The async diffusion output feature moves the D2H (Device→Host) copy and SHM packing from the Worker main thread to a background daemon thread, so the GPU default stream can immediately start the next request's forward pass. This eliminates the GPU bubble caused by synchronous D2H/packing.
**Core benefit**: the previous request's output D2H/packing overlaps with the next request's forward pass.
**Automatically enabled** when `step_execution=False` (default, request-mode). No extra configuration needed. When `step_execution=True` (step-mode), the original synchronous path is used — pump thread and Worker background thread are not started.
## Performance
### HunyuanImage-3.0 (TP4)
| Resolution | Async Off (QPS) | Async On (QPS) | Change |
|---|---|---|---|
| 1024×1024 | 0.4773 | 0.4802 | +0.60% |
| 768×768 | 0.8370 | 0.8533 | +1.95% |
**Analysis**: smaller resolution → larger QPS gain. D2H/packing is relatively fixed overhead; forward is variable (scales with resolution). Smaller images → shorter forward → D2H/packing occupies a larger fraction of total time → more benefit from overlapping it.
- **Path 1**: `execute_model` / `execute_model_batch` → generates `rpc_id`, registers Future, waits for pump to deliver `compute_done`
- **Path 2**: All other RPCs → `_sync_result_buffer` (pump-fed) or `_result_mq` (step-mode, no pump)
5.**record_device_event** (`platforms/`): Cross-platform GPU event recording for side-stream synchronization. Implemented for CUDA, NPU, ROCm, XPU, MUSA; base class returns `None` (safe no-op).
6.**IPC side-stream D2H** (`ipc.py`): `pack_diffusion_output_shm()` and helpers accept `d2h_stream` parameter; use `pin_memory` + `copy_(non_blocking=True)` on the side stream instead of synchronous `.cpu()`.
When `execute_model_batch` returns a `COMPUTE_DONE` with a single `async_output_id` for the entire batch, the executor splits it into per-request `async_output_id`s (format `{batch_id}/{request_id}`) via `_batch_split_map`. When `OUTPUT_READY` arrives, the pump extracts per-request results from the batch output and resolves each request's output future independently. If `OUTPUT_READY` arrives before `execute_batch` registers the split map, `execute_batch` directly adopts the early output.
### Reliability, Lifecycle & Timeout Behavior
1.**Drain Before Memory Release (`drain_async_outputs` / `_ASYNC_OUTPUT_DRAIN_TIMEOUT_S`)**:
When workers transition into sleep states (`handle_sleep_task`) or invoke memory-releasing methods, `WorkerProc.drain_async_outputs()` blocks up to `_ASYNC_OUTPUT_DRAIN_TIMEOUT_S` (10.0s) until all in-flight background D2H/SHM packing tasks finish. This acts as a synchronization barrier preventing worker sleep cycles from releasing device tensors while the side CUDA stream is still actively reading them. Draining typically completes within 10–50 ms under normal operation.
The engine waits for `wait_output_ready()` with a timeout of `_async_output_timeout()` — 600.0s by default, overridable per run with the `VLLM_OMNI_ASYNC_OUTPUT_TIMEOUT` environment variable (a non-numeric or non-positive value is ignored with a warning and the default applies, since this resolves on the request path). If a timeout occurs, the executor logs detailed diagnostic bookkeeping (pending futures, cached outputs, and batch split maps) to identify whether the stall occurred in the worker, pump, or waiter. The packing itself takes milliseconds, but the wait is queued behind the step's GPU work, so its wall-clock duration tracks step time; the bound is generous because a hung engine is surfaced by the worker monitor and `check_health()`, not by this timeout.
-`result_mq` writes are serialized with `_result_mq_lock` to protect the single-writer `MessageQueue` against concurrent writes from the worker main loop (`COMPUTE_DONE`) and background thread (`OUTPUT_READY`).
-`ResultPumpThread` unpacks shared-memory payloads first, and then atomically resolves waiting futures or populates `_completed_outputs` under `_futures_lock`, avoiding race conditions with concurrent `wait_output_ready()` invocations.
4.**Storage-Aware IPC Packing**:
`pack_diffusion_output_shm()` evaluates `max(view_bytes, storage_bytes)` against `_SHM_TENSOR_THRESHOLD` (1 MB) to ensure that per-request slices sharing a large batch storage are moved to POSIX shared memory rather than serialized through pickle over the MessageQueue inline buffer.
Other models with `step_execution=False` are also supported but not yet verified.
### Not Yet Supported (step-mode, `step_execution=True`)
When `step_execution=True` (or `streaming_output=True`, which auto-enables step-mode), models use `execute_stepwise` instead of `execute_model`, which is not in the async Path 1 whitelist. Async output is not applicable.
**Verified**: Helios (`step_execution=True`, does not benefit from this feature).
Other models with `step_execution=True` are also not applicable.
- Each step's `RunnerOutput` must be available synchronously for the scheduler to advance
- Intermediate latent tensors may not need D2H (only the final step needs D2H + postprocess)
- Steps are serial (step N+1 depends on step N output), so the overlap benefit is limited to "final step's D2H overlaps with the next request's forward"
**Planned approach**: first cover only the final step's async D2H, keep intermediate steps synchronous. This matches the request-mode benefit pattern.
## Configuration
No configuration needed. Async output is automatically enabled when `step_execution=False` (default).
