[Perf] Freeze core engine proc heap after init (#24008)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/v1/engine/core.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import gc
 import os
 import queue
 import signal
@@ -536,6 +537,11 @@ class EngineCoreProc(EngineCore):
         self.step_fn = (self.step if self.batch_queue is None else
                         self.step_with_batch_queue)
 
+        # Mark the startup heap as static so that it's ignored by GC.
+        # Reduces pause times of oldest generation collections.
+        gc.collect()
+        gc.freeze()
+
     @contextmanager
     def _perform_handshakes(
         self,