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Better reduction

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This commit is contained in:
Peter Boyle
2026-08-26 23:36:31 -04:00
parent 06c6011229
commit e5f1553232
4 changed files with 98 additions and 36 deletions
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Grid/algorithms/multigrid/DenseCoarseMatrix.h
+20 -7
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@@ -119,7 +119,9 @@ public:
deviceVector<ComplexF> dSlab;
deviceVector<ComplexF> dX; // N x MRHS_MAX
deviceVector<ComplexF> dY; // nrows x MRHS_MAX
deviceVector<ComplexF> dG; // N x MRHS_MAX rank-major staging for the allgather (devSum==4)
deviceVector<ComplexF> dG; // N x MRHS_MAX lex-major staging for the allgather (devSum==4)
deviceVector<int> dLex2Rank;// lex index of a process coordinate -> its rank (allgather block order -> row-block order)
int myLex;
deviceVector<ComplexF> dPartial; // NK x (nrows x MRHS_MAX)
deviceVector<ComplexF*> aptrs; // slab K-chunk pointers (lda = N)
deviceVector<ComplexF*> xptrs; // X K-chunk pointers (ldb = N)
@@ -257,7 +259,18 @@ public:
"DEVICE cartesian ring allreduce (P2P)","DEVICE flat ring allreduce (P2P)",
"DEVICE cartesian ring ALLGATHER (P2P, ~8x fewer bytes than the padded allreduce)"};
GRID_ASSERT(devSum>=0 && devSum<=4);
if ( devSum==4 ) dG.resize((uint64_t)N*MRHS_MAX);
if ( devSum==4 ) {
dG.resize((uint64_t)N*MRHS_MAX);
// allgather delivers blocks in lexicographic-coordinate order; the row
// blocks of x are in RANK order. Same table as BuildRankMajorMap.
int P = grid->ProcessorCount();
std::vector<int> l2r(P);
for(int lp=0; lp<P; lp++){ Coordinate pc(nd); Lexicographic::CoorFromIndex(pc, lp, grid->_processors); l2r[lp] = grid->RankFromProcessorCoor(pc); }
dLex2Rank.resize(P);
acceleratorCopyToDevice(&l2r[0], &dLex2Rank[0], P*sizeof(int));
myLex = CartesianLexIndex(grid);
GRID_ASSERT( l2r[myLex] == grid->ThisRank() );
}
std::cout << GridLogMessage << "DenseCoarseMatrix: slab resident on device ("
<< sbytes/1024./1024. << " MB/rank), split-K NK=" << NK << " (Kc=" << Kc << "); "
<< sumName[devSum] << std::endl;
@@ -972,18 +985,18 @@ public:
std::vector<ComplexF> hG(chunk);
for(int r=0;r<nr;r++)
memcpy(&hG[(uint64_t)r*nrows], &hX[(uint64_t)r*N + (uint64_t)me*nrows], nrows*sizeof(ComplexF));
acceleratorCopyToDevice(&hG[0], &dG[(uint64_t)me*chunk], chunk*sizeof(ComplexF));
acceleratorCopyToDevice(&hG[0], &dG[(uint64_t)myLex*chunk], chunk*sizeof(ComplexF)); // my slot is my LEX index
}
t2 = usecond();
{ GRID_TRACE("DenseAllgather");
CartesianRingAllGather(grid, (ComplexF *)&dG[0], chunk);
// scatter [q][r][i] -> dX[r*N + q*nrows + i]
ComplexF *g = &dG[0]; ComplexF *x = &dX[0];
// scatter lex block L=[r][i] -> dX[r*N + rank(L)*nrows + i]
ComplexF *g = &dG[0]; ComplexF *x = &dX[0]; int *l2r = &dLex2Rank[0];
const int64_t nrw = nrows; const int64_t NN = N; const int nrr = nr;
accelerator_for(idx, (uint64_t)N*nr, 1, {
int64_t r = idx / NN; int64_t gi = idx - r*NN;
int64_t q = gi / nrw; int64_t i = gi - q*nrw;
x[idx] = g[q*(nrw*nrr) + r*nrw + i];
int64_t L = gi / nrw; int64_t i = gi - L*nrw;
x[r*NN + (int64_t)l2r[L]*nrw + i] = g[L*(nrw*nrr) + r*nrw + i];
});
}
t3 = usecond();