[BugFix] Fix ray import error mem cleanup bug (#21381)

Signed-off-by: Travis Johnson <tsjohnso@us.ibm.com>
Signed-off-by: Joe Runde <Joseph.Runde@ibm.com>
Co-authored-by: Travis Johnson <tsjohnso@us.ibm.com>

Author: joerunde

vllm/config.py

@@ -2137,10 +2137,11 @@ def __post_init__(self) -> None:
             elif (current_platform.is_cuda()
                   and cuda_device_count_stateless() < self.world_size):
                 if not ray_found:
-                    raise ValueError("Unable to load Ray which is "
+                    raise ValueError("Unable to load Ray: "
+                                     f"{ray_utils.ray_import_err}. Ray is "
                                      "required for multi-node inference, "
                                      "please install Ray with `pip install "
-                                     "ray`.") from ray_utils.ray_import_err
+                                     "ray`.")
                 backend = "ray"
             elif self.data_parallel_backend == "ray":
                 logger.info("Using ray distributed inference because "

vllm/executor/ray_utils.py

@@ -145,7 +145,9 @@ def override_env_vars(self, vars: Dict[str, str]):
 
 except ImportError as e:
     ray = None  # type: ignore
-    ray_import_err = e
+    # only capture string to avoid variable references in the traceback that can
+    # prevent garbage collection in some cases
+    ray_import_err = str(e)
     RayWorkerWrapper = None  # type: ignore
 
 
@@ -157,8 +159,8 @@ def ray_is_available() -> bool:
 def assert_ray_available():
     """Raise an exception if Ray is not available."""
     if ray is None:
-        raise ValueError("Failed to import Ray, please install Ray with "
-                         "`pip install ray`.") from ray_import_err
+        raise ValueError(f"Failed to import Ray: {ray_import_err}."
+                         "Please install Ray with `pip install ray`.")
 
 
 def _verify_bundles(placement_group: "PlacementGroup",