[BugFix] Preserve spec decoding uniform decode when scheduling (#29759)

Signed-off-by: Nick Hill <nhill@redhat.com>

Author: njhill

tests/v1/e2e/test_spec_decode.py

@@ -191,8 +191,8 @@ def test_suffix_decoding_acceptance(
     # Expect the acceptance rate to improve.
     assert first_accept_rate < last_accept_rate
 
-    # Heuristic: expect at least 85% acceptance rate at the end.
-    assert last_accept_rate > 0.85
+    # Heuristic: expect at least 82.5% acceptance rate at the end.
+    assert last_accept_rate > 0.825
 
     del spec_llm
     torch.cuda.empty_cache()

vllm/v1/core/sched/async_scheduler.py

@@ -33,7 +33,7 @@ def _update_after_schedule(
                 # in this scheduling step.
                 request.num_output_placeholders += 1 + cur_num_spec_tokens
                 # Add placeholders for the new tokens in spec_token_ids.
-                # Wwe will update the actual spec token ids in the worker process.
+                # We will update the actual spec token ids in the worker process.
                 request.spec_token_ids = [-1] * self.num_spec_tokens
 
         scheduler_output.pending_structured_output_tokens = (

vllm/v1/core/sched/scheduler.py

@@ -236,6 +236,22 @@ def schedule(self) -> SchedulerOutput:
         while req_index < len(self.running) and token_budget > 0:
             request = self.running[req_index]
 
+            if (
+                request.num_output_placeholders > 0
+                # This is (num_computed_tokens + 1) - (num_output_placeholders - 1).
+                # Since output placeholders are also included in the computed tokens
+                # count, we subtract (num_output_placeholders - 1) to remove any draft
+                # tokens, so that we can be sure no further steps are needed even if
+                # they are all rejected.
+                and request.num_computed_tokens + 2 - request.num_output_placeholders
+                >= request.num_prompt_tokens + request.max_tokens
+            ):
+                # Async scheduling: Avoid scheduling an extra step when we are sure that
+                # the previous step has reached request.max_tokens. We don't schedule
+                # partial draft tokens since this prevents uniform decode optimizations.
+                req_index += 1
+                continue
+
             num_new_tokens = (
                 request.num_tokens_with_spec
                 + request.num_output_placeholders
@@ -245,18 +261,10 @@ def schedule(self) -> SchedulerOutput:
                 num_new_tokens = self.scheduler_config.long_prefill_token_threshold
             num_new_tokens = min(num_new_tokens, token_budget)
 
-            num_spec_placeholders = max(0, request.num_output_placeholders - 1)
-            max_total_tokens = min(
-                # Avoid scheduling tokens that we're sure won't will be needed based on
-                # request.max_tokens. For this calculation we assume placeholder
-                # speculated output tokens are rejected.
-                request.num_prompt_tokens + request.max_tokens + num_spec_placeholders,
-                # Make sure the input position does not exceed the max model len.
-                # This is necessary when using spec decoding.
-                self.max_model_len,
-            )
+            # Make sure the input position does not exceed the max model len.
+            # This is necessary when using spec decoding.
             num_new_tokens = min(
-                num_new_tokens, max_total_tokens - 1 - request.num_computed_tokens
+                num_new_tokens, self.max_model_len - 1 - request.num_computed_tokens
             )
 
             # Schedule encoder inputs.
@@ -799,15 +807,15 @@ def _make_cached_request_data(
         for idx, req in enumerate(itertools.chain(running_reqs, resumed_reqs)):
             req_id = req.request_id
             req_ids.append(req_id)
-            num_tokens = num_scheduled_tokens[req_id] - len(
-                spec_decode_tokens.get(req_id, ())
-            )
             if self.use_pp:
                 # 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
                 # need to send the sampled tokens back because the model runner
                 # will cache them.
+                num_tokens = num_scheduled_tokens[req_id] - len(
+                    spec_decode_tokens.get(req_id, ())
+                )
                 token_ids = req.all_token_ids[
                     req.num_computed_tokens : req.num_computed_tokens + num_tokens
                 ]