How does the FSB on a muilti-pricessor system work?

[Drenlin]

I've been wondering this. A friend of mine has gotten hold of an old dual-xeon workstation. The processors have a 533MHz FSB, but I'm wondering if that factors to 533 over both processors, or 533 for each one. If the latter, is that roughly the equivalent of 1066, for the processors at least? (I know that's simplifying things a bit)

edit: title fail...I need to learn to touch-type

 

[DCIScouts]

That should be 533MHz for each processor, and no that's not really like 1066 at all. All that means is that there can be a maximum of 533 cycles per second to each processor. It's kind of like saying a 2.4GHz dual-core processor is like a 4.8GHz single-core; not even close.

Think of it this way instead: Instead of having one lane of traffic that can move at 55MPH, you can have two lanes; allowing more traffic to flow. Are things going to be processed faster? Yes, are they going to get there twice as fast? No.

 

[Drenlin]

I figured it wouldn't be exactly the equivalent of a 1066 syatem. I guess what I'm wondering is, will it perform as fast as a dual core of the same speed, with a faster FSB on it.

 

[TrainTrackHack]

^ Not quite.

The FSB is shared among the CPUs/cores. You won't get the rated FSB speed for each CPU, rather, they all use the single FSB. How much bandwidth each CPU gets depends on the load - if only one core is doing work and all the others sit idle, that one core gets all the bandwidth it wants. Likewise, if all the cores are working at full capacity, the bandwidth gets divided across reasonably evenly.

 

[Drenlin]

I see...

What about the newer systems with multiple FSB's? Same thing?

 

[poke349]

There's one FSB for each processor. But no single thread can use both of them for double the bandwidth.

Each socket only gets "its" FSB to use, the other socket can't really use it. So you will only feel the full benefit of the dual FSBs when you're running a multi-threaded load.

I've measured my system, and a single thread can't get more than about 5GB/s of bandwidth. But when I put two threads on it and lock them onto separate sockets, I can get up to 9GB/s combined.