Grid/skills/ref_a2a_emf_work.md

name, description, metadata

name	description	metadata
ref_a2a_emf_work	A2A Extended Meson Field GPU offload work — status, file locations, pending task	node_type type originSessionId memory project 956e80aa-401d-481a-80bb-17f8abe1c131

What was built

Grid/algorithms/blas/A2ASpatialSum.h — batched GEMM spatial sum replacing scalar SIMD accumulation. Included via Grid/algorithms/Algorithms.h.

tests/Test_extended_meson_field.cc — test with class A2AExtendedMesonFieldRef containing:

• CPU reference path (use_blas=false)
• BLAS path (use_blas=true) using A2ASpatialSum
• Per-phase timing with [ref type=N] / [blas type=N] labels
• 4 contraction types (0-3), all verified at machine precision (~4e-16 rel_err)

Pending task: GPU offload class

Goal: Write A2AExtendedMesonFieldGPU in the same test file, replacing all thread_for loops with accelerator_for-based free function kernels.

The thread_for blocks to replace all have the form:

thread_for(r, rd, {
  int so = r * grid->_ostride[orthogdim];
  for (int n = 0; n < e1; n++)
  for (int b = 0; b < e2; b++) {
    int ss = so + n * stride + b;
    // work
  }
});

Replace with accelerator_for(ss, grid->oSites(), Nsimd, { ... }).

Free functions to write (each takes Lattice<T> args, opens views internally):

• A2ALoopPropagator — outerProduct sum (loop build)
• A2APackLeftConjugated — conjugate left fermion fields into Lattice<SpinColourVector_v>
• A2ALoopLeftContractionType0/1/2/3 — per-site loop × loop propagator → tloop
• A2ALoopRightContractionType0/1/2/3 — per-site tloop × right → loopRight[j]

Data structure changes required:

• tloopv: std::vector<SpinColourMatrix_v> → Lattice<SpinColourMatrix_v> (PropagatorField)
• leftv[i]: std::vector<SpinColourVector_v> → Lattice<SpinColourVector_v>
• loopRight[j]: std::vector<SpinColourVector_v> → Lattice<SpinColourVector_v>

Why: std::vector<vobj> is host memory, not GPU accessible. See ref_lattice_vs_vector.

A2ASpatialSum impact: PackLeft/PackRight currently take std::vector<std::vector<vobj>>. Once leftv/loopRight become std::vector<Lattice<vobj>>, those signatures must change to match.

Timing on 8.8.8.16 (N_i=N_j=8, Nloop=4, 1 MPI rank)

Dominant costs:

• loop_build: 4-6 ms (outerProduct over 4 propagators)
• pack_loopright: 0.9-2.2 ms (type-dependent)
• spatial_sum (ref): ~1.5 ms
• A2ASpatialSum TOTAL: 2.5-4.3 ms (PackLeft+PackRight dominate GEMM on small volume)

Related

ref_accelerator_for ref_coalesced_views ref_lattice_vs_vector ref_grid_simt_pattern