Decrease GC tick frequency; only create instance IDs if we'll use them

Both _maybe_gc and random_string_insecure_fast feature in CPU profiles.

synapse/logging/context.py

@@ -693,7 +693,11 @@ class PreserveLoggingContext:
         self, new_context: LoggingContextOrSentinel = SENTINEL_CONTEXT
     ) -> None:
         self._new_context = new_context
-        self._instance_id = random_string_insecure_fast(5)
+        self._instance_id = (
+            random_string_insecure_fast(5)
+            if logcontext_debug_logger.isEnabledFor(logging.DEBUG)
+            else ""
+        )
 
     def __enter__(self) -> None:
         logcontext_debug_logger.debug(
@@ -889,7 +893,11 @@ def run_in_background(
         Note that the returned Deferred does not follow the synapse logcontext
         rules.
     """
-    instance_id = random_string_insecure_fast(5)
+    instance_id = (
+        random_string_insecure_fast(5)
+        if logcontext_debug_logger.isEnabledFor(logging.DEBUG)
+        else ""
+    )
     calling_context = current_context()
     logcontext_debug_logger.debug(
         "run_in_background(%s): called with logcontext=%s", instance_id, calling_context
@@ -1052,7 +1060,11 @@ def make_deferred_yieldable(deferred: "defer.Deferred[T]") -> "defer.Deferred[T]
     restores the old context once the awaitable completes (execution passes from the
     reactor back to the code).
     """
-    instance_id = random_string_insecure_fast(5)
+    instance_id = (
+        random_string_insecure_fast(5)
+        if logcontext_debug_logger.isEnabledFor(logging.DEBUG)
+        else ""
+    )
     logcontext_debug_logger.debug(
         "make_deferred_yieldable(%s): called with logcontext=%s",
         instance_id,

synapse/metrics/_gc.py

@@ -141,7 +141,8 @@ def install_gc_manager() -> None:
     # We can ignore the lint here since this looping call does not hold a `HomeServer`
     # reference so can be cleaned up by other means on shutdown.
     gc_task = task.LoopingCall(_maybe_gc)  # type: ignore[prefer-synapse-clock-looping-call]
-    gc_task.start(0.1)
+    # tick every second. Previously this was every 100ms. Increased to reduce CPU time spent inside _maybe_gc.
+    gc_task.start(1.0)
 
 
 #