What happen when you try to run a new process on a machine which is already loaded with tons of other processes and no available resources?
This is not a good news especially if you are running long running tasks and suddenly you discover that your process has been killed. Argh!
Out of Memory Killer, or fondly called OOM Killer is a kernel mechanism to free up resources when all else fails. The mechanism picks up a process based on an elaborate criteria of how long it’s been running, user privileges, memory consumed, nice status, disk operations etc. and terminates it.
Essentially, OS should have to kill the minimum number of processes to free up most amount of memory (until the system becomes stable).
For this, the OS maintains a score for each running process. The score is
called OOM_Score (available in /proc/<pid>/oom_score
Also, the behavior of how OS handles extra memory requests is configurable. An educative post in this regard is available on SO.