[Beowulf] Opinions of Hyper-threading?

Bruno Coutinho coutinho at dcc.ufmg.br
Tue Feb 26 13:03:22 PST 2008

2008/2/25, Geoff Jacobs <gdjacobs at gmail.com>:
> Bill Broadley wrote:
> > I believe it's actually simultaneous, instructions from 2 different
> > processes can run in the same cycle against 2 different register files.
> >
> > Other chips have vertical multithreading where only 1 process runs in
> > any given cycle.
> Multiple threads residing in any particular stage of execution at any
> particular time. If one thread stalls, the processor can proceed with
> execution on the other thread.
> Somewhat outside the bounds of this list, but would an in order
> processor like Power 6 derive more benefit from SMT?

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