perf(ml-alpha): NVIDIA-grade rewrite of CfC K-loop hot kernels — 2.15× faster
Local L40S profile (perception_overfit smoke) GPU kernel time:
1589ms → 739ms (53.5% reduction, 2.15× speedup).
Wall-clock smoke: 9.6s → 4.94s (1.94× faster).
Per-kernel deltas (nsys --cuda-graph-trace=node):
reduce_axis0: 362ms → 10ms (36× faster)
Block layout: per-column (1 block / output) → 32-wide column tile
(block_dim = 32 × 8). Cross-thread reads were strided by n_tail
(~40K floats = 160KB stride) — one cache line per thread, 8× HBM
bandwidth wasted. New tile gives coalesced 128B transactions per
warp. Block tree-reduce kept (no atomicAdd, per feedback_no_atomicadd).
+1 shared-mem pad to eliminate 32-way bank conflict on the ty reduce.
multi_horizon_heads_grn_bwd_batched: 540ms → 113ms (4.8× faster)
1. Stage h_row[HIDDEN] and a1[5,HEAD_MID] in shared at block entry.
Eliminates ~28K redundant DRAM reads/block across Pass 3 + Pass 5.
2. Pass 5 reorder: k outer / i inner with d_z1[k,m] pinned in
register; writes to grad_w1_scratch are sequential per-thread.
3. Block size 64 → 128 threads. Pass 5/6 now partition over i
(output column): cross-thread writes become COALESCED 128B/warp
(was stride-128 = 512B). Passes 2/3/4 gate on (tid < HEAD_MID).
4. Pass 3 thread role: m_out → m_in/n. Same coalescing fix on
grad_w2 writes AND w2 reads in the d_eta_2 sum.
cfc_step_backward_batched: 351ms → 271ms (1.3× faster)
1. Stage x_b[n_in] and h_old_b[n_hid] in shared (was 128× redundant
DRAM reads per block; now 1× cooperative load).
2. Pass 1 thread role: i (output row) → k (output col). For each
i loop iteration, the warp writes grad_w_in[..., tid] /
grad_w_rec[..., tid] — COALESCED 128B/warp (was stride-128
non-coalesced).
multi_horizon_heads_grn_fwd_batched: 211ms → 204ms
Stage h_row[HIDDEN] in shared — Pass 1 and Pass 3 both consume.
cfc_step_batched (fwd): 95ms → 94ms
Stage x_b and h_old_b in shared.
Shared-mem budgets fit comfortably under the 48KB SM cap (~6KB / ~2KB
respectively). All 9 perception_overfit tests pass — gradient
correctness validated end-to-end (constant-signal overfit, K-loop
capture/replay, stride-4 path, evaluate-only paths).
Discipline:
- Block tree-reduce only, never atomicAdd
- No nvrtc; pre-compiled cubins via build.rs
- Mapped-pinned-only is unaffected (CPU↔GPU contract untouched)
- Single source of truth: replaced kernels in place, no v2 suffixes
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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