Note to self: RTFM for ElasticSearch installation.

As mentioned previously, we're running the ELK stack to monitor our cloud services. Since I wanted to keep running the latest and even beta/pre-release versions of these apps, I've manually installed the apps, as opposed to using apt-get or docker.

Anyway - at some point ElasticSearch started taking up 100% CPU load on the machine and stopped responding a couple of minutes later.
So I stopped ElasticSearch and started it again - this time not in deamon mode. This started showing me the exceptions that were occurring.

As it turns out ElasticSearch wants you to change 2 configuration settings on your machine: number of open files and disable swapping.

It's all here: https://www.elastic.co/guide/en/elasticsearch/reference/1.4/setup-configuration.html 

After doing this - ES runs like a charm again.

By the way - increasing the number of open files on Ubuntu Server is described here: http://posidev.com/blog/2009/06/04/set-ulimit-parameters-on-ubuntu/