2 files changed, 6 insertions, 7 deletions
diff --git a/synapse/util/caches/expiringcache.py b/synapse/util/caches/expiringcache.py
index 4abca91f6d..ce85b2ae11 100644
--- a/synapse/util/caches/expiringcache.py
+++ b/synapse/util/caches/expiringcache.py
@@ -16,6 +16,7 @@
 import logging
 from collections import OrderedDict
 
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.util.caches import register_cache
 
 logger = logging.getLogger(__name__)
@@ -63,7 +64,10 @@ class ExpiringCache(object):
             return
 
         def f():
-            self._prune_cache()
+            return run_as_background_process(
+                "prune_cache_%s" % self._cache_name,
+                self._prune_cache,
+            )
 
         self._clock.looping_call(f, self._expiry_ms / 2)
 
diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index a1f8ff8f10..f2bde74dc5 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -80,12 +80,7 @@ class StreamChangeCache(object):
                 )
             }
 
-            # 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
-            }
+            result = changed_entities.intersection(entities)
 
             self.metrics.inc_hits()
         else: