diff --git a/changelog.d/6126.feature b/changelog.d/6126.feature
new file mode 100644
index 0000000000..1207ba6206
--- /dev/null
+++ b/changelog.d/6126.feature
@@ -0,0 +1 @@
+Group events into larger federation transactions at times of high traffic.
diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index d46f4aaeb1..497485fac2 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -152,9 +152,24 @@ class FederationSender(object):
@defer.inlineCallbacks
def _process_event_queue_loop(self):
+ loop_start_time = self.clock.time_msec()
try:
self._is_processing = True
while True:
+ # if we've been going around this loop for a long time without
+ # catching up, deprioritise transaction transmission. This should mean
+ # that events get batched into fewer transactions, which is more
+ # efficient, and hence give us a chance to catch up
+ if (
+ self.clock.time_msec() - loop_start_time > 60 * 1000
+ and not self._transaction_manager.deprioritise_transmission
+ ):
+ logger.warning(
+ "Event processing loop is getting behind: deprioritising "
+ "transaction transmission"
+ )
+ self._transaction_manager.deprioritise_transmission = True
+
last_token = yield self.store.get_federation_out_pos("events")
next_token, events = yield self.store.get_all_new_events_stream(
last_token, self._last_poked_id, limit=100
@@ -251,6 +266,9 @@ class FederationSender(object):
finally:
self._is_processing = False
+ if self._transaction_manager.deprioritise_transmission:
+ logger.info("Event queue caught up: re-prioritising transmission")
+ self._transaction_manager.deprioritise_transmission = False
def _send_pdu(self, pdu, destinations):
# We loop through all destinations to see whether we already have
diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index fad980b893..b890aaf840 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -189,6 +189,11 @@ class PerDestinationQueue(object):
pending_pdus = []
while True:
+ if self._transaction_manager.deprioritise_transmission:
+ # if the event-processing loop has got behind, sleep to give it
+ # a chance to catch up
+ yield self._clock.sleep(2)
+
# We have to keep 2 free slots for presence and rr_edus
limit = MAX_EDUS_PER_TRANSACTION - 2
diff --git a/synapse/federation/sender/transaction_manager.py b/synapse/federation/sender/transaction_manager.py
index 5b6c79c51a..69679dbf65 100644
--- a/synapse/federation/sender/transaction_manager.py
+++ b/synapse/federation/sender/transaction_manager.py
@@ -49,6 +49,10 @@ class TransactionManager(object):
# HACK to get unique tx id
self._next_txn_id = int(self.clock.time_msec())
+ # the federation sender sometimes sets this to delay transaction transmission,
+ # if the sender gets behind.
+ self.deprioritise_transmission = False
+
@measure_func("_send_new_transaction")
@defer.inlineCallbacks
def send_new_transaction(self, destination, pending_pdus, pending_edus):
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