Speed up cache size calculation

Instead of calculating the size of the cache repeatedly, which can take
a long time now that it can use a callback, instead cache the size and
update that on insertion and deletion.

This requires changing the cache descriptors to have two caches, one for
pending deferreds and the other for the actual values. There's no reason
to evict from the pending deferreds as they won't take up any more
memory.

1 files changed, 12 insertions, 3 deletions

diff --git a/synapse/util/caches/expiringcache.py b/synapse/util/caches/expiringcache.py
index b9ead9cbd5..2987c38a2d 100644
--- a/synapse/util/caches/expiringcache.py
+++ b/synapse/util/caches/expiringcache.py
@@ -56,6 +56,8 @@ class ExpiringCache(object):
 
         self.iterable = iterable
 
+        self._size_estimate = 0
+
     def start(self):
         if not self._expiry_ms:
             # Don't bother starting the loop if things never expire
@@ -70,9 +72,14 @@ class ExpiringCache(object):
         now = self._clock.time_msec()
         self._cache[key] = _CacheEntry(now, value)
 
+        if self.iterable:
+            self._size_estimate += len(value)
+
         # Evict if there are now too many items
         while self._max_len and len(self) > self._max_len:
-            self._cache.popitem(last=False)
+            _key, value = self._cache.popitem(last=False)
+            if self.iterable:
+                self._size_estimate -= len(value.value)
 
     def __getitem__(self, key):
         try:
@@ -109,7 +116,9 @@ 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",
@@ -118,7 +127,7 @@ class ExpiringCache(object):
 
     def __len__(self):
         if self.iterable:
-            return sum(len(value.value) for value in self._cache.itervalues())
+            return self._size_estimate
         else:
             return len(self._cache)