[meta] Improving Presence Performance

[anoadragon453]

Presence is a feature of matrix that allows users to share their online/offline/unavailable status with other remote and local users, as well as present a status message. Traditionally this feature has been turned off in all major deployments due to its enormous performance costs. We'd like to turn it back on, and this issue is a collection of other issues that may help us get there.

There is currently no timeline for these tasks.

• Presence logic for handling newly joined rooms is very inefficient and does duplicate work #8956 (fix #1)
• Synchrotrons always wake up sync streams for presence updates #8955
• Not all presence updates should be sent out over federation immediately. #9415
• Stop sending unneeded presence messages #9339
• Moving presence work off of the master process done in Move presence off master #9761

[richvdh]

See also #3971