The async-diag merge (3c0d26292, building on d9cb14f1b + 673b04a8d) wired the
GPU side of cross-stream eval pipelining correctly (training stream `cuda_stream`,
eval stream `validation_stream`, `cuStreamWaitEvent` barrier on `train_done_event`)
but left a host-side `stream.synchronize()` inside
`gpu_backtest_evaluator::launch_metrics_and_download` that blocked the CPU thread
for the FULL eval drain (~25-30s/epoch on L40S). The host thread is the same one
that submits the next epoch's training kernels via `run_full_step` — so until eval
drained, training submission was gated on it, defeating the dedicated
`validation_stream`.
Fix: split metrics readback into record/consume halves at the buffer-readback layer:
* `launch_metrics_and_record_event` — submits the metrics kernel + DtoD-async
copies of `actions_history_buf` / `intent_mag_buf` / `picked_action_history_buf`
into mapped-pinned mirrors, then records `eval_done_event`. Returns immediately.
* `consume_metrics_after_event` — `event.synchronize()` (the SOLE host wait per
epoch boundary), then `read_volatile`s mapped-pinned `metrics_buf`. After the
event syncs, the four action-distribution helpers read directly from the
now-coherent mapped-pinned mirrors — eliminating four synchronous DtoH copies
that violated `feedback_no_htod_htoh_only_mapped_pinned.md`.
Caller migration (per `feedback_no_partial_refactor.md`):
* `evaluate_dqn_graphed` (existing public API) becomes a thin sync wrapper:
launch_async → consume. ABI unchanged.
* New `evaluate_dqn_graphed_async` for the pipelined path.
* `evaluate` / `evaluate_ppo` / `evaluate_supervised` migrated inline to
launch+consume (no caller pipelines them).
* `compute_validation_loss` replaced by `launch_validation_loss` +
`consume_validation_loss`. Training loop now consumes pending at epoch start
(cached_async_val_loss = epoch N-1's val_sharpe) and launches at epoch end
(sentinel-only return). Final post-loop consume drains the LAST epoch's launch
so smoke tests / hyperopt that read `last_eval_direction_dist` see the most
recent epoch's data.
Mathematical identity preserved: every val_sharpe consumed is bit-identical to
what the prior synchronous flow would have produced for the same epoch — only
the timing of the host parse differs (one epoch later, matching the existing
`cached_async_val_loss` lag semantics; HEALTH_DIAG `val [...]` / `val_dir_dist` /
`val_picked_dir_dist` emit moves with the consume).
Audit: docs/dqn-gpu-hot-path-audit.md Fix 4. Out of scope: periodic chunk-level
`stream.synchronize` calls in `submit_dqn_step_loop_cublas` (lines ~1596 + ~1863)
for kernel-error detection — host-blocking but smaller in aggregate; future work.
Workspace baseline preserved: 13 warnings.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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