Reduce cache size by not storing deferreds

Currently the cache descriptors store deferreds rather than raw values,
this is a simple way of triggering only one database hit and sharing the
result if two callers attempt to get the same value.

However, there are a few caches that simply store a mapping from string
to string (or int). These caches can have a large number of entries,
under the assumption that each entry is small. However, the size of a
deferred (specifically the size of ObservableDeferred) is signigicantly
larger than that of the raw value, 2kb vs 32b.

This PR therefore changes the cache descriptors to store the raw values
rather than the deferreds.

As a side effect cached storage function now either return a deferred or
the actual value, as the cached list decriptor already does. This is
fine as we always end up just yield'ing on the returned value
eventually, which handles that case correctly.

synapse/storage/receipts.py

@@ -47,7 +47,10 @@ class ReceiptsStore(SQLBaseStore):
         # Returns an ObservableDeferred
         res = self.get_users_with_read_receipts_in_room.cache.get((room_id,), None)
 
-        if res and res.called and user_id in res.result:
+        if res:
+            if isinstance(res, defer.Deferred) and res.called:
+                res = res.result
+            if user_id in res:
                 # We'd only be adding to the set, so no point invalidating if the
                 # user is already there
                 return

synapse/util/caches/descriptors.py

@@ -19,7 +19,7 @@ from synapse.util import unwrapFirstError, logcontext
 from synapse.util.caches.lrucache import LruCache
 from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
 
-from . import DEBUG_CACHES, register_cache
+from . import register_cache
 
 from twisted.internet import defer
 from collections import namedtuple
@@ -76,7 +76,7 @@ class Cache(object):
 
         self.cache = LruCache(
             max_size=max_entries, keylen=keylen, cache_type=cache_type,
-            size_callback=(lambda d: len(d.result)) if iterable else None,
+            size_callback=(lambda d: len(d)) if iterable else None,
         )
 
         self.name = name
@@ -96,6 +96,17 @@ class Cache(object):
                 )
 
     def get(self, key, default=_CacheSentinel, callback=None):
+        """Looks the key up in the caches.
+
+        Args:
+            key(tuple)
+            default: What is returned if key is not in the caches. If not
+                specified then function throws KeyError instead
+            callback(fn): Gets called when the entry in the cache is invalidated
+
+        Returns:
+            Either a Deferred or the raw result
+        """
         callbacks = [callback] if callback else []
         val = self._pending_deferred_cache.get(key, _CacheSentinel)
         if val is not _CacheSentinel:
@@ -137,7 +148,7 @@ class Cache(object):
             if self.sequence == entry.sequence:
                 existing_entry = self._pending_deferred_cache.pop(key, None)
                 if existing_entry is entry:
-                    self.cache.set(key, entry.deferred, entry.callbacks)
+                    self.cache.set(key, result, entry.callbacks)
                 else:
                     entry.invalidate()
             else:
@@ -335,20 +346,10 @@ class CacheDescriptor(_CacheDescriptorBase):
             try:
                 cached_result_d = cache.get(cache_key, callback=invalidate_callback)
 
+                if isinstance(cached_result_d, ObservableDeferred):
                     observer = cached_result_d.observe()
-                if DEBUG_CACHES:
-                    @defer.inlineCallbacks
-                    def check_result(cached_result):
-                        actual_result = yield self.function_to_call(obj, *args, **kwargs)
-                        if actual_result != cached_result:
-                            logger.error(
-                                "Stale cache entry %s%r: cached: %r, actual %r",
-                                self.orig.__name__, cache_key,
-                                cached_result, actual_result,
-                            )
-                            raise ValueError("Stale cache entry")
-                        defer.returnValue(cached_result)
-                    observer.addCallback(check_result)
+                else:
+                    observer = cached_result_d
 
             except KeyError:
                 ret = defer.maybeDeferred(
@@ -447,7 +448,9 @@ class CacheListDescriptor(_CacheDescriptorBase):
 
                 try:
                     res = cache.get(tuple(key), callback=invalidate_callback)
-                    if not res.has_succeeded():
+                    if not isinstance(res, ObservableDeferred):
+                        results[arg] = res
+                    elif not res.has_succeeded():
                         res = res.observe()
                         res.addCallback(lambda r, arg: (arg, r), arg)
                         cached_defers[arg] = res