From fa5c2bc082185580e7d65a8759a8b5213933a137 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 12 Jul 2018 11:37:44 +0100
Subject: [PATCH 1/2] Reduce set building in get_entities_changed

This line shows up as about 5% of cpu time on a synchrotron:

    not_known_entities = set(entities) - set(self._entity_to_key)

Presumably the problem here is that _entity_to_key can be largeish, and
building a set for its keys every time this function is called is slow.

Here we rewrite the logic to avoid building so many sets.
---
 synapse/util/caches/stream_change_cache.py | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index 8637867c6..a1f8ff8f1 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -74,14 +74,18 @@ class StreamChangeCache(object):
         assert type(stream_pos) is int
 
         if stream_pos >= self._earliest_known_stream_pos:
-            not_known_entities = set(entities) - set(self._entity_to_key)
+            changed_entities = {
+                self._cache[k] for k in self._cache.islice(
+                    start=self._cache.bisect_right(stream_pos),
+                )
+            }
 
-            result = (
-                {self._cache[k] for k in self._cache.islice(
-                    start=self._cache.bisect_right(stream_pos))}
-                .intersection(entities)
-                .union(not_known_entities)
-            )
+            # we need to include entities which we don't know about, as well as
+            # those which are known to have changed since the stream pos.
+            result = {
+                e for e in entities
+                if e in changed_entities or e not in self._entity_to_key
+            }
 
             self.metrics.inc_hits()
         else:

From 38c5fa7ee410a5415c72054d8dfd6d10165af50a Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 12 Jul 2018 11:45:28 +0100
Subject: [PATCH 2/2] changelog

---
 changelog.d/3521.feature | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/3521.feature

diff --git a/changelog.d/3521.feature b/changelog.d/3521.feature
new file mode 100644
index 000000000..6dced5f2a
--- /dev/null
+++ b/changelog.d/3521.feature
@@ -0,0 +1 @@
+Cache optimisation for /sync requests
\ No newline at end of file