New ``_attention_mesh_mlo_2d.py`` decomposes a
``(mesh_rows x mesh_cols)`` cube sub-mesh into two stages of
bidirectional AllReduce-mlo:
Stage 1 — row reduce (E/W edges, mesh_cols-1 steps)
Stage 2 — col reduce (N/S edges, mesh_rows-1 steps)
After both stages every cube holds the same final ``(m, l, o)`` and
writes the normalized output. The online-softmax mlo merge is
associative, so row-then-col partitioning is mathematically
equivalent to a 1D ring AllReduce-mlo over all
``mesh_rows * mesh_cols`` cubes but uses fewer hops:
- 2x4 (8 cubes / KV-group): 4 steps vs 7 (1.75x faster)
- 4x4 (16 cubes / full SIP): 6 steps vs 15 (2.5x faster)
Motivation: the original 1D ring kernel ``_attention_mesh_mlo.py``
hit ``IpcqInvalidDirection`` at cube 4 when ``n_ranks=8`` on the
4x4 cube mesh — cube 4 has no W neighbor at the row 0/1 boundary.
N/S edges are already installed by ``configure_sfr_intercube_multisip``
so the 2D kernel runs on existing wiring without SFR changes.
The kernel accepts ``cube_start: int = 0`` and subtracts it from
``program_id(axis=1)`` so the ring math uses launch-local rank. This
matters because kernbench's ``program_id(axis=1)`` returns the
physical cube id (ADR-0022), so a launch starting at cube 8 would
otherwise compute ``my_row = 8//4 = 2`` (out of sub-mesh bounds) and
deadlock. Default ``cube_start=0`` keeps the existing
multi_user_decode validation behavior bit-for-bit.
Bench dispatch: ``multi_user_decode`` in milestone-gqa-llama70b now
uses the 2D kernel via a new ``mesh_shape`` column in
``_PANEL_DISPATCH``. At validation ``N_RANKS_MULTI_USER=4``, the
shape is ``(1, 4)`` — a degenerate single-row mesh, equivalent in
step count and op_log structure to the prior 1D ring at n_ranks=4.
The other three panels keep their 1D kernels.
Tests: 4 new unit tests in ``test_mesh_mlo_2d_correctness.py`` —
1x4 (degenerate row), 2x4 (8-KV-group target), 4x4 (full SIP), and
2x4 at cube_start=8 (the second sub-mesh per SIP). Existing
milestone (12 tests) and mesh-kernels-rank-axis (7 tests) suites
stay green — no regression.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
mukesh
4859149392