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https://github.com/paboyle/Grid.git
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Peter Boyle
62 lines
2.6 KiB
Markdown
62 lines
2.6 KiB
Markdown
---
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name: ref_a2a_emf_work
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description: "A2A Extended Meson Field GPU offload work — status, file locations, pending task"
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metadata:
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node_type: memory
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type: project
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originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
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---
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## What was built
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`Grid/algorithms/blas/A2ASpatialSum.h` — batched GEMM spatial sum replacing scalar SIMD accumulation. Included via `Grid/algorithms/Algorithms.h`.
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`tests/Test_extended_meson_field.cc` — test with class `A2AExtendedMesonFieldRef` containing:
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- CPU reference path (`use_blas=false`)
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- BLAS path (`use_blas=true`) using `A2ASpatialSum`
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- Per-phase timing with `[ref type=N]` / `[blas type=N]` labels
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- 4 contraction types (0-3), all verified at machine precision (~4e-16 rel_err)
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## Pending task: GPU offload class
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**Goal**: Write `A2AExtendedMesonFieldGPU` in the same test file, replacing all `thread_for` loops with `accelerator_for`-based free function kernels.
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The `thread_for` blocks to replace all have the form:
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```cpp
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thread_for(r, rd, {
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int so = r * grid->_ostride[orthogdim];
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for (int n = 0; n < e1; n++)
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for (int b = 0; b < e2; b++) {
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int ss = so + n * stride + b;
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// work
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}
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});
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```
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Replace with `accelerator_for(ss, grid->oSites(), Nsimd, { ... })`.
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**Free functions to write** (each takes `Lattice<T>` args, opens views internally):
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- `A2ALoopPropagator` — outerProduct sum (loop build)
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- `A2APackLeftConjugated` — conjugate left fermion fields into `Lattice<SpinColourVector_v>`
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- `A2ALoopLeftContractionType0/1/2/3` — per-site loop × loop propagator → `tloop`
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- `A2ALoopRightContractionType0/1/2/3` — per-site tloop × right → `loopRight[j]`
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**Data structure changes required**:
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- `tloopv`: `std::vector<SpinColourMatrix_v>` → `Lattice<SpinColourMatrix_v>` (PropagatorField)
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- `leftv[i]`: `std::vector<SpinColourVector_v>` → `Lattice<SpinColourVector_v>`
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- `loopRight[j]`: `std::vector<SpinColourVector_v>` → `Lattice<SpinColourVector_v>`
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**Why**: `std::vector<vobj>` is host memory, not GPU accessible. See [[ref_lattice_vs_vector]].
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**`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.
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## Timing on 8.8.8.16 (N_i=N_j=8, Nloop=4, 1 MPI rank)
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Dominant costs:
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- `loop_build`: 4-6 ms (outerProduct over 4 propagators)
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- `pack_loopright`: 0.9-2.2 ms (type-dependent)
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- `spatial_sum` (ref): ~1.5 ms
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- `A2ASpatialSum TOTAL`: 2.5-4.3 ms (PackLeft+PackRight dominate GEMM on small volume)
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## Related
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[[ref_accelerator_for]] [[ref_coalesced_views]] [[ref_lattice_vs_vector]] [[ref_grid_simt_pattern]]
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