Merge pull request #3383 from matrix-org/rav/hacky_speedup_state_group_cache

Improve StateGroupCache efficiency for wildcard lookups

2 files changed, 55 insertions, 26 deletions

diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 986a20400c..cd9821c270 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -526,10 +526,23 @@ class StateGroupWorkerStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def _get_state_for_groups(self, groups, types=None):
-        """Given list of groups returns dict of group -> list of state events
-        with matching types. `types` is a list of `(type, state_key)`, where
-        a `state_key` of None matches all state_keys. If `types` is None then
-        all events are returned.
+        """Gets the state at each of a list of state groups, optionally
+        filtering by type/state_key
+
+        Args:
+            groups (iterable[int]): list of state groups for which we want
+                to get the state.
+            types (None|iterable[(str, None|str)]):
+                indicates the state type/keys required. If None, the whole
+                state is fetched and returned.
+
+                Otherwise, each entry should be a `(type, state_key)` tuple to
+                include in the response. A `state_key` of None is a wildcard
+                meaning that we require all state with that type.
+
+        Returns:
+            Deferred[dict[int, dict[(type, state_key), EventBase]]]
+                a dictionary mapping from state group to state dictionary.
         """
         if types:
             types = frozenset(types)
@@ -538,7 +551,7 @@ class StateGroupWorkerStore(SQLBaseStore):
         if types is not None:
             for group in set(groups):
                 state_dict_ids, _, got_all = self._get_some_state_from_cache(
-                    group, types
+                    group, types,
                 )
                 results[group] = state_dict_ids
 
@@ -559,22 +572,40 @@ class StateGroupWorkerStore(SQLBaseStore):
             # Okay, so we have some missing_types, lets fetch them.
             cache_seq_num = self._state_group_cache.sequence
 
+            # the DictionaryCache knows if it has *all* the state, but
+            # does not know if it has all of the keys of a particular type,
+            # which makes wildcard lookups expensive unless we have a complete
+            # cache. Hence, if we are doing a wildcard lookup, populate the
+            # cache fully so that we can do an efficient lookup next time.
+
+            if types and any(k is None for (t, k) in types):
+                types_to_fetch = None
+            else:
+                types_to_fetch = types
+
             group_to_state_dict = yield self._get_state_groups_from_groups(
-                missing_groups, types
+                missing_groups, types_to_fetch,
             )
 
-            # Now we want to update the cache with all the things we fetched
-            # from the database.
             for group, group_state_dict in iteritems(group_to_state_dict):
                 state_dict = results[group]
-                state_dict.update(group_state_dict)
 
+                # update the result, filtering by `types`.
+                if types:
+                    for k, v in iteritems(group_state_dict):
+                        (typ, _) = k
+                        if k in types or (typ, None) in types:
+                            state_dict[k] = v
+                else:
+                    state_dict.update(group_state_dict)
+
+                # update the cache with all the things we fetched from the
+                # database.
                 self._state_group_cache.update(
                     cache_seq_num,
                     key=group,
-                    value=state_dict,
-                    full=(types is None),
-                    known_absent=types,
+                    value=group_state_dict,
+                    fetched_keys=types_to_fetch,
                 )
 
         defer.returnValue(results)
@@ -681,7 +712,6 @@ class StateGroupWorkerStore(SQLBaseStore):
                 self._state_group_cache.sequence,
                 key=state_group,
                 value=dict(current_state_ids),
-                full=True,
             )
 
             return state_group
diff --git a/synapse/util/caches/dictionary_cache.py b/synapse/util/caches/dictionary_cache.py
index bdc21e348f..95793d466d 100644
--- a/synapse/util/caches/dictionary_cache.py
+++ b/synapse/util/caches/dictionary_cache.py
@@ -107,29 +107,28 @@ class DictionaryCache(object):
         self.sequence += 1
         self.cache.clear()
 
-    def update(self, sequence, key, value, full=False, known_absent=None):
+    def update(self, sequence, key, value, fetched_keys=None):
         """Updates the entry in the cache
 
         Args:
             sequence
-            key
-            value (dict): The value to update the cache with.
-            full (bool): Whether the given value is the full dict, or just a
-                partial subset there of. If not full then any existing entries
-                for the key will be updated.
-            known_absent (set): Set of keys that we know don't exist in the full
-                dict.
+            key (K)
+            value (dict[X,Y]): The value to update the cache with.
+            fetched_keys (None|set[X]): All of the dictionary keys which were
+                fetched from the database.
+
+                If None, this is the complete value for key K. Otherwise, it
+                is used to infer a list of keys which we know don't exist in
+                the full dict.
         """
         self.check_thread()
         if self.sequence == sequence:
             # Only update the cache if the caches sequence number matches the
             # number that the cache had before the SELECT was started (SYN-369)
-            if known_absent is None:
-                known_absent = set()
-            if full:
-                self._insert(key, value, known_absent)
+            if fetched_keys is None:
+                self._insert(key, value, set())
             else:
-                self._update_or_insert(key, value, known_absent)
+                self._update_or_insert(key, value, fetched_keys)
 
     def _update_or_insert(self, key, value, known_absent):
         # We pop and reinsert as we need to tell the cache the size may have