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Allow subsets of dimensions for spatial only sums

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This commit is contained in:
Peter Boyle
2026-08-27 17:03:14 -04:00
parent 074bddd48b
commit 7abc19dc03
1 changed files with 12 additions and 3 deletions
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Grid/communicator/RingAllReduce.h
+12 -3
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@@ -27,8 +27,15 @@ NAMESPACE_BEGIN(Grid);
// communicator-level primitive: needs only CartesianCommunicator.
//
// RingAllReduce(comm, buf, n) flat ring over all P ranks
// CartesianRingAllReduce(comm, buf, n) ring along each processor dimension
// in turn (P_d ranks per ring)
// CartesianRingAllReduce(comm, buf, n, orthogDim=-1)
// ring along each processor dimension
// in turn (P_d ranks per ring).
// orthogDim in 0..Nd-1: that dimension is
// SKIPPED, so the result is the sum over
// all ranks sharing my coordinate in it
// (e.g. orthogDim=3: sum over each
// time-slice of processors separately).
// -1: all dimensions (full allreduce).
//
// Why: Cray MPICH device-buffer MPI_Allreduce aborts above ~8 MB (MPI_FLOAT,
// measured 4.4 MB pass / 13.3 MB fail) and delivers 5.8 GB/s where P2P rings
@@ -95,11 +102,13 @@ void RingAllReduce(CartesianCommunicator *comm, T *buf, uint64_t n)
}
template<class T>
void CartesianRingAllReduce(CartesianCommunicator *comm, T *buf, uint64_t n)
void CartesianRingAllReduce(CartesianCommunicator *comm, T *buf, uint64_t n, int orthogDim=-1)
{
if ( comm->ProcessorCount()==1 || n==0 ) return;
int Nd = comm->_ndimension;
GRID_ASSERT( orthogDim >= -1 && orthogDim < Nd );
for(int d=0;d<Nd;d++){
if ( d==orthogDim ) continue; // leave this dimension unsummed
int P = comm->_processors[d];
if ( P==1 ) continue;
int me = comm->_processor_coor[d];