Only send devicelist updates for users with whom you share an encrypted room?

[ara4n]

[Half-Shot]

Problematically, this also includes appservice users (e.g. every time a freenode user joins a room and you've not seen them before, that's a device list update that has to happen). This could surely be trivially filtered out.

[ShadowJonathan]

I'm currently poking at making a PR for this, as this is inhibiting some performance on my server (with quite a lot of people visible), however, i'm not 100% sure if Element could handle this bug-free.

[ShadowJonathan]

#11974 takes a lot of "pressure" off of the performance issue that this issue likes to address. (i came across it while rebasing a local patchset)

[anoadragon453]

For the federation side of this, I believe you'd only need to restrict this query to encrypted rooms:

synapse/synapse/handlers/device.py

Line 567 in ba79fb4

room_ids = await self.store.get_rooms_for_user(user_id)

Edit: A potential problem would be trying to verify a user whom you don't share an encrypted room with. You wouldn't know whether you had an up-to-date copy of their device list. Perhaps in the case that we don't share an encrypted room with a user, we should always consider that user's devices out of date, and always requery them from the remote homeserver when a client calls /keys/query? Would still be a net gain in performance in theory.

[ShadowJonathan]

Federation isn't a problem for me personally, that is just a slightly increased average load, the real problem is (initial) sync time, as not iterating rooms that aren't encrypted for device list updates cuts that time substantially.

Having worked on related tests in complement in the meantime, I know this is technically not spec (or at least complement) compliant, so that's why I didn't really keep pushing it forward into a PR.