[Beowulf] Opinions of Hyper-threading?

[Joe Landman]

Bruno Coutinho wrote:

> Yes, and more than an out of order processor. A out of order processor, 
> can reorder the instructions whenever a hazard occurs. A in order 
> processor, on the other hand, has to wait. With SMT, it can switch to 
> another thread.
> 
> And today memory access can stall up to hundreds of cycles, so any 
> processor can hide this latency by switching to another thread.

My gosh ... we have re-invented the Tera MTA.  ...

> But the you have to make sure the processor has enough cache and memory 
> bandwidth to handle the increased memory traffic (like Sun Niagara).

The problem with many (cores|threads) is that memory bandwidth wall.  A 
fixed size (B) pipe to memory, with N requesters on that pipe ...



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