[TRTLLM-8084][feat] Enhance overlap scheduler for two-model spec decoding

Signed-off-by: ziyixiong-nv <[email protected]>

Author: ziyixiong-nv

tensorrt_llm/_torch/pyexecutor/model_engine.py

@@ -35,10 +35,12 @@
 from tensorrt_llm.mapping import CpType
 from tensorrt_llm.runtime.generation import CUASSERT
 
+from ..attention_backend.trtllm import TrtllmAttention
 from ..distributed import Distributed
 from ..models.modeling_utils import DecoderModelForCausalLM
 from ..modules.decoder_layer import DecoderLayer
 from ..speculative.drafter import Drafter
+from ..speculative.mtp import SampleStateTensorsMTP
 from ..speculative.speculation_gate import SpeculationGate
 from .executor_request_queue import ExecutorRequestQueue, RequestQueueItem
 from .guided_decoder import GuidedDecoder
@@ -271,8 +273,18 @@ def __init__(self,
 
         if self.dist.pp_size > 1:
             self.event_loop = self._executor_loop_pp
+        elif self.disable_overlap_scheduler:
+            self.event_loop = self._executor_loop
+        elif self.drafter is not None and (
+                not self.drafter.use_static_draft_loop or not issubclass(
+                    self.draft_model_engine.attn_backend, TrtllmAttention)):
+            logger.warning(
+                "Overlap scheduler is disabled for draft model engine with non-CDL or non-TrtllmAttention attention backend."
+            )
+            self.disable_overlap_scheduler = True
+            self.event_loop = self._executor_loop
         else:
-            self.event_loop = self._executor_loop if disable_overlap_scheduler else self._executor_loop_overlap
+            self.event_loop = self._executor_loop_overlap
         if is_trace_enabled("TLLM_TRACE_EXECUTOR_LOOP"):
             self.event_loop = trace_func(self.event_loop)
 
@@ -1045,14 +1057,11 @@ def _prepare_and_schedule_batch(self):
                     0
                 ] * max_total_draft_tokens if max_total_draft_tokens > 0 else []
 
-            # When overlap scheduler is enabled, and we already prepared the draft tokens in the previous batch,
-            # we don't need to initialize py_draft_tokens at this stage because we haven't append the accepted tokens to the request yet.
-            if not self.has_previous_draft_tokens:
-                # If speculation is off, this function sets py_draft_tokens to []
-                # for all active requests. If it's on, we initialize py_draft_tokens
-                # with dummy draft tokens to make the scheduler aware of the fact
-                # that speculation is about to happen.
-                self._prepare_draft_requests()
+            # If speculation is off, this function sets py_draft_tokens to []
+            # for all active requests. If it's on, we initialize py_draft_tokens
+            # with dummy draft tokens to make the scheduler aware of the fact
+            # that speculation is about to happen.
+            self._prepare_draft_requests()
 
         scheduled_batch, fitting_disagg_gen_init_requests, num_fitting_reqs = self._schedule(
         )
@@ -1256,6 +1265,8 @@ def _executor_loop_overlap(self):
         with self._profiler() as profile_step:
             iter_start_time = time.time()
             iter_stats = None
+            target_inputs = None
+            previous_tensors_device = None
             can_forward = False if self.benchmark_req_queues_size > 0 and self.kv_cache_transceiver else True
             while True:
                 profile_step()
@@ -1329,31 +1340,29 @@ def _executor_loop_overlap(self):
                         self.guided_decoder.init_disagg_gen_requests()
 
                     previous_tensors = self.previous_batch and self.previous_batch.sample_state
-                    target_inputs = None
-                    draft_outputs = None
                     # If there are previous draft tokens, we need to update the target requests to accept some draft tokens.
                     # When there's any accepted tokens, we can't directly use the previous batch's outputs in this iteration for the target model,
                     # so we'll set the target model's input to None and skip updating the target requests after target model forward.
                     use_previous_draft_tokens = self.has_previous_draft_tokens
                     if self.drafter is not None and (self.use_spec_decode or
                                                      use_previous_draft_tokens):
-                        target_inputs, draft_outputs, draft_batch = self._handle_speculative_decoding(
-                            scheduled_batch, previous_tensors)
+                        target_inputs = self._handle_speculative_decoding(
+                            scheduled_batch, previous_tensors,
+                            previous_tensors_device)
 
                     # Use the draft_model's outputs if we've launched the draft model.
                     # Otherwise, use the previous batch's outputs.
-                    if target_inputs is not None or use_previous_draft_tokens:
+                    if (target_inputs is not None
+                            and target_inputs.next_draft_tokens
+                            is not None) or use_previous_draft_tokens:
                         previous_tensors_device = target_inputs
                     else:
                         previous_tensors_device = self.previous_batch and self.previous_batch.sample_state and self.previous_batch.sample_state.device
 
                     batch_outputs = self._forward_step(scheduled_batch,
                                                        previous_tensors_device)
 
-                    if target_inputs is not None:
-                        self._process_draft_results(scheduled_batch,
-                                                    draft_outputs, draft_batch)
-                    elif self.previous_batch is not None and not use_previous_draft_tokens:
+                    if self.previous_batch is not None:
                         self._update_requests(self.previous_batch.sample_state)
 
                         if self.block_reuse_enabled and not self.kv_cache_manager.is_vswa and self.kv_cache_transceiver:
@@ -1368,6 +1377,10 @@ def _executor_loop_overlap(self):
                                             (req, block_id,
                                              self.ctx_in_transmission_counter))
 
+                    if self.drafter is not None and self.use_spec_decode:
+                        # Cleanup previous draft resources used in the draft model
+                        self.drafter.cleanup_previous_draft_resources()
+
                     if self.guided_decoder is not None:
                         # add_batch must be called again to have updated new tokens.
                         self.guided_decoder.add_batch(scheduled_batch)
@@ -1402,6 +1415,94 @@ def _executor_loop_overlap(self):
 
                 self._kv_connector_terminate_requests()
 
+    def _accept_draft_tokens(
+        self, scheduled_batch: ScheduledRequests,
+        target_outputs: SampleStateTensors,
+        target_inputs: Optional[SampleStateTensors]
+    ) -> Tuple[SampleStateTensorsMTP, Optional[torch.Tensor]]:
+        """
+        Prepare target device inputs after computing draft token acceptance.
+
+        This function:
+        1. If draft tokens exist: compares sampled tokens with draft tokens to compute acceptance
+        2. If no draft tokens: directly uses the first sampled token
+        3. Creates new_tokens by extracting accepted tokens per request
+
+        Args:
+            scheduled_batch: The scheduled requests
+            target_outputs: Contains new_tokens [max_draft_len + 1, batch_size, beam_width]
+                                or [1, batch_size, beam_width] if no draft tokens
+            target_inputs: Contains next_draft_tokens [batch_size, max_draft_len]
+        Returns:
+            Tuple of:
+            - SampleStateTensorsMTP with new_tokens set to accepted tokens,
+              new_tokens_lens and next_draft_tokens set to None
+            - num_accepted_tokens: [batch_size] tensor with acceptance counts per request,
+              or None if no draft tokens
+        """
+        has_draft_tokens = target_inputs is not None and isinstance(
+            target_inputs, SampleStateTensorsMTP
+        ) and target_inputs.next_draft_tokens is not None
+        target_tokens = target_outputs.new_tokens  # [max_draft_len + 1, batch_size, beam_width] or [1, batch_size, beam_width]
+        new_tokens = torch.zeros_like(target_tokens)
+
+        # Squeeze the beam dimension (beam_width=1 for greedy or single beam)
+        target_tokens = target_tokens.squeeze(
+            -1)  # [max_draft_len + 1, batch_size] or [1, batch_size]
+
+        batch_size = target_tokens.shape[1]
+        device = target_tokens.device
+        # Compute number of accepted tokens per request
+        num_accepted_tokens = torch.zeros(batch_size,
+                                          dtype=torch.int32,
+                                          device=device)
+        # Handle case where there are no draft tokens
+        if has_draft_tokens:
+            # Draft tokens exist, compute acceptance
+            draft_tokens = target_inputs.next_draft_tokens  # [batch_size, max_draft_len]
+            max_draft_len = draft_tokens.shape[1]
+
+            # Compute number of accepted tokens per request
+            # Generation requests: compare with draft tokens to find acceptance
+            num_contexts = len(scheduled_batch.context_requests)
+            if batch_size > num_contexts:
+                # Use .T to transpose: [max_draft_len + 1, num_gens] -> [num_gens, max_draft_len + 1]
+                gen_target_tokens = target_tokens[:,
+                                                  num_contexts:].T  # [num_gens, max_draft_len + 1]
+
+                # Compare draft tokens with target tokens to find acceptance
+                # Use cumprod to find the first rejection point
+                draft_tokens_gen = draft_tokens[
+                    num_contexts:, :].int()  # [num_gens, max_draft_len]
+                num_accepted_tokens[num_contexts:] += torch.cumprod(
+                    (draft_tokens_gen == gen_target_tokens[:, :max_draft_len]
+                     ).int(),
+                    dim=-1).sum(dim=1)
+
+            # Vectorized extraction using advanced indexing (no GPU-CPU sync)
+            # Use num_accepted_tokens as indices to gather the right tokens
+            batch_indices = torch.arange(batch_size, device=device)
+            new_tokens[0, :, 0] = target_tokens[num_accepted_tokens,
+                                                batch_indices]
+        else:
+            # No draft tokens to accept, just use the first (and only) sampled token
+            batch_indices = torch.arange(batch_size, device=device)
+            new_tokens[0, :, 0] = target_tokens[0, batch_indices]
+
+        # Create the updated SampleStateTensorsMTP
+        # new_tokens_lens and next_draft_tokens are left as None
+        result_tensors = SampleStateTensorsMTP(
+            new_tokens=new_tokens,
+            log_probs=target_outputs.log_probs,
+            new_tokens_lens=None,
+            next_draft_tokens=None)
+
+        # Copy logits if available
+        if hasattr(target_outputs, 'logits'):
+            result_tensors.logits = target_outputs.logits
+
+        return result_tensors, num_accepted_tokens
+
     def _process_previous_batch(self):
         if self.kv_cache_transceiver and self.previous_batch.ctx_transmission_reqs:
             for req in self.previous_batch.ctx_transmission_reqs:
@@ -2298,7 +2399,8 @@ def _remove_inflight_ids(self, scheduled_requests):
         for req in scheduled_requests.all_requests():
             self.inflight_req_ids.erase(req.request_id)
 
-    def _handle_speculative_decoding(self, scheduled_batch, previous_tensors):
+    def _handle_speculative_decoding(self, scheduled_batch, previous_tensors,
+                                     target_inputs):
         with request_context(is_draft=self.draft_model_engine is not None,
                              scheduled_requests=scheduled_batch):
             # Do an early checking to see if we need to forward the draft model.
@@ -2308,20 +2410,25 @@ def _handle_speculative_decoding(self, scheduled_batch, previous_tensors):
                 self.previous_batch is not None and self.use_spec_decode
                 and self.drafter.should_forward_draft_model(scheduled_batch))
 
-            if has_draft_batch or self.has_previous_draft_tokens:
-                self._update_requests(self.previous_batch.sample_state)
-                if self.has_previous_draft_tokens:
-                    self._prepare_draft_requests()
+            new_target_inputs = None
+            if has_draft_batch:
+                target_outputs = self.previous_batch.sample_state and self.previous_batch.sample_state.device
+                assert target_outputs is not None, "target_outputs should not be None"
+                new_target_inputs, num_accepted_tokens_device = self._accept_draft_tokens(
+                    scheduled_batch=scheduled_batch,
+                    target_inputs=target_inputs,
+                    target_outputs=target_outputs)
 
             if has_draft_batch:
-                target_inputs, draft_outputs, draft_batch = self.drafter.generate_draft_tokens_with_overlap(
+                self.drafter.generate_draft_tokens_with_overlap(
                     scheduled_batch, self.resource_manager,
-                    previous_tensors.device if previous_tensors else None)
+                    previous_tensors.device if previous_tensors else None,
+                    new_target_inputs, num_accepted_tokens_device)
 
-                self.has_previous_draft_tokens = target_inputs is not None and target_inputs.next_draft_tokens is not None
+                # Pad draft tokens to the max draft length for CUDA graph compatibility
+                self.has_previous_draft_tokens = new_target_inputs is not None and new_target_inputs.next_draft_tokens is not None
             else:
                 self.has_previous_draft_tokens = False
-                target_inputs, draft_outputs, draft_batch = None, None, None
                 # We are not running the draft model. Remove the draft tokens and turn off spec
                 # decode so that the requests get handled correctly.
                 # One corner case: when we have at least one context request, we have to keep spec
@@ -2334,34 +2441,7 @@ def _handle_speculative_decoding(self, scheduled_batch, previous_tensors):
                     for request in scheduled_batch.all_requests():
                         request.py_draft_tokens = []
 
-        return target_inputs, draft_outputs, draft_batch
-
-    def _process_draft_results(self, scheduled_batch, draft_outputs,
-                               draft_batch):
-        """
-        Append the draft tokens to the target requests, and clean up the draft resources.
-        """
-        with request_context(is_draft=self.draft_model_engine is not None,
-                             scheduled_requests=scheduled_batch):
-            req_id_to_old_request = {
-                req.py_request_id: req
-                for req in scheduled_batch.all_requests()
-            }
-
-            if self.drafter.use_static_draft_loop:
-                self.drafter.process_static_draft_outputs(
-                    draft_outputs, draft_batch, req_id_to_old_request)
-            elif draft_outputs is not None:
-                self.drafter.process_dynamic_draft_outputs(
-                    draft_outputs, req_id_to_old_request)
-
-            # Pad draft tokens to the max draft length. This is for CUDA graph compatibility.
-            self.drafter.pad_draft_tokens_for_cuda_graph(scheduled_batch)
-            # add_batch must be called again to restore to target requests with updated draft tokens.
-            if self.guided_decoder is not None:
-                self.guided_decoder.add_batch(scheduled_batch)
-                if hasattr(self.drafter, "guided_decoder"):
-                    self.guided_decoder.rollback_draft_tokens()
+        return new_target_inputs
 
 
 class DisaggPPTerminationHandler:

tensorrt_llm/_torch/pyexecutor/py_executor.py

@@ -277,7 +277,9 @@ def _group_requests_by_strategy_key(
     for req_index, req in enumerate(requests):
         strategy = _request_strategy(req, vocab_size=vocab_size)
         strategy_key = strategy_to_key(strategy)
-        speculation_needs_probs = req.py_draft_logits is not None and strategy is not GREEDY
+        # In the overlap path, py_draft_logits is not updated yet,
+        # so we use get_draft_token_length() for the checking.
+        speculation_needs_probs = get_draft_token_length(req) > 0 and strategy is not GREEDY
         group_dict_entry = group_dict[(strategy_key, speculation_needs_probs)]
         group_dict_entry[0].append(req_index)
         group_dict_entry[1].append(strategy)