[Mpi-forum] MPI One-Sided Communication

Underwood, Keith D keith.d.underwood at intel.com
Fri Apr 24 21:22:19 CDT 2009

>No, that's not true in the slightest for what Vinod is referring to.
>NWChem's CCSD(T) code hit 0.357 petaflops at 0.55% efficiency on
>Jaguar and this method is anything but simple.  The most scalable
>portion is a 7-loop accumulation wherein the first loop computes very
>large intermediates (by design, it fills up the memory) which must be
>communicated all over the place in a non-trivial way along with other
>intermediates and permanent data inside of another set of do loops.
>Once all the buffers get pushed around, the inner loops are just a
>bunch of DGEMMs.

Ahem.  I think you just proved Greg's point.  Jaguar is a Cray XT series.  While I rather like that machine (I was part of the team at Sandia at the time), it has atrocious one-sided performance for small messages.  If you can do it with big messages, you can do it with MPI-1 two sided.   

>See the attached papers for the details of the algorithm.  The
>GA/ARMCI implementation is less than 1000 lines of code.  Please let
>me know when you can match this with MPI-1.

At this point, we are just bickering about where the work is, right?  So, the app wants to program in a GA model for ease of use.  Great.  That isn't a performance argument. 

>> I think this nicely illustrates my point that I don't know of any
>> existing HPC application that really needs one-sided hardware to get
>> great performance.*
>Frankly, this is just a religious war already waged in HPCWire
>(Myricom versus IBM).  It appears you're saying that IBM and Cray
>supporting powerful one-sided hardware is a waste of money.  Is that
>why their machines, SGI's and those running Infiniband (excellent
>RDMA) compose the first 26 slots on the Top500 list?  Anti-one-sided
>Myricom's best showing is #27.

Um, yeah, Infiniband isn't up there because of its one-sided support.  Neither is the Cray machine.  Oh, wait, neither are the IBM machines.  


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