Adds the third 4-cases panel: KV replicated per cube (8× memory
waste), PEs SP on S_kv within each cube, intra-CUBE 8-way reduce on
(m, ℓ, O), and no inter-CUBE comm (every cube ends with full answer;
designated writer = cube 0). Reuses the row-chain + col-bridge
intra-CUBE pattern that anchors Case 4 (21 ipcq_copy per cube × 8
cubes = 168 total). 12 tests pass (4 Case 4 + 4 Case 2 + 4 Case 3).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Second case in the GQA decode 4-cases comparative study per
GQA_full_deck.pptx slide 11. Case 2 replicates K, V across all 8
cubes × 8 PEs (the slide-11 8 KB/tok/PE memory waste) and has zero
inter-rank comm. For B=1 (single-user decode default), only PE 0
of CUBE 0 has work — the inherent PE-TP waste slide 11 calls out.
Changes:
- New kernel: src/kernbench/benches/_gqa_attention_decode_cube_repl_pe_tp.py
Simplest of the 4 cases. Active rank loads full Q/K/V from HBM,
computes attention via S_kv tile sweep with online-softmax merge,
writes O. All non-(0,0) ranks early-return. No tl.send/recv.
- src/kernbench/benches/milestone_gqa_decode_4cases.py:
- Add panel single_kv_group_decode_gqa_cube_repl_pe_tp (Case 2)
to _PANELS and _PANEL_DISPATCH.
- Add _run_decode_panel_cube_repl_pe_tp helper: DPPolicy K/V/Q/O
= cube=replicate, pe=replicate (models 8× memory waste).
- Extend _make_bench_fn to dispatch kind="decode_cube_repl_pe_tp"
to the new runner.
- tests/attention/test_milestone_gqa_decode_4cases.py:
4 new tests assert Case 2 contract: panel registered, smoke
completion, zero ipcq_copy (no comm), single dma_write from cube 0.
Verification: 8/8 tests pass (4 Case 4 anchor + 4 new Case 2).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
First milestone of the decode 4-cases comparative study per
GQA_full_deck.pptx slides 11-17. Case 4 (Cube-SP × PE-SP, the optimal
case per slide 11) is structurally the existing _gqa_attention_decode_long
at sub_w=4 (Increment 2's lrab-adapted center-root reduce). This commit
wires it into a dedicated bench so the remaining cases land alongside.
Changes:
- New bench: src/kernbench/benches/milestone_gqa_decode_4cases.py
Houses the 4 case panels under one milestone-gqa-decode-4cases entry
(gated by GQA_DECODE_4CASES_RUN=1; output to
1H_milestone_output/gqa_decode_4cases/sweep.json). Cases 1-3 are
TBD in subsequent sub-increments (5C.A/B/C).
- New panel: single_kv_group_decode_gqa_cube_sp_pe_sp
C=8, P=8, sub_w=4, T_q=1, S_kv=131_072, d_head=128, h_q=8, h_kv=1.
- src/kernbench/benches/milestone_gqa_headline.py: _run_decode_panel
extended with keyword-only sub_w/T_q/d_head/h_q/h_kv overrides
(defaults preserve existing-panel behaviour).
- tests/attention/test_milestone_gqa_decode_4cases.py: 4 new tests
asserting registration, smoke completion, reduce-to-root at the lrab
center cube (cube 6), and the predicted 189-ipcq Case-4 traffic
pattern (168 intra-CUBE + 21 inter-CUBE lrab Phase 1+2).
- tests/attention/test_milestone_gqa_headline.py: rename
test_sweep_json_has_four_panels -> test_sweep_json_has_expected_panels
and switch hardcoded 4 to len(PANELS) (the panel set grew to 5
with Increment 5's single_kv_group_prefill_gqa_c8_p8).
Deviation noted: slide 13 prescribes AllReduce on (m,ℓ,O); our kernel
does reduce-to-root (only the lrab center cube has the answer) per
ADR-0060 §4. Treated as the kernbench Case-4 baseline.
Verification: all 4 new tests pass; 90 regression tests pass; the
previously-failing test_sweep_json_has_four_panels now passes under
its renamed form.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
mukesh