Merge pull request #3033 from matrix-org/erikj/calculate_state_metrics

Add counter metrics for calculating state delta

synapse/storage/events.py

@@ -53,6 +53,22 @@ event_counter = metrics.register_counter(
     "persisted_events_sep", labels=["type", "origin_type", "origin_entity"]
 )
 
+# The number of times we are recalculating the current state
+state_delta_counter = metrics.register_counter(
+    "state_delta",
+)
+# The number of times we are recalculating state when there is only a
+# single forward extremity
+state_delta_single_event_counter = metrics.register_counter(
+    "state_delta_single_event",
+)
+# The number of times we are reculating state when we could have resonably
+# calculated the delta when we calculated the state for an event we were
+# persisting.
+state_delta_reuse_delta_counter = metrics.register_counter(
+    "state_delta_reuse_delta",
+)
+
 
 def encode_json(json_object):
     if USE_FROZEN_DICTS:
@@ -368,7 +384,8 @@ class EventsStore(EventsWorkerStore):
                                 room_id, ev_ctx_rm, latest_event_ids
                             )
 
-                            if new_latest_event_ids == set(latest_event_ids):
+                            latest_event_ids = set(latest_event_ids)
+                            if new_latest_event_ids == latest_event_ids:
                                 # No change in extremities, so no change in state
                                 continue
 
@@ -389,6 +406,26 @@ class EventsStore(EventsWorkerStore):
                                 if all_single_prev_not_state:
                                     continue
 
+                            state_delta_counter.inc()
+                            if len(new_latest_event_ids) == 1:
+                                state_delta_single_event_counter.inc()
+
+                                # This is a fairly handwavey check to see if we could
+                                # have guessed what the delta would have been when
+                                # processing one of these events.
+                                # What we're interested in is if the latest extremities
+                                # were the same when we created the event as they are
+                                # now. When this server creates a new event (as opposed
+                                # to receiving it over federation) it will use the
+                                # forward extremities as the prev_events, so we can
+                                # guess this by looking at the prev_events and checking
+                                # if they match the current forward extremities.
+                                for ev, _ in ev_ctx_rm:
+                                    prev_event_ids = set(e for e, _ in ev.prev_events)
+                                    if latest_event_ids == prev_event_ids:
+                                        state_delta_reuse_delta_counter.inc()
+                                        break
+
                             logger.info(
                                 "Calculating state delta for room %s", room_id,
                             )