Add Speculative Decoding Eagle3 topk > 1 (#5318)

Co-authored-by: Stefan He <hebiaobuaa@gmail.com>
Co-authored-by: Yubo Wang <yubowang2019@gmail.com>

python/sglang/srt/model_executor/model_runner.py

@@ -221,7 +221,16 @@ class ModelRunner:
         server_args = self.server_args
 
         if server_args.attention_backend is None:
-            # By default, use flashinfer for non-mla attention and triton for mla attention
+            """
+            We auto select the fastest attention backend according to the current offering
+            1. Models with MHA Architecture (e.g: Llama, QWen)
+                1.1 We will turn on FA3 on hopper unless user use spec decode with topk > 1 or page_size > 1.
+                1.2 In other cases, we will use flashinfer if available, otherwise use triton.
+            2. Models with MLA Architecture and using FA3
+                2.1 We will use FA3 backend on hopper.
+                2.2 Otherwise, we will use triton backend.
+            """
+
             if not self.use_mla_backend:
                 if (
                     is_hopper_with_cuda_12_3()
@@ -234,9 +243,7 @@ class ModelRunner:
                         "flashinfer" if is_flashinfer_available() else "triton"
                     )
             else:
-                if is_hopper_with_cuda_12_3() and is_no_spec_infer_or_topk_one(
-                    server_args
-                ):
+                if is_hopper_with_cuda_12_3():
                     server_args.attention_backend = "fa3"
                 else:
                     server_args.attention_backend = "triton"

python/sglang/srt/server_args.py

@@ -359,7 +359,18 @@ class ServerArgs:
 
             if self.page_size > 1 and self.speculative_eagle_topk > 1:
                 self.speculative_eagle_topk = 1
-                logger.info("speculative_eagle_topk is changed to 1 when page_size > 1")
+                logger.info(
+                    "speculative_eagle_topk is adjusted to 1 when page_size > 1"
+                )
+
+            if (
+                self.speculative_eagle_topk == 1
+                and self.speculative_num_draft_tokens != self.speculative_num_steps + 1
+            ):
+                logger.info(
+                    "speculative_num_draft_tokens is adjusted to speculative_num_steps + 1 when speculative_eagle_topk == 1"
+                )
+                self.speculative_num_draft_tokens = self.speculative_num_steps + 1
 
             # The token generated from the verify step is counted.
             # If sepculative_num_steps >= speculative_num_draft_tokens, the additional tokens will definitely be discarded.

python/sglang/srt/utils.py

@@ -1909,6 +1909,8 @@ def is_page_size_one(server_args):
     return server_args.page_size == 1
 
 
+# TODO(hebiao064): Accelerate FA3 Spec Decode with topk > 1.
+# TODO(hebiao064): Improve the acc rate for FA3 Spec Decode with topk == 1 and page_size > 1.
 def is_no_spec_infer_or_topk_one(server_args):
     return server_args.speculative_eagle_topk is None or (
         server_args.speculative_eagle_topk is not None