2 files changed, 89 insertions, 16 deletions
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index d13011d138..a09eaa4379 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -32,6 +32,7 @@ from typing import (
 )
 
 import attr
+from prometheus_client import Histogram
 from signedjson.key import decode_verify_key_bytes
 from signedjson.sign import verify_signed_json
 from unpaddedbase64 import decode_base64
@@ -79,6 +80,24 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+# Added to debug performance and track progress on optimizations
+backfill_processing_before_timer = Histogram(
+    "synapse_federation_backfill_processing_before_time_seconds",
+    "sec",
+    [],
+    buckets=(
+        1.0,
+        5.0,
+        10.0,
+        20.0,
+        30.0,
+        40.0,
+        60.0,
+        80.0,
+        "+Inf",
+    ),
+)
+
 
 def get_domains_from_state(state: StateMap[EventBase]) -> List[Tuple[str, int]]:
     """Get joined domains from state
@@ -138,6 +157,7 @@ class FederationHandler:
     def __init__(self, hs: "HomeServer"):
         self.hs = hs
 
+        self.clock = hs.get_clock()
         self.store = hs.get_datastores().main
         self._storage_controllers = hs.get_storage_controllers()
         self._state_storage_controller = self._storage_controllers.state
@@ -197,12 +217,39 @@ class FederationHandler:
                 return. This is used as part of the heuristic to decide if we
                 should back paginate.
         """
+        # 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()
+
         async with self._room_backfill.queue(room_id):
-            return await self._maybe_backfill_inner(room_id, current_depth, limit)
+            return await self._maybe_backfill_inner(
+                room_id,
+                current_depth,
+                limit,
+                processing_start_time=processing_start_time,
+            )
 
     async def _maybe_backfill_inner(
-        self, room_id: str, current_depth: int, limit: int
+        self,
+        room_id: str,
+        current_depth: int,
+        limit: int,
+        *,
+        processing_start_time: int,
     ) -> bool:
+        """
+        Checks whether the `current_depth` is at or approaching any backfill
+        points in the room and if so, will backfill. We only care about
+        checking backfill points that happened before the `current_depth`
+        (meaning less than or equal to the `current_depth`).
+
+        Args:
+            room_id: The room to backfill in.
+            current_depth: The depth to check at for any upcoming backfill points.
+            limit: The max number of events to request from the remote federated server.
+            processing_start_time: The time when `maybe_backfill` started
+                processing. Only used for timing.
+        """
         backwards_extremities = [
             _BackfillPoint(event_id, depth, _BackfillPointType.BACKWARDS_EXTREMITY)
             for event_id, depth in await self.store.get_oldest_event_ids_with_depth_in_room(
@@ -433,6 +480,11 @@ class FederationHandler:
 
             return False
 
+        processing_end_time = self.clock.time_msec()
+        backfill_processing_before_timer.observe(
+            (processing_start_time - processing_end_time) / 1000
+        )
+
         success = await try_backfill(likely_domains)
         if success:
             return True
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index dd0d610fe9..f40b071a74 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -29,7 +29,7 @@ from typing import (
     Tuple,
 )
 
-from prometheus_client import Counter
+from prometheus_client import Counter, Histogram
 
 from synapse import event_auth
 from synapse.api.constants import (
@@ -98,6 +98,26 @@ soft_failed_event_counter = Counter(
     "Events received over federation that we marked as soft_failed",
 )
 
+# Added to debug performance and track progress on optimizations
+backfill_processing_after_timer = Histogram(
+    "synapse_federation_backfill_processing_after_time_seconds",
+    "sec",
+    [],
+    buckets=(
+        1.0,
+        5.0,
+        10.0,
+        20.0,
+        30.0,
+        40.0,
+        60.0,
+        80.0,
+        120.0,
+        180.0,
+        "+Inf",
+    ),
+)
+
 
 class FederationEventHandler:
     """Handles events that originated from federation.
@@ -604,20 +624,21 @@ class FederationEventHandler:
         if not events:
             return
 
-        # if there are any events in the wrong room, the remote server is buggy and
-        # should not be trusted.
-        for ev in events:
-            if ev.room_id != room_id:
-                raise InvalidResponseError(
-                    f"Remote server {dest} returned event {ev.event_id} which is in "
-                    f"room {ev.room_id}, when we were backfilling in {room_id}"
-                )
+        with backfill_processing_after_timer.time():
+            # if there are any events in the wrong room, the remote server is buggy and
+            # should not be trusted.
+            for ev in events:
+                if ev.room_id != room_id:
+                    raise InvalidResponseError(
+                        f"Remote server {dest} returned event {ev.event_id} which is in "
+                        f"room {ev.room_id}, when we were backfilling in {room_id}"
+                    )
 
-        await self._process_pulled_events(
-            dest,
-            events,
-            backfilled=True,
-        )
+            await self._process_pulled_events(
+                dest,
+                events,
+                backfilled=True,
+            )
 
     @trace
     async def _get_missing_events_for_pdu(