use_presence: Default to False

[rht]

This is a pre-emptive measure so that a user's first install experience is smoother.

Tracking issue with use_presence's performance problem:
#3971.

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.
• Pull request includes a sign off
• Code style is correct (run the linters)

[rht]

I will fix the test cases if there is a consensus that presence should be disabled by default.

[rht]

Actually, since this default value being set to False is temporary, there is less hassle with just saying in INSTALL.md that use_presence should be off.

[clokep]

I don't think we can just change the default here since it would cause people using this feature to have it automatically disabled when upgrading?

[rht]

Yeah, better if just recommend in INSTALL.md to turn it off for now?

[clokep]

Yeah, better if just recommend in INSTALL.md to turn it off for now?

This is mentioned in the README already: https://github.com/matrix-org/synapse#help-synapse-is-slow-and-eats-all-my-ramcpu

All of this could be a bit better organized intro a troubleshooting section, but I don't think that belongs in the INSTALL file.

I'm going to close this for now since the proposed solution is not backwards compatible.

[rht]

I have seen that text in README. Unfortunately it is a remedy after people have noticed their RAM and CPU shot up. It's better to warn people in advance in INSTALL.md.