synapse-product/synapse
Eric Eastwood 51d732db3b
Optimize how we calculate likely_domains during backfill (#13575)
Optimize how we calculate `likely_domains` during backfill because I've seen this take 17s in production just to `get_current_state` which is used to `get_domains_from_state` (see case [*2. Loading tons of events* in the `/messages` investigation issue](https://github.com/matrix-org/synapse/issues/13356)).

There are 3 ways we currently calculate hosts that are in the room:

 1. `get_current_state` -> `get_domains_from_state`
    - Used in `backfill` to calculate `likely_domains` and `/timestamp_to_event` because it was cargo-culted from `backfill`
    - This one is being eliminated in favor of `get_current_hosts_in_room` in this PR 🕳
 1. `get_current_hosts_in_room`
    - Used for other federation things like sending read receipts and typing indicators
 1. `get_hosts_in_room_at_events`
    - Used when pushing out events over federation to other servers in the `_process_event_queue_loop`

Fix https://github.com/matrix-org/synapse/issues/13626

Part of https://github.com/matrix-org/synapse/issues/13356

Mentioned in [internal doc](https://docs.google.com/document/d/1lvUoVfYUiy6UaHB6Rb4HicjaJAU40-APue9Q4vzuW3c/edit#bookmark=id.2tvwz3yhcafh)


### Query performance

#### Before

The query from `get_current_state` sucks just because we have to get all 80k events. And we see almost the exact same performance locally trying to get all of these events (16s vs 17s):
```
synapse=# SELECT type, state_key, event_id FROM current_state_events WHERE room_id = '!OGEhHVWSdvArJzumhm:matrix.org';
Time: 16035.612 ms (00:16.036)

synapse=# SELECT type, state_key, event_id FROM current_state_events WHERE room_id = '!OGEhHVWSdvArJzumhm:matrix.org';
Time: 4243.237 ms (00:04.243)
```

But what about `get_current_hosts_in_room`: When there is 8M rows in the `current_state_events` table, the previous query in `get_current_hosts_in_room` took 13s from complete freshness (when the events were first added). But takes 930ms after a Postgres restart or 390ms if running back to back to back.

```sh
$ psql synapse
synapse=# \timing on
synapse=# SELECT COUNT(DISTINCT substring(state_key FROM '@[^:]*:(.*)$'))
FROM current_state_events
WHERE
    type = 'm.room.member'
    AND membership = 'join'
    AND room_id = '!OGEhHVWSdvArJzumhm:matrix.org';
 count
-------
  4130
(1 row)

Time: 13181.598 ms (00:13.182)

synapse=# SELECT COUNT(*) from current_state_events where room_id = '!OGEhHVWSdvArJzumhm:matrix.org';
 count
-------
 80814

synapse=# SELECT COUNT(*) from current_state_events;
  count
---------
 8162847

synapse=# SELECT pg_size_pretty( pg_total_relation_size('current_state_events') );
 pg_size_pretty
----------------
 4702 MB
```

#### After

I'm not sure how long it takes from complete freshness as I only really get that opportunity once (maybe restarting computer but that's cumbersome) and it's not really relevant to normal operating times. Maybe you get closer to the fresh times the more access variability there is so that Postgres caches aren't as exact. Update: The longest I've seen this run for is 6.4s and 4.5s after a computer restart.

After a Postgres restart, it takes 330ms and running back to back takes 260ms.

```sh
$ psql synapse
synapse=# \timing on
Timing is on.
synapse=# SELECT
    substring(c.state_key FROM '@[^:]*:(.*)$') as host
FROM current_state_events c
/* Get the depth of the event from the events table */
INNER JOIN events AS e USING (event_id)
WHERE
    c.type = 'm.room.member'
    AND c.membership = 'join'
    AND c.room_id = '!OGEhHVWSdvArJzumhm:matrix.org'
GROUP BY host
ORDER BY min(e.depth) ASC;
Time: 333.800 ms
```

#### Going further

To improve things further we could add a `limit` parameter to `get_current_hosts_in_room`. Realistically, we don't need 4k domains to choose from because there is no way we're going to query that many before we a) probably get an answer or b) we give up. 

Another thing we can do is optimize the query to use a index skip scan:

 - https://wiki.postgresql.org/wiki/Loose_indexscan
 - Index Skip Scan, https://commitfest.postgresql.org/37/1741/
 - https://www.timescale.com/blog/how-we-made-distinct-queries-up-to-8000x-faster-on-postgresql/
2022-08-30 01:38:14 -05:00
..
_scripts Support registration_shared_secret in a file (#13614) 2022-08-25 16:27:46 +00:00
api synapse.api.auth.Auth cleanup: make permission-related methods use Requester instead of the UserID (#13024) 2022-08-22 14:17:59 +01:00
app Add experimental configuration option to allow disabling legacy Prometheus metric names. (#13540) 2022-08-24 11:35:54 +00:00
appservice Add prometheus counters for content types other than events (#13175) 2022-07-13 15:18:20 +01:00
config Generate missing configuration files at startup (#13615) 2022-08-26 11:26:06 +00:00
crypto Instrument _check_sigs_and_hash_and_fetch to trace time spent in child concurrent calls (#13588) 2022-08-23 21:53:37 -05:00
events Instrument _check_sigs_and_hash_and_fetch to trace time spent in child concurrent calls (#13588) 2022-08-23 21:53:37 -05:00
federation Faster Room Joins: fix /make_knock blocking indefinitely when the room in question is a partial-stated room. (#13583) 2022-08-24 09:09:59 +00:00
handlers Optimize how we calculate likely_domains during backfill (#13575) 2022-08-30 01:38:14 -05:00
http synapse.api.auth.Auth cleanup: make permission-related methods use Requester instead of the UserID (#13024) 2022-08-22 14:17:59 +01:00
logging Instrument the federation/backfill part of /messages (#13489) 2022-08-16 12:39:40 -05:00
metrics Add experimental configuration option to allow disabling legacy Prometheus metric names. (#13540) 2022-08-24 11:35:54 +00:00
module_api Update module API "update room membership" method to allow for remote joins (#13441) 2022-08-05 09:37:58 +00:00
push Fix breaking event sending due to bad push rule (#13547) 2022-08-17 11:02:38 +00:00
replication Speed up fetching large numbers of push rules (#13592) 2022-08-23 13:15:43 +01:00
res Add viewport directive to HTML templates to optimise for mobile (#13493) 2022-08-11 14:04:20 +01:00
rest Drop support for delegating email validation, round 2 (#13596) 2022-08-23 11:40:00 +00:00
server_notices Directly lookup local membership instead of getting all members in a room first (get_users_in_room mis-use) (#13608) 2022-08-24 14:13:12 -05:00
spam_checker_api Fix import in module_api module and docs on the new check_event_for_spam signature (#12918) 2022-05-31 12:04:53 +02:00
state Rename event_map to unpersisted_events (#13603) 2022-08-24 21:06:31 +01:00
static Add viewport directive to HTML templates to optimise for mobile (#13493) 2022-08-11 14:04:20 +01:00
storage Optimize how we calculate likely_domains during backfill (#13575) 2022-08-30 01:38:14 -05:00
streams Instrument /messages for understandable traces in Jaeger (#13368) 2022-08-03 10:57:38 -05:00
util Add experimental configuration option to allow disabling legacy Prometheus metric names. (#13540) 2022-08-24 11:35:54 +00:00
__init__.py Fix Synapse git info missing in version strings (#12973) 2022-06-07 15:24:11 +01:00
event_auth.py Implement MSC3848: Introduce errcodes for specific event sending failures (#13343) 2022-07-27 13:44:40 +01:00
notifier.py Notifier: accept callbacks to fire on room joins (#13254) 2022-07-13 19:48:24 +01:00
py.typed Mark Module API error imports as re-exported and mark Synapse as containing type annotations (#11054) 2021-10-13 08:42:41 +01:00
server.py Move the "email unsubscribe" resource, refactor the macaroon generator & simplify the access token verification logic. (#12986) 2022-06-14 09:12:08 -04:00
types.py Fix destination_is errors seen in sentry. (#13041) 2022-06-14 18:28:26 +01:00
visibility.py Add some miscellaneous comments around sync (#13474) 2022-08-10 12:58:20 +01:00