Only lock when we're backfilling (#16159)

changelog.d/16159.misc

@@ -0,0 +1 @@
+Reduce scope of locks when paginating to alleviate DB contention.

synapse/handlers/federation.py

@@ -60,6 +60,7 @@ from synapse.events import EventBase
 from synapse.events.snapshot import EventContext, UnpersistedEventContextBase
 from synapse.events.validator import EventValidator
 from synapse.federation.federation_client import InvalidResponseError
+from synapse.handlers.pagination import PURGE_PAGINATION_LOCK_NAME
 from synapse.http.servlet import assert_params_in_dict
 from synapse.logging.context import nested_logging_context
 from synapse.logging.opentracing import SynapseTags, set_tag, tag_args, trace
@@ -152,6 +153,7 @@ class FederationHandler:
         self._device_handler = hs.get_device_handler()
         self._bulk_push_rule_evaluator = hs.get_bulk_push_rule_evaluator()
         self._notifier = hs.get_notifier()
+        self._worker_locks = hs.get_worker_locks_handler()
 
         self._clean_room_for_join_client = ReplicationCleanRoomRestServlet.make_client(
             hs
@@ -200,7 +202,7 @@ class FederationHandler:
     @trace
     @tag_args
     async def maybe_backfill(
-        self, room_id: str, current_depth: int, limit: int
+        self, room_id: str, current_depth: int, limit: int, record_time: bool = True
     ) -> bool:
         """Checks the database to see if we should backfill before paginating,
         and if so do.
@@ -213,15 +215,19 @@ class FederationHandler:
             limit: The number of events that the pagination request will
                 return. This is used as part of the heuristic to decide if we
                 should back paginate.
+            record_time: Whether to record the time it takes to backfill.
 
         Returns:
             True if we actually tried to backfill something, otherwise False.
         """
         # Starting the processing time here so we can include the room backfill
         # linearizer lock queue in the timing
-        processing_start_time = self.clock.time_msec()
+        processing_start_time = self.clock.time_msec() if record_time else 0
 
         async with self._room_backfill.queue(room_id):
+            async with self._worker_locks.acquire_read_write_lock(
+                PURGE_PAGINATION_LOCK_NAME, room_id, write=False
+            ):
                 return await self._maybe_backfill_inner(
                     room_id,
                     current_depth,
@@ -305,12 +311,21 @@ class FederationHandler:
         # of history that extends all the way back to where we are currently paginating
         # and it's within the 100 events that are returned from `/backfill`.
         if not sorted_backfill_points and current_depth != MAX_DEPTH:
+            # Check that we actually have later backfill points, if not just return.
+            have_later_backfill_points = await self.store.get_backfill_points_in_room(
+                room_id=room_id,
+                current_depth=MAX_DEPTH,
+                limit=1,
+            )
+            if not have_later_backfill_points:
+                return False
+
             logger.debug(
                 "_maybe_backfill_inner: all backfill points are *after* current depth. Trying again with later backfill points."
             )
             run_as_background_process(
                 "_maybe_backfill_inner_anyway_with_max_depth",
-                self._maybe_backfill_inner,
+                self.maybe_backfill,
                 room_id=room_id,
                 # We use `MAX_DEPTH` so that we find all backfill points next
                 # time (all events are below the `MAX_DEPTH`)
@@ -319,7 +334,7 @@ class FederationHandler:
                 # We don't want to start another timing observation from this
                 # nested recursive call. The top-most call can record the time
                 # overall otherwise the smaller one will throw off the results.
-                processing_start_time=None,
+                record_time=False,
             )
             # We return `False` because we're backfilling in the background and there is
             # no new events immediately for the caller to know about yet.

synapse/handlers/pagination.py

@@ -487,9 +487,6 @@ class PaginationHandler:
 
         room_token = from_token.room_key
 
-        async with self._worker_locks.acquire_read_write_lock(
-            PURGE_PAGINATION_LOCK_NAME, room_id, write=False
-        ):
         (membership, member_event_id) = (None, None)
         if not use_admin_priviledge:
             (
@@ -604,13 +601,11 @@ class PaginationHandler:
                 or missing_too_many_events
                 or not_enough_events_to_fill_response
             ):
-                    did_backfill = (
-                        await self.hs.get_federation_handler().maybe_backfill(
+                did_backfill = await self.hs.get_federation_handler().maybe_backfill(
                     room_id,
                     curr_topo,
                     limit=pagin_config.limit,
                 )
-                    )
 
                 # If we did backfill something, refetch the events from the database to
                 # catch anything new that might have been added since we last fetched.