(whostmgr (whostmgr)): The subprocess ended prematurely because it received the “KILL” (9) signal.

When suddenly when i accessed an webpage hosted on a cPanel/WHM server, i got Error 500, it was sudden error during browsing a website so it have to be something with the server, not with the script.

I had opened a WHM and i clicked Apache Status page, and it said:

"No response from subprocess (whostmgr (whostmgr)): The subprocess ended prematurely because it received the “KILL” (9) signal."

whm > tweak settings > "The maximum memory a cPanel process can use before it is killed off (in megabytes)." -- i have 1024

So from within command line i did:

free -m



So there seems to be some free RAM...?

I did "service cpanel restart" and it returned several errors containing phrasse "The system failed to fork a new process because of an error: Cannot allocate memory"

I remembered that couple of times i had memory issue and restarting MySQL helped to free RAM, so i did command "service mysql restart" and then cpanel restart worked and Error 500 is away.
/etc/my.cnf , did my best to tune it according to the mysql tunning script advices

Please which Linux commands i should do next time this issue happen so i tweak the configurations so this issue no longer happen?