[Feature] Fully support for async scheduling + PP, 30.8% E2E throughput...

[Feature] Fully support for async scheduling + PP, 30.8% E2E throughput improvement, 31.8% TPOT improvement (#32618)
Signed-off-by: yewentao256 <zhyanwentao@126.com>
Signed-off-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>
Signed-off-by: Nick Hill <nickhill123@gmail.com>
Co-authored-by: Nick Hill <nickhill123@gmail.com>

tests/test_config.py

@@ -42,6 +42,22 @@ def test_compile_config_repr_succeeds():
     assert "inductor_passes" in val
 
 
+def test_async_scheduling_with_pipeline_parallelism_is_allowed():
+    cfg = VllmConfig(
+        scheduler_config=SchedulerConfig(
+            max_model_len=8192,
+            is_encoder_decoder=False,
+            async_scheduling=True,
+        ),
+        parallel_config=ParallelConfig(
+            pipeline_parallel_size=2,
+            distributed_executor_backend="mp",
+            nnodes=2,
+        ),
+    )
+    assert cfg.scheduler_config.async_scheduling is True
+
+
 @dataclass
 class _TestConfigFields:
     a: int

tests/v1/core/test_scheduler.py

@@ -127,6 +127,21 @@ def test_schedule_multimodal_requests():
         assert len(encoder_input) == 1
 
 
+def test_async_scheduling_pp_allows_rescheduling_with_output_placeholders():
+    """Async scheduling + PP: allow multi-step in-flight scheduling per request"""
+    scheduler = create_scheduler(async_scheduling=True, pipeline_parallel_size=2)
+    (req,) = create_requests(num_requests=1, num_tokens=8)
+    scheduler.add_request(req)
+
+    _ = scheduler.schedule()
+    assert req.num_output_placeholders > 0
+
+    # before any update_from_output, we still expect the request can be
+    # scheduled again (multi-step in-flight).
+    output = scheduler.schedule()
+    assert req.request_id in output.num_scheduled_tokens
+
+
 def test_schedule_partial_requests():
     """Test scheduling behavior with partial requests.
 

vllm/v1/core/sched/scheduler.py

@@ -344,13 +344,6 @@ class Scheduler(SchedulerInterface):
         while req_index < len(self.running) and token_budget > 0:
             request = self.running[req_index]
 
-            # do not schedule another step for the same request while it still has
-            # output placeholders for PP.
-            # TODO: support PP + async scheduling without this limit
-            if self.use_pp and request.num_output_placeholders > 0:
-                req_index += 1
-                continue
-
             if (
                 request.num_output_placeholders > 0
                 # This is (num_computed_tokens + 1) - (num_output_placeholders - 1).
@@ -1003,7 +996,10 @@ class Scheduler(SchedulerInterface):
         for idx, req in enumerate(itertools.chain(running_reqs, resumed_reqs)):
             req_id = req.request_id
             req_ids.append(req_id)
-            if self.use_pp:
+            # NOTE: In PP+async scheduling, we consume token ids via a direct GPU
+            # broadcast path (`input_batch.prev_sampled_token_ids`), so we can
+            # omit this payload.
+            if self.use_pp and not self.scheduler_config.async_scheduling:
                 # When using PP, the scheduler sends the sampled tokens back,
                 # because there's no direct communication between the first-
                 # stage worker and the last-stage worker. Otherwise, we don't

vllm/v1/worker/gpu_model_runner.py

@@ -1010,9 +1010,13 @@ class GPUModelRunner(
             req_state.num_computed_tokens = num_computed_tokens
 
             if not is_last_rank:
-                # When using PP, the scheduler sends the sampled tokens back,
-                # because there's no direct communication between the first-
-                # stage worker and the last-stage worker.
+                if not req_data.new_token_ids:
+                    # Async scheduled PP: Sampled tokens propagated via GPU broadcast.
+                    new_token_ids: list[int] = []
+                else:
+                    # Non-async scheduling with PP: The scheduler sends
+                    # sampled token ids back because there's no direct communication
+                    # between the first-stage worker and the last-stage worker.
                     new_token_ids = req_data.new_token_ids[i]
                     # Add the sampled token(s) from the previous step (if any).
                     # This doesn't include "unverified" tokens like spec tokens.
@@ -1023,7 +1027,9 @@ class GPUModelRunner(
                         # Avoid slicing list in most common case.
                         req_state.output_token_ids.append(new_token_ids[-1])
                     elif num_new_tokens > 0:
-                    req_state.output_token_ids.extend(new_token_ids[-num_new_tokens:])
+                        req_state.output_token_ids.extend(
+                            new_token_ids[-num_new_tokens:]
+                        )
             elif num_output_tokens < len(req_state.output_token_ids):
                 # Some output tokens were discarded due to a sync-KV-load
                 # failure. Align the cached state.
@@ -3577,7 +3583,9 @@ class GPUModelRunner(
         self.kv_connector_output = None
 
         if self.execute_model_state is None:
-            # Nothing to do (PP non-final rank case), output isn't used.
+            # receive sampled token ids from the last PP rank.
+            if self.use_async_scheduling and get_pp_group().world_size > 1:
+                self._pp_receive_prev_sampled_token_ids_to_input_batch()
             if not kv_connector_output:
                 return None  # type: ignore[return-value]
 
@@ -3618,6 +3626,12 @@ class GPUModelRunner(
         self._update_states_after_model_execute(
             sampler_output.sampled_token_ids, scheduler_output
         )
+        if self.use_async_scheduling:
+            pp = get_pp_group()
+            if pp.world_size > 1 and pp.is_last_rank:
+                self._pp_broadcast_prev_sampled_token_ids(
+                    sampler_output.sampled_token_ids
+                )
 
         self._draft_token_ids = None
         self._draft_token_req_ids = None
@@ -3753,6 +3767,45 @@ class GPUModelRunner(
 
         return async_output
 
+    def _pp_broadcast_prev_sampled_token_ids(
+        self, sampled_token_ids: torch.Tensor
+    ) -> None:
+        """Broadcast sampled token ids (GPU) from last PP stage"""
+        pp = get_pp_group()
+        assert pp.is_last_rank
+        # `prev_sampled_token_ids` is expected to have shape [num_reqs, 1].
+        assert sampled_token_ids.dim() == 2 and sampled_token_ids.shape[-1] == 1, (
+            "PP+async expects sampled_token_ids to have shape [num_reqs, 1]"
+        )
+        torch.distributed.broadcast(
+            sampled_token_ids, src=pp.rank, group=pp.device_group
+        )
+
+    def _pp_receive_prev_sampled_token_ids_to_input_batch(self) -> None:
+        """Receive sampled token ids broadcast from last PP stage"""
+        pp = get_pp_group()
+        assert not pp.is_last_rank
+        num_reqs = self.input_batch.num_reqs
+        # `prev_sampled_token_ids` is expected to have shape [num_reqs, 1].
+        recv = torch.empty((num_reqs, 1), dtype=torch.int32, device=self.device)
+        torch.distributed.broadcast(recv, src=pp.last_rank, group=pp.device_group)
+        self.input_batch.prev_sampled_token_ids = recv
+
+        # construct `prev_req_id_to_index` here so `_prepare_input_ids`
+        # can map req_id -> previous batch row
+        discard_req_indices = np.nonzero(self.discard_request_mask.np[:num_reqs])[0]
+        discard_req_indices_set = set(discard_req_indices)
+        prev_req_id_to_index: dict[str, int] = {}
+        for i, req_id in enumerate(self.input_batch.req_ids):
+            if i in discard_req_indices_set:
+                continue
+            prev_req_id_to_index[req_id] = i
+            # PP+async scheduling: advance per-request local cached output length by
+            # appending a placeholder (-1) token id.
+            if (req_state := self.requests.get(req_id)) is not None:
+                req_state.output_token_ids.append(-1)
+        self.input_batch.prev_req_id_to_index = prev_req_id_to_index
+
     def take_draft_token_ids(self) -> DraftTokenIds | None:
         if not self.num_spec_tokens or not self._draft_token_req_ids:
             return None