Re: [OMPI users] Questions about non-blocking collective calls...

On Dec 17, 2015, at 1:39 PM, Eric Chamberland wrote: > > Just to be clear: we *always* call MPI_Wait. Now the question was about > *when* to do it. Ok. Remember that the various flavors of MPI_Test are acceptable, too. And it's ok to call

Re: [OMPI users] Questions about non-blocking collective calls...

Le 2015-12-17 12:45, Jeff Squyres (jsquyres) a écrit : On Dec 17, 2015, at 8:57 AM, Eric Chamberland wrote: But I would like to know if the MPI I am using is able to do message progression or not: So how do an end-user like me can knows that? Does-it rely

Re: [OMPI users] Questions about non-blocking collective calls...

On Dec 17, 2015, at 8:57 AM, Eric Chamberland wrote: > > But I would like to know if the MPI I am using is able to do message > progression or not: So how do an end-user like me can knows that? Does-it > rely on hardware? Is there a #define by OpenMPI that

Re: [OMPI users] Questions about non-blocking collective calls...

Hi Gilles, Le 2015-10-21 20:31, Gilles Gouaillardet a écrit : #3 difficult question ... first, keep in mind there is currently no progress thread in Open MPI. that means messages can be received only when MPI_Wait* or MPI_Test* is invoked. you might hope messages are received when doing some

Re: [OMPI users] Questions about non-blocking collective calls...

Eric, A tree based implementation for a gather is not that critical, because the root will always have to gather the entire data, so from it’s perspective going from a star to a tree-based topology is basically exchanging latencies for bandwidth (a little bit more complicated in practice). In

Re: [OMPI users] Questions about non-blocking collective calls...

Hi Gilles and Josh, I think my reply apply to both of your answers which I thank you for. On 21/10/15 08:31 PM, Gilles Gouaillardet wrote: Eric, #2 maybe not ... a tree based approach has O(log(n)) scaling (compared to O(n) scaling with your linear method. so at scale, MPI_Igather will

Re: [OMPI users] Questions about non-blocking collective calls...

Instead of posting a single, big MPI_Igather, why not post and simultaneously progress multiple, small MPI_Igathers? In this way, you can pipeline multiple outstanding collectives and do post collective processing as soon as requests complete. Without network hardware offload capability, Gilles'

Re: [OMPI users] Questions about non-blocking collective calls...

Eric, #1 MPI_Igather uses only one MPI_Request, and it is marked as completed when all data has been received. so no, you cannot process data as it is received. (and btw, that would require extra subroutines to ask which piece of data has already been received) #2 maybe not ... a tree based