Make it possible to configure repeat interval of background jobs

[litetex]

Description:

I noticed that prune_old_user_ips is executed quite often (every 5s) and wanted to lower the execution interval a bit to reduce server load, however it looks like it's currently not possible to do this at runtime.

It would be nice if the repeat interval of background jobs would be configurable somewhere.

[devonh]

Good idea. This particular timer being every 5s seems quite aggressive, depending on the number of users on your server.
For Synapse instances with a large number of users, it may make sense for this cleanup to be run more frequently. But for servers that don't have a massive number of users, pruning old user ips every 5s is overkill.

Doing some rough calculations:

• With the default user_ips_max_age of 28 days
• 28d x 24h x 60m x 60s / 5s = ~500,000 timer ticks in 28 days
• So unless your Synapse instance has at least 500,000 users becoming inactive every single month (assuming somewhat equal distribution), most of the timer ticks will be doing a DB transaction with no effect