Thanks Jed.
On Fri, Jan 3, 2014 at 4:14 PM, Jed Brown <[email protected]> wrote: > John Mousel <[email protected]> writes: > > > I would like to investigate using pipelined GMRES and IBCGS on Kraken > but > > it seems from the FAQ page that Cray MPT-5.6 is required for the > pipelined > > methods to be effective. Checking the module avail, I see that they only > > have up to MPT-5.3.6. Is there any benefit at all to these methods if I > > can't configure with MPT-5.6? > > IBCGS does not use pipelining so MPT version is irrelevant. For > pipelined GMRES/CG/CR, you need MPT-5.6 or later to use the asynchronous > interface (MPI_Iallreduce). Without that, the pipelined methods will do > fewer reductions, but they will be synchronous instead of asynchronous, > thus likely not paying off. > > Unfortunately, even MPT-5.6 does not managed to make significant > asynchronous progress. So when you use MPI_Iallreduce, almost nothing > happens until you call MPI_Wait. The Cray MPI team says they think this > is a software/implementation problem rather than a hardware problem, and > they are hopeful that they'll be able to release a new MPT that does a > good job of making asynchronous progress. > > This probably means that PGMRES won't provide large benefits until > Cray's MPI team writes some code and you switch to a machine that uses > that new code. >
