From 1cb84aaab5231532105f5a12f01387c9cca27a6a Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@element.io>
Date: Thu, 12 Sep 2024 22:36:16 +0100
Subject: [PATCH] Sliding Sync: Increase concurrency of sliding sync a bit
 (#17696)

For initial requests a typical page size is 20 rooms, so we may as well
do the batching as 20.

This should speed up bigger syncs a little bit.
---
 changelog.d/17696.misc                      |  1 +
 synapse/handlers/sliding_sync/__init__.py   |  2 +-
 synapse/handlers/sliding_sync/extensions.py | 10 +++++++++-
 3 files changed, 11 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/17696.misc

diff --git a/changelog.d/17696.misc b/changelog.d/17696.misc
new file mode 100644
index 000000000..a2f1b1f39
--- /dev/null
+++ b/changelog.d/17696.misc
@@ -0,0 +1 @@
+Speed up sliding sync requests a bit where there are many room changes.
diff --git a/synapse/handlers/sliding_sync/__init__.py b/synapse/handlers/sliding_sync/__init__.py
index 04493494a..c3b5bbbf6 100644
--- a/synapse/handlers/sliding_sync/__init__.py
+++ b/synapse/handlers/sliding_sync/__init__.py
@@ -267,7 +267,7 @@ class SlidingSyncHandler:
 
         if relevant_rooms_to_send_map:
             with start_active_span("sliding_sync.generate_room_entries"):
-                await concurrently_execute(handle_room, relevant_rooms_to_send_map, 10)
+                await concurrently_execute(handle_room, relevant_rooms_to_send_map, 20)
 
         extensions = await self.extensions.get_extensions_response(
             sync_config=sync_config,
diff --git a/synapse/handlers/sliding_sync/extensions.py b/synapse/handlers/sliding_sync/extensions.py
index 6f37cc346..7c2f8a256 100644
--- a/synapse/handlers/sliding_sync/extensions.py
+++ b/synapse/handlers/sliding_sync/extensions.py
@@ -38,6 +38,7 @@ from synapse.types.handlers.sliding_sync import (
     SlidingSyncConfig,
     SlidingSyncResult,
 )
+from synapse.util.async_helpers import concurrently_execute
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -534,7 +535,10 @@ class SlidingSyncExtensionHandler:
             # For rooms we've previously sent down, but aren't up to date, we
             # need to use the from token from the room status.
             if previously_rooms:
-                for room_id, receipt_token in previously_rooms.items():
+                # Fetch any missing rooms concurrently.
+
+                async def handle_previously_room(room_id: str) -> None:
+                    receipt_token = previously_rooms[room_id]
                     # TODO: Limit the number of receipts we're about to send down
                     # for the room, if its too many we should TODO
                     previously_receipts = (
@@ -546,6 +550,10 @@ class SlidingSyncExtensionHandler:
                     )
                     fetched_receipts.extend(previously_receipts)
 
+                await concurrently_execute(
+                    handle_previously_room, previously_rooms.keys(), 20
+                )
+
             if initial_rooms:
                 # We also always send down receipts for the current user.
                 user_receipts = (