Reduce one step decode for draft model. (#11561)

python/sglang/srt/layers/attention/aiter_backend.py

@@ -1064,7 +1064,7 @@ class AiterMultiStepDraftBackend:
             device=model_runner.device,
         )
         self.attn_backends = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 AiterAttnBackend(
                     model_runner,
@@ -1107,7 +1107,7 @@ class AiterMultiStepDraftBackend:
             self.page_size,
         )
 
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             forward_batch.spec_info.kv_indptr = self.kv_indptr[i, : bs + 1]
             forward_batch.spec_info.kv_indices = kv_indices_buffer[i][
                 : seq_lens_sum * self.topk + bs * (i + 1)
@@ -1141,7 +1141,7 @@ class AiterMultiStepDraftBackend:
             dtype=torch.int32,
             device=self.device,
         )
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(
                 max_bs, max_num_tokens, kv_indices_buf=self.cuda_graph_kv_indices[i]
             )

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -2320,7 +2320,7 @@ class FlashAttentionMultiStepBackend:
         self.topk = topk
         self.speculative_num_steps = speculative_num_steps
         self.attn_backends = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 FlashAttentionBackend(
                     model_runner,
@@ -2335,7 +2335,7 @@ class FlashAttentionMultiStepBackend:
             self.attn_backends[i].init_forward_metadata(forward_batch)
 
     def init_cuda_graph_state(self, max_bs: int, max_num_tokens: int):
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(max_bs, max_num_tokens)
 
     def init_forward_metadata_capture_cuda_graph(

python/sglang/srt/layers/attention/flashinfer_backend.py

@@ -1405,7 +1405,7 @@ class FlashInferMultiStepDraftBackend:
             (max_bs,), dtype=torch.int32, device=model_runner.device
         )
         self.attn_backends: List[FlashInferAttnBackend] = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 FlashInferAttnBackend(
                     model_runner,
@@ -1493,7 +1493,7 @@ class FlashInferMultiStepDraftBackend:
             device="cuda",
         )
 
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(
                 max_bs, max_num_tokens, kv_indices_buf=self.cuda_graph_kv_indices[i]
             )

python/sglang/srt/layers/attention/flashinfer_mla_backend.py

@@ -916,7 +916,7 @@ class FlashInferMLAMultiStepDraftBackend:
         )
 
         self.attn_backends = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 FlashInferMLAAttnBackend(
                     model_runner,
@@ -998,7 +998,7 @@ class FlashInferMLAMultiStepDraftBackend:
             device="cuda",
         )
 
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(
                 max_bs, max_num_tokens, kv_indices_buf=self.cuda_graph_kv_indices[i]
             )

python/sglang/srt/layers/attention/flashmla_backend.py

@@ -478,7 +478,7 @@ class FlashMLAMultiStepDraftBackend:
         )
 
         self.attn_backends = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 FlashMLABackend(
                     model_runner,
@@ -506,7 +506,7 @@ class FlashMLAMultiStepDraftBackend:
         self.common_template(forward_batch, call_fn)
 
     def init_cuda_graph_state(self, max_bs: int, max_num_tokens: int):
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(
                 max_bs, max_num_tokens, block_kv_indices=None
             )

python/sglang/srt/layers/attention/triton_backend.py

@@ -918,7 +918,7 @@ class TritonMultiStepDraftBackend:
             device=model_runner.device,
         )
         self.attn_backends: List[TritonAttnBackend] = []
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends.append(
                 TritonAttnBackend(
                     model_runner,
@@ -969,7 +969,7 @@ class TritonMultiStepDraftBackend:
         if call_fn is None:
             return
 
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             forward_batch.spec_info.kv_indptr = self.kv_indptr[i, : bs + 1]
             forward_batch.spec_info.kv_indices = kv_indices_buffer[i][
                 : seq_lens_sum * self.topk + bs * (i + 1)
@@ -1009,7 +1009,8 @@ class TritonMultiStepDraftBackend:
             dtype=torch.int32,
             device=self.device,
         )
-        for i in range(self.speculative_num_steps):
+
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(
                 max_bs,
                 max_num_tokens,

python/sglang/srt/layers/attention/trtllm_mha_backend.py

@@ -637,7 +637,7 @@ class TRTLLMHAAttnMultiStepDraftBackend(FlashInferMultiStepDraftBackend):
         self, model_runner: ModelRunner, topk: int, speculative_num_steps: int
     ):
         super().__init__(model_runner, topk, speculative_num_steps)
-        for i in range(speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i] = TRTLLMHAAttnBackend(
                 model_runner,
                 skip_prefill=True,
@@ -651,7 +651,7 @@ class TRTLLMHAAttnMultiStepDraftBackend(FlashInferMultiStepDraftBackend):
             self.attn_backends[i].init_forward_metadata(forward_batch)
 
     def init_cuda_graph_state(self, max_bs: int, max_num_tokens: int):
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i].init_cuda_graph_state(max_bs, max_num_tokens)
 
     def init_forward_metadata_capture_cuda_graph(

python/sglang/srt/layers/attention/trtllm_mla_backend.py

@@ -735,7 +735,7 @@ class TRTLLMMLAMultiStepDraftBackend(FlashInferMLAMultiStepDraftBackend):
     ):
         super().__init__(model_runner, topk, speculative_num_steps)
 
-        for i in range(self.speculative_num_steps):
+        for i in range(self.speculative_num_steps - 1):
             self.attn_backends[i] = TRTLLMMLABackend(
                 model_runner,
                 skip_prefill=True,

python/sglang/srt/speculative/draft_utils.py

+import logging
+
+from sglang.srt.server_args import ServerArgs, get_global_server_args
+from sglang.srt.utils.common import is_blackwell
+
+logger = logging.getLogger(__name__)
+
+
+class DraftBackendFactory:
+    def __init__(
+        self,
+        server_args: ServerArgs,
+        draft_model_runner,
+        topk: int,
+        speculative_num_steps: int,
+    ):
+        self.server_args = server_args
+        self.draft_model_runner = draft_model_runner
+        self.topk = topk
+        self.speculative_num_steps = speculative_num_steps
+
+    def _create_backend(
+        self, backend_name: str, backend_map: dict, error_template: str
+    ):
+        backend_type = getattr(self.server_args, backend_name)
+        if backend_type is None:
+            backend_type = self.server_args.attention_backend
+
+        if backend_type not in backend_map:
+            raise ValueError(error_template.format(backend_type=backend_type))
+
+        return backend_map[backend_type]()
+
+    def create_decode_backend(self):
+        if self.speculative_num_steps == 1:
+
+            class DummyAttnBackend:
+                def __init__(self):
+                    pass
+
+                def init_forward_metadata(*args, **kwargs):
+                    pass
+
+            return DummyAttnBackend()
+
+        backend_map = {
+            "flashinfer": self._create_flashinfer_decode_backend,
+            "triton": self._create_triton_decode_backend,
+            "aiter": self._create_aiter_decode_backend,
+            "fa3": self._create_fa3_decode_backend,
+            "hybrid_linear_attn": (
+                self._create_fa3_decode_backend
+                if not is_blackwell()
+                else self._create_triton_decode_backend
+            ),
+            "flashmla": self._create_flashmla_decode_backend,
+            "trtllm_mha": self._create_trtllm_mha_decode_backend,
+            "trtllm_mla": self._create_trtllm_mla_decode_backend,
+        }
+
+        return self._create_backend(
+            "decode_attention_backend",
+            backend_map,
+            "EAGLE is not supported in decode attention backend {backend_type}",
+        )
+
+    def create_draft_extend_backend(self):
+        backend_map = {
+            "flashinfer": self._create_flashinfer_prefill_backend,
+            "triton": self._create_triton_prefill_backend,
+            "aiter": self._create_aiter_prefill_backend,
+            "fa3": self._create_fa3_prefill_backend,
+            "hybrid_linear_attn": (
+                self._create_fa3_prefill_backend
+                if not is_blackwell()
+                else self._create_triton_prefill_backend
+            ),
+            "flashmla": self._create_flashmla_prefill_backend,
+            "trtllm_mha": self._create_trtllm_mha_prefill_backend,
+            "trtllm_mla": self._create_trtllm_mla_prefill_backend,
+        }
+        backend_name = (
+            "decode_attention_backend"
+            if self.server_args.speculative_attention_mode == "decode"
+            else "prefill_attention_backend"
+        )
+        return self._create_backend(
+            backend_name,
+            backend_map,
+            "EAGLE is not supported in attention backend {backend_type}",
+        )
+
+    def _create_flashinfer_decode_backend(self):
+        if not get_global_server_args().use_mla_backend:
+            from sglang.srt.layers.attention.flashinfer_backend import (
+                FlashInferMultiStepDraftBackend,
+            )
+
+            self.has_prefill_wrapper_verify = True
+            return FlashInferMultiStepDraftBackend(
+                self.draft_model_runner, self.topk, self.speculative_num_steps
+            )
+        else:
+            from sglang.srt.layers.attention.flashinfer_mla_backend import (
+                FlashInferMLAMultiStepDraftBackend,
+            )
+
+            self.has_prefill_wrapper_verify = True
+            return FlashInferMLAMultiStepDraftBackend(
+                self.draft_model_runner, self.topk, self.speculative_num_steps
+            )
+
+    def _create_triton_decode_backend(self):
+        from sglang.srt.layers.attention.triton_backend import (
+            TritonMultiStepDraftBackend,
+        )
+
+        return TritonMultiStepDraftBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_aiter_decode_backend(self):
+        from sglang.srt.layers.attention.aiter_backend import AiterMultiStepDraftBackend
+
+        return AiterMultiStepDraftBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_fa3_decode_backend(self):
+        from sglang.srt.layers.attention.flashattention_backend import (
+            FlashAttentionMultiStepBackend,
+        )
+
+        return FlashAttentionMultiStepBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_flashmla_decode_backend(self):
+        from sglang.srt.layers.attention.flashmla_backend import (
+            FlashMLAMultiStepDraftBackend,
+        )
+
+        return FlashMLAMultiStepDraftBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_trtllm_mha_decode_backend(self):
+        from sglang.srt.layers.attention.trtllm_mha_backend import (
+            TRTLLMHAAttnMultiStepDraftBackend,
+        )
+
+        self.has_prefill_wrapper_verify = True
+        return TRTLLMHAAttnMultiStepDraftBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_trtllm_mla_decode_backend(self):
+        if not get_global_server_args().use_mla_backend:
+            raise ValueError(
+                "trtllm_mla backend requires MLA model (use_mla_backend=True)."
+            )
+
+        from sglang.srt.layers.attention.trtllm_mla_backend import (
+            TRTLLMMLAMultiStepDraftBackend,
+        )
+
+        self.has_prefill_wrapper_verify = True
+        return TRTLLMMLAMultiStepDraftBackend(
+            self.draft_model_runner, self.topk, self.speculative_num_steps
+        )
+
+    def _create_flashinfer_prefill_backend(self):
+        if not get_global_server_args().use_mla_backend:
+            from sglang.srt.layers.attention.flashinfer_backend import (
+                FlashInferAttnBackend,
+            )
+
+            return FlashInferAttnBackend(self.draft_model_runner, skip_prefill=False)
+        else:
+            from sglang.srt.layers.attention.flashinfer_mla_backend import (
+                FlashInferMLAAttnBackend,
+            )
+
+            return FlashInferMLAAttnBackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_triton_prefill_backend(self):
+        from sglang.srt.layers.attention.triton_backend import TritonAttnBackend
+
+        return TritonAttnBackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_aiter_prefill_backend(self):
+        from sglang.srt.layers.attention.aiter_backend import AiterAttnBackend
+
+        return AiterAttnBackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_fa3_prefill_backend(self):
+        from sglang.srt.layers.attention.flashattention_backend import (
+            FlashAttentionBackend,
+        )
+
+        return FlashAttentionBackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_trtllm_mha_prefill_backend(self):
+        from sglang.srt.layers.attention.trtllm_mha_backend import TRTLLMHAAttnBackend
+
+        return TRTLLMHAAttnBackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_trtllm_mla_prefill_backend(self):
+        if not get_global_server_args().use_mla_backend:
+            raise ValueError(
+                "trtllm_mla backend requires MLA model (use_mla_backend=True)."
+            )
+
+        from sglang.srt.layers.attention.trtllm_mla_backend import TRTLLMMLABackend
+
+        return TRTLLMMLABackend(self.draft_model_runner, skip_prefill=False)
+
+    def _create_flashmla_prefill_backend(self):
+        logger.warning(
+            "flashmla prefill backend is not yet supported for draft extend."
+        )
+        return None

python/sglang/srt/speculative/eagle_worker.py

@@ -27,7 +27,8 @@ from sglang.srt.model_executor.forward_batch_info import (
     ForwardBatch,
     ForwardMode,
 )
-from sglang.srt.server_args import ServerArgs, get_global_server_args
+from sglang.srt.server_args import ServerArgs
+from sglang.srt.speculative.draft_utils import DraftBackendFactory
 from sglang.srt.speculative.eagle_draft_cuda_graph_runner import (
     EAGLEDraftCudaGraphRunner,
 )
@@ -195,204 +196,22 @@ class EAGLEWorker(TpModelWorker):
         self.has_prefill_wrapper_verify = False
         self.draft_extend_attn_backend = None
 
-        # Initialize decode attention backend
-        self.draft_attn_backend = self._create_decode_backend()
-
-        # Initialize draft extend attention backend (respects speculative_attention_mode setting)
-        self.draft_extend_attn_backend = self._create_draft_extend_backend()
-
-        self.draft_model_runner.draft_attn_backend = self.draft_attn_backend
-
-    def _create_backend(
-        self, backend_name: str, backend_map: dict, error_template: str
-    ):
-        backend_type = getattr(self.server_args, backend_name)
-        if backend_type is None:
-            backend_type = self.server_args.attention_backend
-
-        if backend_type not in backend_map:
-            raise ValueError(error_template.format(backend_type=backend_type))
-
-        return backend_map[backend_type]()
-
-    def _create_decode_backend(self):
-        backend_map = {
-            "flashinfer": self._create_flashinfer_decode_backend,
-            "triton": self._create_triton_decode_backend,
-            "aiter": self._create_aiter_decode_backend,
-            "fa3": self._create_fa3_decode_backend,
-            "hybrid_linear_attn": (
-                self._create_fa3_decode_backend
-                if not is_blackwell()
-                else self._create_triton_decode_backend
-            ),
-            "flashmla": self._create_flashmla_decode_backend,
-            "trtllm_mha": self._create_trtllm_mha_decode_backend,
-            "trtllm_mla": self._create_trtllm_mla_decode_backend,
-        }
-
-        return self._create_backend(
-            "decode_attention_backend",
-            backend_map,
-            "EAGLE is not supported in decode attention backend {backend_type}",
-        )
-
-    def _create_draft_extend_backend(self):
-        backend_map = {
-            "flashinfer": self._create_flashinfer_prefill_backend,
-            "triton": self._create_triton_prefill_backend,
-            "aiter": self._create_aiter_prefill_backend,
-            "fa3": self._create_fa3_prefill_backend,
-            "hybrid_linear_attn": (
-                self._create_fa3_prefill_backend
-                if not is_blackwell()
-                else self._create_triton_prefill_backend
-            ),
-            "flashmla": self._create_flashmla_prefill_backend,
-            "trtllm_mha": self._create_trtllm_mha_prefill_backend,
-            "trtllm_mla": self._create_trtllm_mla_prefill_backend,
-        }
-        backend_name = (
-            "decode_attention_backend"
-            if self.server_args.speculative_attention_mode == "decode"
-            else "prefill_attention_backend"
-        )
-        return self._create_backend(
-            backend_name,
-            backend_map,
-            "EAGLE is not supported in attention backend {backend_type}",
-        )
-
-    def _create_flashinfer_decode_backend(self):
-        if not get_global_server_args().use_mla_backend:
-            from sglang.srt.layers.attention.flashinfer_backend import (
-                FlashInferMultiStepDraftBackend,
-            )
-
-            self.has_prefill_wrapper_verify = True
-            return FlashInferMultiStepDraftBackend(
-                self.draft_model_runner, self.topk, self.speculative_num_steps
-            )
-        else:
-            from sglang.srt.layers.attention.flashinfer_mla_backend import (
-                FlashInferMLAMultiStepDraftBackend,
-            )
-
-            self.has_prefill_wrapper_verify = True
-            return FlashInferMLAMultiStepDraftBackend(
-                self.draft_model_runner, self.topk, self.speculative_num_steps
-            )
-
-    def _create_triton_decode_backend(self):
-        from sglang.srt.layers.attention.triton_backend import (
-            TritonMultiStepDraftBackend,
-        )
-
-        return TritonMultiStepDraftBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
-        )
-
-    def _create_aiter_decode_backend(self):
-        from sglang.srt.layers.attention.aiter_backend import AiterMultiStepDraftBackend
-
-        return AiterMultiStepDraftBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
-        )
-
-    def _create_fa3_decode_backend(self):
-        from sglang.srt.layers.attention.flashattention_backend import (
-            FlashAttentionMultiStepBackend,
-        )
-
-        return FlashAttentionMultiStepBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
-        )
-
-    def _create_flashmla_decode_backend(self):
-        from sglang.srt.layers.attention.flashmla_backend import (
-            FlashMLAMultiStepDraftBackend,
-        )
-
-        return FlashMLAMultiStepDraftBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
-        )
-
-    def _create_trtllm_mha_decode_backend(self):
-        from sglang.srt.layers.attention.trtllm_mha_backend import (
-            TRTLLMHAAttnMultiStepDraftBackend,
-        )
-
-        self.has_prefill_wrapper_verify = True
-        return TRTLLMHAAttnMultiStepDraftBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
-        )
-
-    def _create_trtllm_mla_decode_backend(self):
-        if not get_global_server_args().use_mla_backend:
-            raise ValueError(
-                "trtllm_mla backend requires MLA model (use_mla_backend=True)."
-            )
-
-        from sglang.srt.layers.attention.trtllm_mla_backend import (
-            TRTLLMMLAMultiStepDraftBackend,
-        )
-
-        self.has_prefill_wrapper_verify = True
-        return TRTLLMMLAMultiStepDraftBackend(
-            self.draft_model_runner, self.topk, self.speculative_num_steps
+        draft_backend_factory = DraftBackendFactory(
+            self.server_args,
+            self.draft_model_runner,
+            self.topk,
+            self.speculative_num_steps,
         )
 
-    def _create_flashinfer_prefill_backend(self):
-        if not get_global_server_args().use_mla_backend:
-            from sglang.srt.layers.attention.flashinfer_backend import (
-                FlashInferAttnBackend,
-            )
-
-            return FlashInferAttnBackend(self.draft_model_runner, skip_prefill=False)
-        else:
-            from sglang.srt.layers.attention.flashinfer_mla_backend import (
-                FlashInferMLAAttnBackend,
-            )
-
-            return FlashInferMLAAttnBackend(self.draft_model_runner, skip_prefill=False)
-
-    def _create_triton_prefill_backend(self):
-        from sglang.srt.layers.attention.triton_backend import TritonAttnBackend
-
-        return TritonAttnBackend(self.draft_model_runner, skip_prefill=False)
-
-    def _create_aiter_prefill_backend(self):
-        from sglang.srt.layers.attention.aiter_backend import AiterAttnBackend
-
-        return AiterAttnBackend(self.draft_model_runner, skip_prefill=False)
+        # Initialize decode attention backend
+        self.draft_attn_backend = draft_backend_factory.create_decode_backend()
 
-    def _create_fa3_prefill_backend(self):
-        from sglang.srt.layers.attention.flashattention_backend import (
-            FlashAttentionBackend,
+        # Initialize draft extend attention backend (respects speculative_attention_mode setting)
+        self.draft_extend_attn_backend = (
+            draft_backend_factory.create_draft_extend_backend()
         )
 
-        return FlashAttentionBackend(self.draft_model_runner, skip_prefill=False)
-
-    def _create_trtllm_mha_prefill_backend(self):
-        from sglang.srt.layers.attention.trtllm_mha_backend import TRTLLMHAAttnBackend
-
-        return TRTLLMHAAttnBackend(self.draft_model_runner, skip_prefill=False)
-
-    def _create_trtllm_mla_prefill_backend(self):
-        if not get_global_server_args().use_mla_backend:
-            raise ValueError(
-                "trtllm_mla backend requires MLA model (use_mla_backend=True)."
-            )
-
-        from sglang.srt.layers.attention.trtllm_mla_backend import TRTLLMMLABackend
-
-        return TRTLLMMLABackend(self.draft_model_runner, skip_prefill=False)
-
-    def _create_flashmla_prefill_backend(self):
-        logger.warning(
-            "flashmla prefill backend is not yet supported for draft extend."
-        )
-        return None
+        self.draft_model_runner.draft_attn_backend = self.draft_attn_backend
 
     def init_cuda_graphs(self):
         """Capture cuda graphs."""