1 files changed, 25 insertions, 4 deletions
diff --git a/synapse/util/caches/expiringcache.py b/synapse/util/caches/expiringcache.py
index cddf1ed515..2726b67b6d 100644
--- a/synapse/util/caches/expiringcache.py
+++ b/synapse/util/caches/expiringcache.py
@@ -18,6 +18,7 @@ from collections import OrderedDict
 
 from six import iteritems, itervalues
 
+from synapse.config import cache as cache_config
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.util.caches import register_cache
 
@@ -51,15 +52,16 @@ class ExpiringCache(object):
                 an item on access. Defaults to False.
             iterable (bool): If true, the size is calculated by summing the
                 sizes of all entries, rather than the number of entries.
-
         """
         self._cache_name = cache_name
 
+        self._original_max_size = max_len
+
+        self._max_size = int(max_len * cache_config.properties.default_factor_size)
+
         self._clock = clock
 
-        self._max_len = max_len
         self._expiry_ms = expiry_ms
-
         self._reset_expiry_on_get = reset_expiry_on_get
 
         self._cache = OrderedDict()
@@ -82,9 +84,11 @@ class ExpiringCache(object):
     def __setitem__(self, key, value):
         now = self._clock.time_msec()
         self._cache[key] = _CacheEntry(now, value)
+        self.evict()
 
+    def evict(self):
         # Evict if there are now too many items
-        while self._max_len and len(self) > self._max_len:
+        while self._max_size and len(self) > self._max_size:
             _key, value = self._cache.popitem(last=False)
             if self.iterable:
                 self.metrics.inc_evictions(len(value.value))
@@ -170,6 +174,23 @@ class ExpiringCache(object):
         else:
             return len(self._cache)
 
+    def set_cache_factor(self, factor: float) -> bool:
+        """
+        Set the cache factor for this individual cache.
+
+        This will trigger a resize if it changes, which may require evicting
+        items from the cache.
+
+        Returns:
+            bool: Whether the cache changed size or not.
+        """
+        new_size = int(self._original_max_size * factor)
+        if new_size != self._max_size:
+            self._max_size = new_size
+            self.evict()
+            return True
+        return False
+
 
 class _CacheEntry(object):
     __slots__ = ["time", "value"]