Optimise notifier mk2 (#17766)

Based on #17765.

Basically the idea is to reduce the overhead of calling
`ObservableDeferred` in a loop. The two gains are: a) just using a list
of deferreds rather than the machinery of `ObservableDeferred`, and b)
only calling `PreseverLoggingContext` once.

`PreseverLoggingContext` in particular is expensive to call a lot as
each time it needs to call `get_thread_resource_usage` twice, so that it
an update the CPU metrics of the log context.

changelog.d/17766.misc

@@ -0,0 +1 @@
+Increase performance of the notifier when there are many syncing users.

synapse/notifier.py

@@ -41,6 +41,7 @@
 from prometheus_client import Counter
 
 from twisted.internet import defer
+from twisted.internet.defer import Deferred
 
 from synapse.api.constants import EduTypes, EventTypes, HistoryVisibility, Membership
 from synapse.api.errors import AuthError
@@ -52,6 +53,7 @@
 from synapse.metrics import LaterGauge
 from synapse.streams.config import PaginationConfig
 from synapse.types import (
+    ISynapseReactor,
     JsonDict,
     MultiWriterStreamToken,
     PersistedEventPosition,
@@ -61,7 +63,9 @@
     StreamToken,
     UserID,
 )
-from synapse.util.async_helpers import ObservableDeferred, timeout_deferred
+from synapse.util.async_helpers import (
+    timeout_deferred,
+)
 from synapse.util.metrics import Measure
 from synapse.util.stringutils import shortstr
 from synapse.visibility import filter_events_for_client
@@ -90,18 +94,6 @@ def count(func: Callable[[T], bool], it: Iterable[T]) -> int:
     return n
 
 
-class _NotificationListener:
-    """This represents a single client connection to the events stream.
-    The events stream handler will have yielded to the deferred, so to
-    notify the handler it is sufficient to resolve the deferred.
-    """
-
-    __slots__ = ["deferred"]
-
-    def __init__(self, deferred: "defer.Deferred"):
-        self.deferred = deferred
-
-
 class _NotifierUserStream:
     """This represents a user connected to the event stream.
     It tracks the most recent stream token for that user.
@@ -114,11 +106,13 @@ class _NotifierUserStream:
 
     def __init__(
         self,
+        reactor: ISynapseReactor,
         user_id: str,
         rooms: StrCollection,
         current_token: StreamToken,
         time_now_ms: int,
     ):
+        self.reactor = reactor
         self.user_id = user_id
         self.rooms = set(rooms)
 
@@ -130,28 +124,31 @@ def __init__(
         self.current_token = current_token
         self.last_notified_ms = time_now_ms
 
-        self.notify_deferred: ObservableDeferred[StreamToken] = ObservableDeferred(
-            defer.Deferred()
-        )
+        # Set of listeners that we need to wake up when there has been a change.
+        self.listeners: Set[Deferred[StreamToken]] = set()
 
-    def notify(
+    def update_and_fetch_deferreds(
         self,
         current_token: StreamToken,
         time_now_ms: int,
-    ) -> None:
-        """Notify any listeners for this user of a new event from an
-        event source.
+    ) -> Collection["Deferred[StreamToken]"]:
+        """Update the stream for this user because of a new event from an
+        event source, and return the set of deferreds to wake up.
+
         Args:
             current_token: The new current token.
             time_now_ms: The current time in milliseconds.
+
+        Returns:
+            The set of deferreds that need to be called.
         """
         self.current_token = current_token
         self.last_notified_ms = time_now_ms
-        notify_deferred = self.notify_deferred
 
-        with PreserveLoggingContext():
-            self.notify_deferred = ObservableDeferred(defer.Deferred())
-            notify_deferred.callback(self.current_token)
+        listeners = self.listeners
+        self.listeners = set()
+
+        return listeners
 
     def remove(self, notifier: "Notifier") -> None:
         """Remove this listener from all the indexes in the Notifier
@@ -165,9 +162,9 @@ def remove(self, notifier: "Notifier") -> None:
         notifier.user_to_user_stream.pop(self.user_id)
 
     def count_listeners(self) -> int:
-        return len(self.notify_deferred.observers())
+        return len(self.listeners)
 
-    def new_listener(self, token: StreamToken) -> _NotificationListener:
+    def new_listener(self, token: StreamToken) -> "Deferred[StreamToken]":
         """Returns a deferred that is resolved when there is a new token
         greater than the given token.
 
@@ -177,10 +174,17 @@ def new_listener(self, token: StreamToken) -> _NotificationListener:
         """
         # Immediately wake up stream if something has already since happened
         # since their last token.
-        if self.current_token != token:
-            return _NotificationListener(defer.succeed(self.current_token))
-        else:
-            return _NotificationListener(self.notify_deferred.observe())
+        if token != self.current_token:
+            return defer.succeed(self.current_token)
+
+        # Create a new deferred and add it to the set of listeners. We add a
+        # cancel handler to remove it from the set again, to handle timeouts.
+        deferred: "Deferred[StreamToken]" = Deferred(
+            canceller=lambda d: self.listeners.discard(d)
+        )
+        self.listeners.add(deferred)
+
+        return deferred
 
 
 @attr.s(slots=True, frozen=True, auto_attribs=True)
@@ -233,6 +237,7 @@ def __init__(self, hs: "HomeServer"):
         # List of callbacks to be notified when a lock is released
         self._lock_released_callback: List[Callable[[str, str, str], None]] = []
 
+        self.reactor = hs.get_reactor()
         self.clock = hs.get_clock()
         self.appservice_handler = hs.get_application_service_handler()
         self._pusher_pool = hs.get_pusherpool()
@@ -329,12 +334,20 @@ async def on_un_partial_stated_room(
         user_streams = self.room_to_user_streams.get(room_id, set())
         time_now_ms = self.clock.time_msec()
         current_token = self.event_sources.get_current_token()
+
+        listeners: List["Deferred[StreamToken]"] = []
         for user_stream in user_streams:
             try:
-                user_stream.notify(current_token, time_now_ms)
+                listeners.extend(
+                    user_stream.update_and_fetch_deferreds(current_token, time_now_ms)
+                )
             except Exception:
                 logger.exception("Failed to notify listener")
 
+        with PreserveLoggingContext():
+            for listener in listeners:
+                listener.callback(current_token)
+
         users_woken_by_stream_counter.labels(StreamKeyType.UN_PARTIAL_STATED_ROOMS).inc(
             len(user_streams)
         )
@@ -538,12 +551,24 @@ def on_new_event(
 
             time_now_ms = self.clock.time_msec()
             current_token = self.event_sources.get_current_token()
+            listeners: List["Deferred[StreamToken]"] = []
             for user_stream in user_streams:
                 try:
-                    user_stream.notify(current_token, time_now_ms)
+                    listeners.extend(
+                        user_stream.update_and_fetch_deferreds(
+                            current_token, time_now_ms
+                        )
+                    )
                 except Exception:
                     logger.exception("Failed to notify listener")
 
+            # We resolve all these deferreds in one go so that we only need to
+            # call `PreserveLoggingContext` once, as it has a bunch of overhead
+            # (to calculate performance stats)
+            with PreserveLoggingContext():
+                for listener in listeners:
+                    listener.callback(current_token)
+
             users_woken_by_stream_counter.labels(stream_key).inc(len(user_streams))
 
             self.notify_replication()
@@ -582,6 +607,7 @@ async def wait_for_events(
             if room_ids is None:
                 room_ids = await self.store.get_rooms_for_user(user_id)
             user_stream = _NotifierUserStream(
+                reactor=self.reactor,
                 user_id=user_id,
                 rooms=room_ids,
                 current_token=current_token,
@@ -604,8 +630,8 @@ async def wait_for_events(
                         # Now we wait for the _NotifierUserStream to be told there
                         # is a new token.
                         listener = user_stream.new_listener(prev_token)
-                        listener.deferred = timeout_deferred(
-                            listener.deferred,
+                        listener = timeout_deferred(
+                            listener,
                             (end_time - now) / 1000.0,
                             self.hs.get_reactor(),
                         )
@@ -618,7 +644,7 @@ async def wait_for_events(
                         )
 
                         with PreserveLoggingContext():
-                            await listener.deferred
+                            await listener
 
                         log_kv(
                             {