Collaborative editing doesn't work and confluence service crashing

[5140883]

This problem happened several times, and I have read almost all the community articles, cannot find a solution. Please help!

Confluence version: 6.15.8 (with unlimited license)

Problem description:

1. Suddenly users cannot edit or create page in website. When user edit/ create a page, error message pops up, "Cannot save the page. Please check the communication with server" , or "Loading slow, please ask your admin".

2. In confluence server, 2 processes (confluence main process & synchrony process) still alive, but synchrony process takes too much CPU up to 2000%+ (less than 100% in normal)

3. I tried to get thread info by "jstack <synchrony PID>, it failed with:

194141: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

4. Restart service doesn't work. The synchrony process eats  almost all the CPU in several minitues after restarting.

5. Service looks well after I disabled "Collaborative editing" in admin area (and CPU goes down after a while)

6. Reboot the server works, everything is back to normal.

Please help! If more info needed, please tell me. Thanks!

[5140883]

Add some info:

1. No 'OutOfMemory' info found in logs

2. Xms=48G Xmx=48G

3. Bundled JDK version: 

openjdk version "1.8.0_222"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.222-b10, mixed mode)