1 files changed, 35 insertions, 18 deletions

diff --git a/synapse/storage/databases/main/metrics.py b/synapse/storage/databases/main/metrics.py
index 686052bd83..92099f95ce 100644
--- a/synapse/storage/databases/main/metrics.py
+++ b/synapse/storage/databases/main/metrics.py
@@ -12,10 +12,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import typing
-from collections import Counter
 
-from synapse.metrics import BucketCollector
+from synapse.metrics import GaugeBucketCollector
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import DatabasePool
@@ -23,6 +21,26 @@ from synapse.storage.databases.main.event_push_actions import (
     EventPushActionsWorkerStore,
 )
 
+# Collect metrics on the number of forward extremities that exist.
+_extremities_collecter = GaugeBucketCollector(
+    "synapse_forward_extremities",
+    "Number of rooms on the server with the given number of forward extremities"
+    " or fewer",
+    buckets=[1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500],
+)
+
+# we also expose metrics on the "number of excess extremity events", which is
+# (E-1)*N, where E is the number of extremities and N is the number of state
+# events in the room. This is an approximation to the number of state events
+# we could remove from state resolution by reducing the graph to a single
+# forward extremity.
+_excess_state_events_collecter = GaugeBucketCollector(
+    "synapse_excess_extremity_events",
+    "Number of rooms on the server with the given number of excess extremity "
+    "events, or fewer",
+    buckets=[0] + [1 << n for n in range(12)],
+)
+
 
 class ServerMetricsStore(EventPushActionsWorkerStore, SQLBaseStore):
     """Functions to pull various metrics from the DB, for e.g. phone home
@@ -32,18 +50,6 @@ class ServerMetricsStore(EventPushActionsWorkerStore, SQLBaseStore):
     def __init__(self, database: DatabasePool, db_conn, hs):
         super().__init__(database, db_conn, hs)
 
-        # Collect metrics on the number of forward extremities that exist.
-        # Counter of number of extremities to count
-        self._current_forward_extremities_amount = (
-            Counter()
-        )  # type: typing.Counter[int]
-
-        BucketCollector(
-            "synapse_forward_extremities",
-            lambda: self._current_forward_extremities_amount,
-            buckets=[1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"],
-        )
-
         # Read the extrems every 60 minutes
         def read_forward_extremities():
             # run as a background process to make sure that the database transactions
@@ -58,14 +64,25 @@ class ServerMetricsStore(EventPushActionsWorkerStore, SQLBaseStore):
         def fetch(txn):
             txn.execute(
                 """
-                select count(*) c from event_forward_extremities
-                group by room_id
+                SELECT t1.c, t2.c
+                FROM (
+                    SELECT room_id, COUNT(*) c FROM event_forward_extremities
+                    GROUP BY room_id
+                ) t1 LEFT JOIN (
+                    SELECT room_id, COUNT(*) c FROM current_state_events
+                    GROUP BY room_id
+                ) t2 ON t1.room_id = t2.room_id
                 """
             )
             return txn.fetchall()
 
         res = await self.db_pool.runInteraction("read_forward_extremities", fetch)
-        self._current_forward_extremities_amount = Counter([x[0] for x in res])
+
+        _extremities_collecter.update_data(x[0] for x in res)
+
+        _excess_state_events_collecter.update_data(
+            (x[0] - 1) * x[1] for x in res if x[1]
+        )
 
     async def count_daily_messages(self):
         """