1 files changed, 26 insertions, 15 deletions
diff --git a/synapse/util/caches/expiringcache.py b/synapse/util/caches/expiringcache.py
index 080388958f..2987c38a2d 100644
--- a/synapse/util/caches/expiringcache.py
+++ b/synapse/util/caches/expiringcache.py
@@ -15,6 +15,7 @@
 
 from synapse.util.caches import register_cache
 
+from collections import OrderedDict
 import logging
 
 
@@ -23,7 +24,7 @@ logger = logging.getLogger(__name__)
 
 class ExpiringCache(object):
     def __init__(self, cache_name, clock, max_len=0, expiry_ms=0,
-                 reset_expiry_on_get=False):
+                 reset_expiry_on_get=False, iterable=False):
         """
         Args:
             cache_name (str): Name of this cache, used for logging.
@@ -36,6 +37,8 @@ class ExpiringCache(object):
                 evicted based on time.
             reset_expiry_on_get (bool): If true, will reset the expiry time for
                 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
@@ -47,9 +50,13 @@ class ExpiringCache(object):
 
         self._reset_expiry_on_get = reset_expiry_on_get
 
-        self._cache = {}
+        self._cache = OrderedDict()
 
-        self.metrics = register_cache(cache_name, self._cache)
+        self.metrics = register_cache(cache_name, self)
+
+        self.iterable = iterable
+
+        self._size_estimate = 0
 
     def start(self):
         if not self._expiry_ms:
@@ -65,15 +72,14 @@ class ExpiringCache(object):
         now = self._clock.time_msec()
         self._cache[key] = _CacheEntry(now, value)
 
-        # Evict if there are now too many items
-        if self._max_len and len(self._cache.keys()) > self._max_len:
-            sorted_entries = sorted(
-                self._cache.items(),
-                key=lambda item: item[1].time,
-            )
+        if self.iterable:
+            self._size_estimate += len(value)
 
-            for k, _ in sorted_entries[self._max_len:]:
-                self._cache.pop(k)
+        # Evict if there are now too many items
+        while self._max_len and len(self) > self._max_len:
+            _key, value = self._cache.popitem(last=False)
+            if self.iterable:
+                self._size_estimate -= len(value.value)
 
     def __getitem__(self, key):
         try:
@@ -99,7 +105,7 @@ class ExpiringCache(object):
             # zero expiry time means don't expire. This should never get called
             # since we have this check in start too.
             return
-        begin_length = len(self._cache)
+        begin_length = len(self)
 
         now = self._clock.time_msec()
 
@@ -110,15 +116,20 @@ class ExpiringCache(object):
                 keys_to_delete.add(key)
 
         for k in keys_to_delete:
-            self._cache.pop(k)
+            value = self._cache.pop(k)
+            if self.iterable:
+                self._size_estimate -= len(value.value)
 
         logger.debug(
             "[%s] _prune_cache before: %d, after len: %d",
-            self._cache_name, begin_length, len(self._cache)
+            self._cache_name, begin_length, len(self)
         )
 
     def __len__(self):
-        return len(self._cache)
+        if self.iterable:
+            return self._size_estimate
+        else:
+            return len(self._cache)
 
 
 class _CacheEntry(object):