Freeze allocated objects on startup. (#6953)

* commit 'fc87d2ffb':
  Freeze allocated objects on startup. (#6953)

Author: anoadragon453

changelog.d/6953.misc

@@ -0,0 +1 @@
+Reduce time spent doing GC by freezing objects on startup.

synapse/app/_base.py

@@ -279,6 +279,15 @@ def handle_sighup(*args, **kwargs):
 
         setup_sentry(hs)
         setup_sdnotify(hs)
+
+        # We now freeze all allocated objects in the hopes that (almost)
+        # everything currently allocated are things that will be used for the
+        # rest of time. Doing so means less work each GC (hopefully).
+        #
+        # This only works on Python 3.7
+        if sys.version_info >= (3, 7):
+            gc.collect()
+            gc.freeze()
     except Exception:
         traceback.print_exc(file=sys.stderr)
         reactor = hs.get_reactor()