Speed up processing of federation stream RDATA rows.

changelog.d/7584.misc

@@ -0,0 +1 @@
+Speed up processing of federation stream RDATA rows.

synapse/app/generic_worker.py

@@ -863,9 +863,24 @@ async def update_token(self, token):
         a FEDERATION_ACK back to the master, and stores the token that we have processed
          in `federation_stream_position` so that we can restart where we left off.
         """
-        try:
-            self.federation_position = token
+        self.federation_position = token
+
+        # We save and send the ACK to master asynchronously, so we don't block
+        # processing on persistence. We don't need to do this operation for
+        # every single RDATA we receive, we just need to do it periodically.
+
+        if self._fed_position_linearizer.is_queued(None):
+            # There is already a task queued up to save and send the token, so
+            # no need to queue up another task.
+            return
 
+        run_as_background_process("_save_and_send_ack", self._save_and_send_ack)
+
+    async def _save_and_send_ack(self):
+        """Save the current federation position in the database and send an ACK
+        to master with where we're up to.
+        """
+        try:
             # We linearize here to ensure we don't have races updating the token
             #
             # XXX this appears to be redundant, since the ReplicationCommandHandler

synapse/replication/tcp/handler.py

@@ -116,6 +116,8 @@ def __init__(self, hs):
         # batching works.
         self._pending_batches = {}  # type: Dict[str, List[Any]]
 
+        self._queued_events = {}  # type: Dict[str, List[Any]]
+
         # The factory used to create connections.
         self._factory = None  # type: Optional[ReconnectingClientFactory]
 

synapse/util/async_helpers.py

@@ -225,6 +225,18 @@ def __init__(self, name=None, max_count=1, clock=None):
             {}
         )  # type: Dict[str, Sequence[Union[int, Dict[defer.Deferred, int]]]]
 
+    def is_queued(self, key) -> bool:
+        """Checks whether there is a process queued up waiting
+        """
+        entry = self.key_to_defer.get(key)
+        if not entry:
+            # No entry so nothing is waiting.
+            return False
+
+        # There are waiting deferreds only in the OrderedDict of deferreds is
+        # non-empty.
+        return bool(entry[1])
+
     def queue(self, key):
         # we avoid doing defer.inlineCallbacks here, so that cancellation works correctly.
         # (https://twistedmatrix.com/trac/ticket/4632 meant that cancellations were not