Optionally disable state groups caching

The theory behind this is that the overhead and high churn rate of the
cache result in greater CPU and memory wastage than is offloaded from
the database.

Note this still caches groups on insert which means multiple room inserts
in a short period (ie during room creation) will still benefit from the
cache.

Author: Fizzadar

synapse/storage/databases/state/store.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import logging
+import os
 from typing import TYPE_CHECKING, Collection, Dict, Iterable, List, Optional, Set, Tuple
 
 import attr
@@ -40,6 +41,10 @@
 
 MAX_STATE_DELTA_HOPS = 100
 
+BEEPER_DISABLE_STATE_GROUPS_CACHE = bool(
+    os.environ.get("BEEPER_DISABLE_STATE_GROUPS_CACHE")
+)
+
 
 @attr.s(slots=True, frozen=True, auto_attribs=True)
 class _GetStateGroupDelta:
@@ -358,6 +363,9 @@ def _insert_into_cache(
                 last lookup in cache
         """
 
+        if BEEPER_DISABLE_STATE_GROUPS_CACHE:
+            return
+
         # We need to work out which types we've fetched from the DB for the
         # member vs non-member caches. This should be as accurate as possible,
         # but can be an underestimate (e.g. when we have wild cards)