I have discovered a problem.
I have a bunch of forums all running absolutely fine on 1vCPU 3.5GB RAM VMs.
Based on previous discussions, it seems that 1GB plus swap is adequate to run a low volume forum.
So I experimented by moving a number of forums to 1.8GB RAM plus 3GB SSD swap VMs.
They ran fine for a while with basically zero swap usage. Then suddenly out of the blue they started thrashing. Swap usage shot up to 60% with low CPU % and the web server because non responsive.
Rebooting doesn’t help. It immediately started thrashing again.
So I ended up moving them back to the high RAM sizes.
Since the situation persists thru reboots, I suspect it has something to do with sidekiq or redis…