Sliding Sync: Slight optimization when fetching state for the room (get_events_as_list(...))

[MadLittleMods]

Spawning from @kegsay pointing out that the Sliding Sync endpoint doesn't handle a large room with a lot of state well on initial sync (requesting all state via required_state: [ ["*","*"] ]) (it just takes forever).

After investigating further, the slow part is just get_events_as_list(...) fetching all of the current state ID's out for the room (which can be 100k+ events for rooms with a lot of membership). This is just a slow thing in Synapse in general and the same thing happens in Sync v2 or the /state endpoint.

---

The only idea I had to improve things was to use batch_iter to only try fetching a fixed amount at a time instead of working with large maps, lists, and sets. This doesn't seem to have much effect though.

There is already a batch_iter(event_ids, 200) in _fetch_event_rows(...) for when we actually have to touch the database and that's inside a queue to deduplicate work.

I did notice one slight optimization to use get_events_as_list(...) directly instead of get_events(...). get_events(...) just turns the result from get_events_as_list(...) into a dict and since we're just iterating over the events, we don't need the dict/map.

Dev notes

Run the new tests

SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.storage.databases.main.test_events_worker.GetEventsTestCase

SYNAPSE_POSTGRES=1 SYNAPSE_POSTGRES_USER=postgres SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.storage.databases.main.test_events_worker.GetEventsTestCase

Enable Jaeger tracing in Twisted Trial tests:

Run Jaeger locally with Docker: https://www.jaegertracing.io/docs/1.6/getting-started/#all-in-one-docker-image

Add the following to tests/unittest.py in setUp(...):

        from synapse.logging.opentracing import init_tracer

        # Start the tracer
        init_tracer(self.hs)  # noqa

Add the following to your tests (in tests/rest/client/test_asdf.py for example):

    def default_config(self) -> JsonDict:
        config = super().default_config()
        config["opentracing"] = {
            "enabled": True,
            "jaeger_config": {
                "sampler": {"type": "const", "param": 1},
                "logging": False,
            },
        }
        return config

Add a sleep to the end of the test you're running so the traces have a chance to flow to Jaeger before the process exits.

import time
time.sleep(6)

If you're not tracing an endpoint/servlet (which will already have a logging context and tracing span), you will need to wrap your function under test in a logging context for the tracing to be happy. Also be sure to add @trace decorators to what you want to trace downstream.

def test_asdf(self) -> None:
    with LoggingContext("test") as _ctx:
        my_custom_function()

Run your tests.

Visit http://localhost:16686/ and choose the test master service and find the traces you're interesting in.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

One slight optimization to use get_events_as_list(...) directly instead of get_events(...). get_events(...) just turns the result from get_events_as_list(...) into a dict and since we're just iterating over the events, we don't need the dict/map.

[MadLittleMods]

New test because I used it as a test bench and get consistent traces (previous iteration with tracing). The tracing stuff has been removed.

Feel free to nit to remove the whole thing.

[MadLittleMods]

Collection because we iterate over event_ids multiple times (even before I added in the tracing tag)

[MadLittleMods]

This one was a proper Iterable before and only used once but updated to be a Collection now that we also read from it for the tracing tag.