set default attention backend for deterministic inference (#11801)

python/sglang/srt/server_args.py

@@ -44,6 +44,7 @@ from sglang.srt.utils import (
     is_remote_url,
     is_sm90_supported,
     is_sm100_supported,
+    is_sm120_supported,
     is_triton_kernels_available,
     is_valid_ipv6_address,
     json_list_type,
@@ -1411,9 +1412,23 @@ class ServerArgs:
             )
 
             # Check attention backend
-            if self.attention_backend not in DETERMINISTIC_ATTENTION_BACKEND_CHOICES:
+            if self.attention_backend is None:
+                # User didn't specify attention backend, fallback based on GPU architecture
+                if is_sm100_supported() or is_sm120_supported():
+                    # Blackwell and newer architectures
+                    self.attention_backend = "flashinfer"
+                else:
+                    # Hopper (SM90) and older architectures
+                    self.attention_backend = "fa3"
+                logger.warning(
+                    f"Attention backend not specified. Falling back to '{self.attention_backend}' for deterministic inference. "
+                    f"You can explicitly set --attention-backend to one of {DETERMINISTIC_ATTENTION_BACKEND_CHOICES}."
+                )
+            elif self.attention_backend not in DETERMINISTIC_ATTENTION_BACKEND_CHOICES:
+                # User explicitly specified an incompatible attention backend
                 raise ValueError(
-                    f"Currently only {DETERMINISTIC_ATTENTION_BACKEND_CHOICES} attention backends are supported for deterministic inference."
+                    f"Currently only {DETERMINISTIC_ATTENTION_BACKEND_CHOICES} attention backends are supported for deterministic inference, "
+                    f"but you explicitly specified '{self.attention_backend}'."
                 )
 
             # Currently, only FA3 supports radix cache. Support for other backends is in progress

python/sglang/srt/utils/common.py

@@ -174,6 +174,15 @@ def is_blackwell():
     return torch.cuda.get_device_capability()[0] == 10
 
 
+@lru_cache(maxsize=1)
+def is_sm120_supported(device=None) -> bool:
+    if not is_cuda_alike():
+        return False
+    return (torch.cuda.get_device_capability(device)[0] == 12) and (
+        torch.version.cuda >= "12.8"
+    )
+
+
 @lru_cache(maxsize=1)
 def is_sm100_supported(device=None) -> bool:
     if not is_cuda_alike():