Better memory monitoring

[artmakh]

This PR improves memory monitoring on Linux-based hosts.

Node's os.freemem doesn't represent how much ram is available for processes. It returns what is shown in cat /proc/meminfo MemFree, which is calculated from memory used by different applications + buffers + Linux disk in-memory caching. This means that's it's ok to have MemFree close to 0 on Linux host, because Linux can always unload disk caches and buffers from memory without any harm to running processes. To clarify this, Linux developers added new field in /proc/meminfo called MemAvailable. This filed represent real free memory capacity for applications on host, without including disk caching.

So, if monitoring should answer the question Is it enough free RAM on my host to run an application? it should use MemAvailable parameter. That's why I think it should be better to switch memory monitoring on this parameter, instead of MemFree.

Unfortunately, there is no way to get MemAvailable from node.os, but thankfully systeminformation is already in use, and you can get MemAvailable there.

Also here is a little bit more information about free vs available https://www.linuxatemyram.com/