Add speculator attention backend switch (#9981)

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

@@ -22,17 +22,45 @@ class HybridAttnBackend(AttentionBackend):
         self.prefill_backend = prefill_backend
         self.decode_backend = decode_backend
 
-    def init_forward_metadata(self, forward_batch: ForwardBatch):
-        if forward_batch.forward_mode.is_decode_or_idle():
-            self.decode_backend.init_forward_metadata(forward_batch)
+    def _select_backend(self, forward_mode: ForwardMode) -> AttentionBackend:
+        """
+        Select the appropriate attention backend based on the forward mode.
+
+        Args:
+            forward_mode: The current forward mode indicating the operation type
+
+        Returns:
+            The selected attention backend (prefill or decode)
+
+        Note:
+            - decode_or_idle: Always uses decode backend
+            - target_verify or draft_extend: Uses decode backend if speculative_attention_backend is "decode", otherwise prefill backend
+            - prefill: Always uses prefill backend
+        """
+        if forward_mode.is_decode_or_idle():
+            return self.decode_backend
+        elif forward_mode.is_target_verify() or forward_mode.is_draft_extend():
+            return (
+                self.decode_backend
+                if self.model_runner.server_args.speculative_attention_backend
+                == "decode"
+                else self.prefill_backend
+            )
         else:
-            self.prefill_backend.init_forward_metadata(forward_batch)
+            return self.prefill_backend
+
+    def init_forward_metadata(self, forward_batch: ForwardBatch):
+        backend = self._select_backend(forward_batch.forward_mode)
+        backend.init_forward_metadata(forward_batch)
 
     def init_cuda_graph_state(self, max_bs: int, max_num_tokens: int):
         self.decode_backend.init_cuda_graph_state(max_bs, max_num_tokens)
-        if self.model_runner.server_args.speculative_algorithm is not None:
-            # When speculative decoding is enabled, we also need to initialize the
-            # prefill backend's cuda graph state to support target_verify.
+        if (
+            self.model_runner.server_args.speculative_algorithm is not None
+            and self.model_runner.server_args.speculative_attention_backend == "prefill"
+        ):
+            # When speculative decoding is enabled, we need to initialize the backend
+            # that will be used for target_verify.
             self.prefill_backend.init_cuda_graph_state(max_bs, max_num_tokens)
 
     def init_forward_metadata_capture_cuda_graph(
@@ -45,26 +73,16 @@ class HybridAttnBackend(AttentionBackend):
         forward_mode: ForwardMode,
         spec_info: Optional[Union[EagleDraftInput, EagleVerifyInput]],
     ):
-        if forward_mode.is_decode_or_idle():
-            self.decode_backend.init_forward_metadata_capture_cuda_graph(
-                bs,
-                num_tokens,
-                req_pool_indices,
-                seq_lens,
-                encoder_lens,
-                forward_mode,
-                spec_info,
-            )
-        else:
-            self.prefill_backend.init_forward_metadata_capture_cuda_graph(
-                bs,
-                num_tokens,
-                req_pool_indices,
-                seq_lens,
-                encoder_lens,
-                forward_mode,
-                spec_info,
-            )
+        backend = self._select_backend(forward_mode)
+        backend.init_forward_metadata_capture_cuda_graph(
+            bs,
+            num_tokens,
+            req_pool_indices,
+            seq_lens,
+            encoder_lens,
+            forward_mode,
+            spec_info,
+        )
 
     def init_forward_metadata_replay_cuda_graph(
         self,
@@ -77,28 +95,17 @@ class HybridAttnBackend(AttentionBackend):
         spec_info: Optional[Union[EagleDraftInput, EagleVerifyInput]],
         seq_lens_cpu: Optional[torch.Tensor],
     ):
-        if forward_mode.is_decode_or_idle():
-            self.decode_backend.init_forward_metadata_replay_cuda_graph(
-                bs,
-                req_pool_indices,
-                seq_lens,
-                seq_lens_sum,
-                encoder_lens,
-                forward_mode,
-                spec_info,
-                seq_lens_cpu,
-            )
-        else:
-            self.prefill_backend.init_forward_metadata_replay_cuda_graph(
-                bs,
-                req_pool_indices,
-                seq_lens,
-                seq_lens_sum,
-                encoder_lens,
-                forward_mode,
-                spec_info,
-                seq_lens_cpu,
-            )
+        backend = self._select_backend(forward_mode)
+        backend.init_forward_metadata_replay_cuda_graph(
+            bs,
+            req_pool_indices,
+            seq_lens,
+            seq_lens_sum,
+            encoder_lens,
+            forward_mode,
+            spec_info,
+            seq_lens_cpu,
+        )
 
     def get_cuda_graph_seq_len_fill_value(self):
         return self.decode_backend.get_cuda_graph_seq_len_fill_value()
@@ -127,6 +134,7 @@ class HybridAttnBackend(AttentionBackend):
         save_kv_cache: bool = True,
         **kwargs,
     ):
-        return self.prefill_backend.forward_extend(
+        backend = self._select_backend(forward_batch.forward_mode)
+        return backend.forward_extend(
             q, k, v, layer, forward_batch, save_kv_cache, **kwargs
         )

python/sglang/srt/managers/schedule_batch.py

@@ -98,6 +98,7 @@ GLOBAL_SERVER_ARGS_KEYS = [
     "sampling_backend",
     "speculative_accept_threshold_single",
     "speculative_accept_threshold_acc",
+    "speculative_attention_backend",
     "torchao_config",
     "triton_attention_reduce_in_fp32",
     "num_reserved_decode_tokens",

python/sglang/srt/models/deepseek_v2.py

@@ -1045,6 +1045,15 @@ class DeepseekV2AttentionMLA(nn.Module):
         # Determine attention backend used by current forward batch
         if forward_batch.forward_mode.is_decode_or_idle():
             attention_backend = global_server_args_dict["decode_attention_backend"]
+        elif (
+            forward_batch.forward_mode.is_target_verify()
+            or forward_batch.forward_mode.is_draft_extend()
+        ):
+            # Use the specified backend for speculative operations (both verify and draft extend)
+            if global_server_args_dict["speculative_attention_backend"] == "decode":
+                attention_backend = global_server_args_dict["decode_attention_backend"]
+            else:  # default to prefill
+                attention_backend = global_server_args_dict["prefill_attention_backend"]
         else:
             attention_backend = global_server_args_dict["prefill_attention_backend"]
         self.current_attention_backend = attention_backend

python/sglang/srt/server_args.py

@@ -262,6 +262,7 @@ class ServerArgs:
     speculative_accept_threshold_single: float = 1.0
     speculative_accept_threshold_acc: float = 1.0
     speculative_token_map: Optional[str] = None
+    speculative_attention_backend: str = "prefill"
 
     # Expert parallelism
     ep_size: int = 1
@@ -1561,6 +1562,13 @@ class ServerArgs:
             help="The path of the draft model's small vocab table.",
             default=ServerArgs.speculative_token_map,
         )
+        parser.add_argument(
+            "--speculative-attention-backend",
+            type=str,
+            choices=["prefill", "decode"],
+            help="Attention backend to use for speculative decoding operations (both target verify and draft extend). 'prefill' (default) or 'decode'.",
+            default=ServerArgs.speculative_attention_backend,
+        )
 
         # Expert parallelism
         parser.add_argument(

python/sglang/srt/speculative/eagle_worker.py

@@ -191,7 +191,7 @@ class EAGLEWorker(TpModelWorker):
         # Initialize decode attention backend
         self.draft_attn_backend = self._create_decode_backend()
 
-        # Initialize prefill attention backend
+        # Initialize draft extend attention backend (respects speculative_attention_backend setting)
         self.draft_extend_attn_backend = self._create_draft_extend_backend()
 
         self.draft_model_runner.draft_attn_backend = self.draft_attn_backend
@@ -234,11 +234,15 @@ class EAGLEWorker(TpModelWorker):
             "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_backend == "decode"
+            else "prefill_attention_backend"
+        )
         return self._create_backend(
-            "prefill_attention_backend",
+            backend_name,
             backend_map,
-            "EAGLE is not supported in prefill attention backend {backend_type}",
+            "EAGLE is not supported in attention backend {backend_type}",
         )
 
     def _create_flashinfer_decode_backend(self):