Bump batch size from 50 to 1000 for _get_e2e_cross_signing_signatures_for_devices query (#18939)

Co-authored-by: Eric Eastwood <[email protected]>

Author: anoadragon453

changelog.d/18939.misc

@@ -0,0 +1 @@
+Reduce overall number of calls to `_get_e2e_cross_signing_signatures_for_devices` by increasing the batch size of devices the query is called with, reducing DB load.

synapse/storage/databases/main/end_to_end_keys.py

@@ -354,7 +354,10 @@ async def get_e2e_device_keys_and_signatures(
             if d is not None and d.keys is not None
         )
 
-        for batch in batch_iter(signature_query, 50):
+        # 1000 is an arbitrary batch size. It helped performance on a very
+        # large-scale deployment (matrix.org), but has not been tested against
+        # any other setup.
+        for batch in batch_iter(signature_query, 1000):
             cross_sigs_result = (
                 await self._get_e2e_cross_signing_signatures_for_devices(batch)
             )