From f03ea34a3d35f540039bc7167c5759f62f0d8167 Mon Sep 17 00:00:00 2001
From: Yuhao Yang <47235274+yhyang201@users.noreply.github.com>
Date: Sat, 29 Nov 2025 23:05:54 +0800
Subject: [PATCH] add runtime check for PyTorch 2.9.1 + CuDNN < 9.15 to prevent
 Conv3d performance issues (#14119)

---
 .github/workflows/pr-test.yml    |  2 +-
 python/sglang/srt/server_args.py | 45 ++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/pr-test.yml b/.github/workflows/pr-test.yml
index 148f30937..a8907ab97 100644
--- a/.github/workflows/pr-test.yml
+++ b/.github/workflows/pr-test.yml
@@ -453,8 +453,8 @@ jobs:
 
         - name: Install dependencies
           run: |
-            CUSTOM_BUILD_SGL_KERNEL=${{needs.check-changes.outputs.sgl_kernel}} bash scripts/ci/ci_install_dependency.sh
             pip install "bitsandbytes>=0.44.0"
+            CUSTOM_BUILD_SGL_KERNEL=${{needs.check-changes.outputs.sgl_kernel}} bash scripts/ci/ci_install_dependency.sh
         - name: Run test
           timeout-minutes: 30
           run: |
diff --git a/python/sglang/srt/server_args.py b/python/sglang/srt/server_args.py
index abe466632..7a718795a 100644
--- a/python/sglang/srt/server_args.py
+++ b/python/sglang/srt/server_args.py
@@ -3979,6 +3979,11 @@ class ServerArgs:
         # Check LoRA
         self.check_lora_server_args()
 
+        # torch 2.9.1 has compatibility issues with cuDNN 9.14 and below,
+        # causing extremely slow nn.Conv3d performance.
+        # TODO(yhyang201): Remove this check when sglang no longer uses torch 2.9.1.
+        self.check_torch_2_9_1_cudnn_compatibility()
+
         # Check speculative decoding
         if self.speculative_algorithm is not None:
             assert (
@@ -4054,6 +4059,46 @@ class ServerArgs:
         if self.model_impl == "mindspore":
             assert is_npu(), "MindSpore model impl is only supported on Ascend npu."
 
+    def check_torch_2_9_1_cudnn_compatibility(self):
+        if self.get_model_config().is_multimodal:
+            import torch
+
+            torch_version = torch.__version__.split("+", 1)[0]
+            if torch_version == "2.9.1":
+                cudnn_version = None
+                try:
+                    cudnn_version = torch.backends.cudnn.version()
+                except Exception:
+                    cudnn_version = None
+                if cudnn_version is not None:
+                    version_float = float(str(cudnn_version)[:3]) / 100
+                    if version_float < 9.15:
+                        RED = "\033[91m"
+                        BOLD = "\033[1m"
+                        RESET = "\033[0m"
+                        msg = (
+                            f"{RED}{BOLD}"
+                            "CRITICAL WARNING: PyTorch 2.9.1 & CuDNN Compatibility Issue Detected\n"
+                            "--------------------------------------------------------------------------------\n"
+                            f"Current Environment: PyTorch {torch.__version__} | CuDNN {version_float:.2f}\n\n"
+                            "Issue:     There is a KNOWN BUG in PyTorch 2.9.1's `nn.Conv3d` implementation\n"
+                            "           when used with CuDNN versions older than 9.15. This can cause\n"
+                            "           SEVERE PERFORMANCE DEGRADATION and EXCESSIVE MEMORY USAGE.\n\n"
+                            "Reference: https://github.com/pytorch/pytorch/issues/168167\n\n"
+                            "Solution:  You MUST upgrade CuDNN to version 9.15+ to ensure correctness.\n\n"
+                            "Run the following command immediately to fix:\n"
+                            "    pip install nvidia-cudnn-cu12==9.16.0.29\n"
+                            "--------------------------------------------------------------------------------\n"
+                            f"{RESET}"
+                        )
+                        raise RuntimeError(msg)
+                else:
+                    RED = "\033[91m"
+                    RESET = "\033[0m"
+                    logger.warning(
+                        f"{RED}WARNING: Could not determine CuDNN version for torch==2.9.1. Please ensure CuDNN >= 9.15 to avoid nn.Conv3d bugs.{RESET}"
+                    )
+
     def check_lora_server_args(self):
         assert self.max_loras_per_batch > 0, "max_loras_per_batch must be positive"