[Beowulf] Opinions of Hyper-threading?
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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