If there are no people on the server, the java process is on 10% CPU usage. As soon as someone joins the game the usage will instantly go to 100%. I'm 100% sure it is not a problem with my server as it happens on multiple machines with different specs. MC-63590 Is also referring to this issue but setting the max-server-tick to -1 is not a solution as it will only stop the watchdog from stopping the server. The problem is that since 1.8.1 one of the threads is always using 100% CPU as soon as someone joins the server.
I'm having the same problem as the one in MC-2747, so I don't think I have to send a new crash report. 🙂
aaaah, I didn't know I had a mod installed, sorry!
EDIT: it's also happening in vanilla so the mod wasn't the problem.
When I set the difficulty to peaceful the CPU usage will go back to normal it seems, now steadily at 20%.
Might be an issue with mob spawning?