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Your Linode was OOMing, meaning something inside your node is consuming all of the available virtual memory. You can see this for yourself by logging into Lish and viewing the output of the "logview" command within the console [0].

These messages you see written to your console indicate that your Linode had exhausted its allotted RAM resources and invoked the OOM-Killer. The OOM-Killer is designed to identify the highest scoring processes and kill them in order to free up memory required by the system.

When you rebooted your Linode, it came back online - however, you will want to take measures to optimize your services to reduce your overall RAM and swap usage. To begin with, I'd suggest implementing the low-memory settings advised in our Troubleshooting guide to minimize your risks of OOMing again [1].

The process which is taking all your memory is almost always Apache in prefork mode using mod-php. (You can use the "top" command to check, then press capital M).

Try switching to Nginx + PHP-FPM. See the thread below this one:

https://forum.linode.com/viewtopic.php?f=20&t=11978

Thanks for your reply, is there any way to improve the server configuration without switching to Nginx, because I don't want to make website offline, I don't have secondary hosting.

If your Apache is >= 2.4, you can use Apache + PHP-FPM, and it would also be an improvement over your current setup.

The offline time depends on how fast you can change the configuration and restart Apache.

If you use a little bit of scripting, it should not take more than a few seconds, a minute at most.
Use a test linode to experiment.

we can now run secure, fast, and dependable PHP code using only the stock apache httpd and php.net releases; no more messing around with suphp or suexec - or, indeed, mod_php.