[Beowulf] Opinions of Hyper-threading?

Mark Hahn hahn at mcmaster.ca
Wed Feb 27 19:42:50 EST 2008


> alternatives?  Well, at the instruction level, we have the VLIW --
> prepacked workloads known not to intefer with each other. What about at the

there's a good reason VLIW never made much of a splash - even EPIC is 
pretty much past-tense.  the reason is that static schedules don't work
well with things like caches or even early-out ALU's.  they are fine 
when your ops are consistent/deterministic in execution time - heck,
we should consider VLIW to be a sort of mixed-grill SIMD or vector approach.

as far as I can tell, current GPUs are an example of how far this can be
taken, and at what cost.  you get a set of very in-order core-like units
whose memory model is massively constrained by the need to remain in lock-step.
very good for data-parallel workloads like graphics, but not as easy to 
apply to more complicated data access patterns or conditional flows.

(since I'm ranting about GPUs, I'm really surprised those guys haven't 
dived deeply into the processor-in-memory model.  I guess it says something
significant about how differently specialized cpu vs dram fabs are. 
especially when you consider that GPU boards use custom for-GPU-only dram.)
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