[Beowulf] Opinions of Hyper-threading?

Mark Hahn hahn at mcmaster.ca
Tue Feb 26 18:58:10 EST 2008


>> 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.  ...

I think the reason we both know what that name means is that 
they had (have?) a nugget of truth.  after all, a multiplier 
unit on a chip doesn't really care on which thread's behalf 
it's doing work.  MTA is perhaps a bit far towards the pure 
gatling-gun approach, but I think we can all agree that ultimately
any program is just a big hairy dataflow graph.

>> 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 ...

I think that's why almost everyone agrees with the elegance of AMD's 
system architecture - memory attached to and thus scaling with ncpus.
and yes, there's a lot of work already going on regarding making caches
more intelligent - predicting the multireference or sharing properties
of a cache block, for instance, to choose when to move it and between
which caches in a big system.

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