synapse-product/changelog.d
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
..
.gitignore Correct attrs package name in requirements (#3492) 2018-07-07 10:46:59 +10:00
13540.misc Add experimental configuration option to allow disabling legacy Prometheus metric names. (#13540) 2022-08-24 11:35:54 +00:00
13573.misc Cache user IDs instead of profile objects (#13573) 2022-08-23 09:49:59 +00:00
13575.misc Optimize how we calculate likely_domains during backfill (#13575) 2022-08-30 01:38:14 -05:00
13583.bugfix 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
13585.bugfix When loading current ids, sort by stream_id to avoid incorrect overwrite and avoid errors caused by sorting alphabetical instance name which can be null (#13585) 2022-08-24 12:53:46 -05:00
13586.misc Comment about a better future where we can get the state diff between two events (#13586) 2022-08-24 18:59:27 -05:00
13588.misc Instrument _check_sigs_and_hash_and_fetch to trace time spent in child concurrent calls (#13588) 2022-08-23 21:53:37 -05:00
13591.misc Speed up @cachedList (#13591) 2022-08-23 14:53:27 +00:00
13592.misc Speed up fetching large numbers of push rules (#13592) 2022-08-23 13:15:43 +01:00
13597.misc Rewrite get push actions queries (#13597) 2022-08-24 10:12:51 +01:00
13600.misc Fix regression caused by #13573 (#13600) 2022-08-23 14:14:05 +00:00
13602.doc Write about the chain cover a little. (#13602) 2022-08-23 17:41:55 +00:00
13603.misc Rename event_map to unpersisted_events (#13603) 2022-08-24 21:06:31 +01:00
13605.misc Update get_users_in_room mis-use to get hosts with dedicated get_current_hosts_in_room (#13605) 2022-08-24 14:15:37 -05:00
13606.misc Use dedicated get_local_users_in_room to find local users when calculating join_authorised_via_users_server of a /make_join request (#13606) 2022-08-24 11:14:28 -05:00
13608.misc 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
13614.feature Support registration_shared_secret in a file (#13614) 2022-08-25 16:27:46 +00:00
13615.feature Generate missing configuration files at startup (#13615) 2022-08-26 11:26:06 +00:00
13616.bugfix register_new_matrix_user: read server url from config (#13616) 2022-08-25 15:29:08 +01:00
13632.bugfix Move the execution of the retention purge_jobs to the main worker (#13632) 2022-08-26 08:38:10 +01:00
13639.misc Print complement failure results last (#13639) 2022-08-28 20:05:30 +00:00
13640.doc Improve documentation around user registration (#13640) 2022-08-26 13:29:31 +00:00