Larrabee delayed: anyone know what's happening?
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From: Del Cecchi` on 3 Jan 2010 23:16 Robert Myers wrote: (snip) > > > Yes, and if your goal is to claw your way to the top of the all- > important Top 500 list, you're not going to waste money on > communication that doesn't help with a linpack benchmark. > > (snip) > Robert. > But of course no one pays the big bucks for Blue Gene or other ibm cousins to get their name in the paper but to get a job done. It might be a job that will get done wrong or not need to be done at all, but they want to get it done. del
From: William Clodius on 3 Jan 2010 23:25 Mayan Moudgill <mayan(a)bestweb.net> wrote: > nmm1(a)cam.ac.uk wrote: > <snip> > > > > In general, it doesn't, but it more-or-less does for the one you are > > doing - which is NOT the way to do multi-dimensional FFTs! It is > > almost always much faster to transpose, so you are doing vector > > operations in the FFT at each stage. > > > > > The initial bit-reversal is trivial to parallelize (at least for 1D > FFTs). It will (at most) involve a single copy of the array over the > entire network. I think Nick is saying that to improve locality it is essential to transpose the dimensions of the array as you cycle through each dimension in a multi-dimensional array in a multi-dimensional FFT. -- Bill Clodius los the lost and net the pet to email
From: Mayan Moudgill on 3 Jan 2010 23:47 William Clodius wrote: > Mayan Moudgill <mayan(a)bestweb.net> wrote: > > >>nmm1(a)cam.ac.uk wrote: >><snip> >> >>>In general, it doesn't, but it more-or-less does for the one you are >>>doing - which is NOT the way to do multi-dimensional FFTs! It is >>>almost always much faster to transpose, so you are doing vector >>>operations in the FFT at each stage. >>> >> >> >>The initial bit-reversal is trivial to parallelize (at least for 1D >>FFTs). It will (at most) involve a single copy of the array over the >>entire network. > > > I think Nick is saying that to improve locality it is essential to > transpose the dimensions of the array as you cycle through each > dimension in a multi-dimensional array in a multi-dimensional FFT. > OK; like I said, I'm not familiar with multi-D FFTs.
From: Robert Myers on 4 Jan 2010 01:40 On Jan 3, 11:16 pm, Del Cecchi` <dcecchinos...(a)att.net> wrote: > Robert Myers wrote: > > (snip) > > > > > Yes, and if your goal is to claw your way to the top of the all- > > important Top 500 list, you're not going to waste money on > > communication that doesn't help with a linpack benchmark. > > (snip) > > Robert. > > But of course no one pays the big bucks for Blue Gene or other ibm > cousins to get their name in the paper but to get a job done. > > It might be a job that will get done wrong or not need to be done at > all, but they want to get it done. > I'm a tad more cynical than you are. It's true that there were issues that the labs claimed needed to be addressed urgently. The labs have at times come under ferocious criticism for slow or no progress on the problem you showed a graphic from, and the labs have come under ferocious criticism for some of the systems they've bought from IBM. At the time of the first hurry-up Blue Gene purchase, Japan had wiggled its way to the top of toe Top 500 list. You can talk all you want, but I can show you briefings that talk about Earth Simulator being at the Top of the Top 500 list. Of what *possible* relevance did that have to the Rayleigh Taylor instability or the stewardship stockpile program? None whatsoever, of course. Clearing that black mark off America's preeminence in high technology was something that every senator and representative could understand. Continuing to emphasize American leadership on that score is something that political leaders continue to understand. Under the circumstances, a "we're working on it" from anyone less than IBM would not have done. I admire your restraint in responding, as it must feel to you that I want to belittle IBM. I'm not sure that IBM even *wants* that business. Want it or not, they've got it, and I am sure that a long string of administrations has leaned on IBM to make sure that it is given a priority that IBM otherwise would not. My real problem is not with IBM, but with the politics of big science. Robert.
From: Robert Myers on 4 Jan 2010 01:46
On Jan 3, 11:25 pm, wclod...(a)lost-alamos.pet (William Clodius) wrote: > I think Nick is saying that to improve locality it is essential to > transpose the dimensions of the array as you cycle through each > dimension in a multi-dimensional array in a multi-dimensional FFT. But it sure wasn't necessary on a Cray-1--a side benefit of not having cache and having easily worked-around limitations on an arbitrary stride in memory. But, of course, none of this matters any more, because no one has to bother with multi-dimensional FFT's. Or, at least, they'd better not. Robert. |