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 _______________________________________________ Beowulf mailing list, [email protected] To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
