On 24 May 2012 12:40, George Bosilca <bosi...@eecs.utk.edu> wrote: > On May 24, 2012, at 11:22 , Jeff Squyres wrote: > >> On May 24, 2012, at 11:10 AM, Lisandro Dalcin wrote: >> >>>> So I checked them all, and I found SCATTERV, GATHERV, and REDUCE_SCATTER >>>> all had the issue. Now fixed on the trunk, and will be in 1.6.1. >>> >>> Please be careful with REDUCE_SCATTER[_BLOCK] . My understanding of >>> the MPI standard is that the the length of the recvcounts array is the >>> local group size >>> (http://www.mpi-forum.org/docs/mpi22-report/node113.htm#Node113) >> >> >> I read that this morning and it made my head hurt. >> >> I read it to be: reduce the data in the local group, scatter the results to >> the remote group. >> >> As such, the reduce COUNT is sum(recvcounts), and is used for the reduction >> in the local group. Then use recvcounts to scatter it to the remote group. >> >> …right? > > Right, you reduce locally but you scatter remotely. As such the size of the > recvcounts buffer is the remote size. As in the local group you do a reduce > (where every process participate with the same amount of data) you only need > a total count which in this case is the sum of all recvcounts. This > requirement is enforced by the fact that the input buffer is of size sum of > all recvcounts, which make sense only if you know the remote group receives > counts.
The standard says this: "Within each group, all processes provide the same recvcounts argument, and provide input vectors of sum_i^n recvcounts[i] elements stored in the send buffers, where n is the size of the group" So, I read " Within each group, ... where n is the size of the group" as being the LOCAL group size. > > I don't see much difference with the other collective. The generic behavior > is that you apply the operation on the local group but the result is moved > into the remote group. > Well, for me this one DO IS different (for example, SCATTER is unidirectional for intercomunicators, but REDUCE_SCATTER is bidirectional). The "recvbuff" is a local buffer, but you understand "recvcounts" as remote. Mmm, the standard is really confusing in this point... -- Lisandro Dalcin --------------- CIMEC (INTEC/CONICET-UNL) Predio CONICET-Santa Fe Colectora RN 168 Km 472, Paraje El Pozo 3000 Santa Fe, Argentina Tel: +54-342-4511594 (ext 1011) Tel/Fax: +54-342-4511169
