stuff, but it's not as good as dual core. SMT incurs it's own overhead. HT enabled P4/Xeon processors actually run SLOWER when they run very serial code. Some benchmarks actually run faster on P4 systems when HT is disabled. You might remember that Apple caught a lot of heat when they benchmarked the first Gen G5s because the Dell box they ran against had HT disabled. The thing the stupid PC fan boys never bothered to accept was that the benchmarks in question ran faster with HT off. (Dell actually has documentation for benching their workstations and it specifically tells you to disable HT).
Bottom line is, with a pervasively Multi-threaded operating system like OS X, SMT would probably offer a noticeable benefit. Also, the PPC 970 is a 'wider' processor than the P4 or Athlon 64. That is, it has more semi-descrete execution units so it would lend its self to SMT better than something like the P4. In fact, the efficiency of SMT on the P4 is about -10% to 10 or 15%. Yes, it can be 10% slower with HT (SMT) turned on.
I haven't heard recent info on production Power5 boxes but IBM was claiming up to 35% boost with their SMT implementation just prior to the release of production boxes.
And finally, the coolest thing..
If IBM released an SMT enabled dual core chip and Apple went Dual-Dual with them, it would look to the OS like there was 8 logical processors in the box.