[Beowulf] Opinions of Hyper-threading?

Geoff Jacobs gdjacobs at gmail.com
Wed Feb 27 09:50:02 EST 2008

Ashley Pittman wrote:
> I saw a talk which said SMT was worth a maximum of 20% on power5 and
> often performed worse than if it had been tured off.  This correlates
> well with my experience of it on Intel CPUs.
> http://www.hpcx.ac.uk/about/events/annual2006/slides/hague.pdf
> It seems most people, myself included, benchmark(ed) with hyperthreading
> disabled in the bois/at boot time and again with hyperthreading enabled
> and jobs scheduled to the meta-cpu's.  Not surprisingly the performance
> often isn't all that different despite having twice as many cpu's
> however the variance is much higher when it's enabled.
> I believe there should be a third way whereby the virtual cpu's are
> enabled and running but not used to run parallel jobs, more to run any
> background tasks the OS should happen to throw at them, if we were to go
> down this road I could use the reclaimed cycles to do something sensible
> with marshaling data for non-blocking MPI operations.  At least part of
> the reason this wasn't tested before is scheduler support for
> hyperthreading and CPU binding, by the time kernel support was good
> enough to do the tests I'd have liked to have done the window was closed
> and hardware technology had moved on.

I was wondering about the Power 6 because it is an in order design. To
me, hyperthreading and OoOE are optimizing in the same area, and I
wanted to know if SMT is more beneficial where there is no OoOE.

Geoffrey D. Jacobs

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