nsys profile of multi_fold_convergence on L40S identified backtest_state_gather as
the #1 GPU consumer at 37.2% (596 ms / 60,588 calls — kernel launch latency
dominated compute) and the per-step TLOB cuBLAS gemvx calls as #2 at 25%
(242K calls). Both share the same per-step amplification: chunk_len=512 separate
gather launches + 512 separate TLOB.forward calls (4 SGEMMs each) per chunk
before any Q-values can be computed.
This commit replaces the per-step gather + DtoD pattern with a single batched
launch, and reuses the same chunked buffer for a single chunk-wide TLOB forward.
Per-chunk launch reduction: from 2*chunk_len + chunk_len*7 to 1 + 7 for the
gather+TLOB phase (4608 -> 8 with chunk_len=512, a 576x reduction).
New kernel `backtest_state_gather_chunk` (experience_kernels.cu):
- Writes [chunk_len, N, padded_sd] directly into chunked_states_buf
- chunk_len * N threads, 1 thread per output row
- Mathematically identical to per-step gather: portfolio_buf and plan_isv_buf
are CONSTANT within a chunk (env_step + plan_state_isv update at chunk
boundary only). Each thread reads independent feature offsets, no atomics,
no reordering.
Chunked val TLOB (gpu_backtest_evaluator.rs + metrics.rs):
- Val TLOB instance now sized to DQN_BACKTEST_CHUNK_SIZE * n_windows via new
GpuBacktestEvaluator::val_tlob_batch_size() helper.
- submit_dqn_step_loop_cublas calls tlob.forward(chunked_states, batch) ONCE
per chunk on the chunk-wide buffer instead of chunk_len times on states_buf.
- Partial last chunks reuse the same buffers (forward(b) accepts any
b <= construction_batch).
Borrow restructure:
- Removed top-of-function `let ch_states = self.chunked_states_buf.as_ref()?`
binding (TLOB needs &mut). Replaced with per-chunk ch_states_base raw u64
device pointer extracted in tight scope, reused by Phase 1 (gather) and
Phase 2+3 (compute_q_values_to + last_step_states_ptr). The pointer is
stable across the chunk because the Option<CudaSlice<f32>> does not
reallocate.
Per-step gather kernel `backtest_state_gather` retained unchanged for
evaluate() / evaluate_ppo() / evaluate_supervised() paths that still need
a single-step writer (closure-based callers with no chunked buffer).
Audit doc dqn-gpu-hot-path-audit.md updated with Fix 18 entry per Invariant 7.
Build: SQLX_OFFLINE=true cargo check -p ml --lib clean (12 warnings, baseline).
Tests: cargo test -p ml --lib --no-run clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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