Merge pull request #4991 from matrix-org/erikj/stagger_push_startup

Make starting pushers faster during start up

changelog.d/4991.feature

@@ -0,0 +1 @@
+Reduce CPU usage starting pushers during start up.

synapse/push/emailpusher.py

@@ -72,8 +72,15 @@ class EmailPusher(object):
 
         self._is_processing = False
 
-    def on_started(self):
-        if self.mailer is not None:
+    def on_started(self, should_check_for_notifs):
+        """Called when this pusher has been started.
+
+        Args:
+            should_check_for_notifs (bool): Whether we should immediately
+                check for push to send. Set to False only if it's known there
+                is nothing to send
+        """
+        if should_check_for_notifs and self.mailer is not None:
             self._start_processing()
 
     def on_stop(self):

synapse/push/httppusher.py

@@ -112,7 +112,15 @@ class HttpPusher(object):
         self.data_minus_url.update(self.data)
         del self.data_minus_url['url']
 
-    def on_started(self):
+    def on_started(self, should_check_for_notifs):
+        """Called when this pusher has been started.
+
+        Args:
+            should_check_for_notifs (bool): Whether we should immediately
+                check for push to send. Set to False only if it's known there
+                is nothing to send
+        """
+        if should_check_for_notifs:
             self._start_processing()
 
     def on_new_notifications(self, min_stream_ordering, max_stream_ordering):

synapse/push/pusherpool.py

@@ -21,6 +21,7 @@ from twisted.internet import defer
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.push import PusherConfigException
 from synapse.push.pusher import PusherFactory
+from synapse.util.async_helpers import concurrently_execute
 
 logger = logging.getLogger(__name__)
 
@@ -197,7 +198,7 @@ class PusherPool:
                 p = r
 
         if p:
-            self._start_pusher(p)
+            yield self._start_pusher(p)
 
     @defer.inlineCallbacks
     def _start_pushers(self):
@@ -208,10 +209,14 @@ class PusherPool:
         """
         pushers = yield self.store.get_all_pushers()
         logger.info("Starting %d pushers", len(pushers))
-        for pusherdict in pushers:
-            self._start_pusher(pusherdict)
+
+        # Stagger starting up the pushers so we don't completely drown the
+        # process on start up.
+        yield concurrently_execute(self._start_pusher, pushers, 10)
+
         logger.info("Started pushers")
 
+    @defer.inlineCallbacks
     def _start_pusher(self, pusherdict):
         """Start the given pusher
 
@@ -248,7 +253,22 @@ class PusherPool:
         if appid_pushkey in byuser:
             byuser[appid_pushkey].on_stop()
         byuser[appid_pushkey] = p
-        p.on_started()
+
+        # Check if there *may* be push to process. We do this as this check is a
+        # lot cheaper to do than actually fetching the exact rows we need to
+        # push.
+        user_id = pusherdict["user_name"]
+        last_stream_ordering = pusherdict["last_stream_ordering"]
+        if last_stream_ordering:
+            have_notifs = yield self.store.get_if_maybe_push_in_range_for_user(
+                user_id, last_stream_ordering,
+            )
+        else:
+            # We always want to default to starting up the pusher rather than
+            # risk missing push.
+            have_notifs = True
+
+        p.on_started(have_notifs)
 
     @defer.inlineCallbacks
     def remove_pusher(self, app_id, pushkey, user_id):

synapse/storage/event_push_actions.py

@@ -386,6 +386,36 @@ class EventPushActionsWorkerStore(SQLBaseStore):
         # Now return the first `limit`
         defer.returnValue(notifs[:limit])
 
+    def get_if_maybe_push_in_range_for_user(self, user_id, min_stream_ordering):
+        """A fast check to see if there might be something to push for the
+        user since the given stream ordering. May return false positives.
+
+        Useful to know whether to bother starting a pusher on start up or not.
+
+        Args:
+            user_id (str)
+            min_stream_ordering (int)
+
+        Returns:
+            Deferred[bool]: True if there may be push to process, False if
+            there definitely isn't.
+        """
+
+        def _get_if_maybe_push_in_range_for_user_txn(txn):
+            sql = """
+                SELECT 1 FROM event_push_actions
+                WHERE user_id = ? AND stream_ordering > ?
+                LIMIT 1
+            """
+
+            txn.execute(sql, (user_id, min_stream_ordering,))
+            return bool(txn.fetchone())
+
+        return self.runInteraction(
+            "get_if_maybe_push_in_range_for_user",
+            _get_if_maybe_push_in_range_for_user_txn,
+        )
+
     def add_push_actions_to_staging(self, event_id, user_id_actions):
         """Add the push actions for the event to the push action staging area.