Title: Avoid Linux locking up in low memory situations using earlyoom

	Title: Avoid Linux locking up in low memory situations using earlyoom
	Author: Solène
	Date: 28 September 2022
	Tags: linux nixos portoftheweek
	Description: This article presents the program earlyoom to prevent a
	Linux system to lock up in low memory situations.

	# Introduction

	Within operating system kernels, at least for Linux and the BSDs, there
	is a mechanism called "out of memory killer" which is triggered when
	the system is running out of memory and some room must be made to make
	the system responsive again.

	However, in practice this OOM mechanism doesn't work well. If the
	system is running out of memory, it will become totally unresponsive,
	and sometimes the OOM killer will help, but it may take like 30
	minutes, but sometimes it may be stuck forever.

	Today, I stumbled upon a nice project called "earlyoom", which is an
	OOM manager working in the user land instead of inside the kernel,
	which gives it a lot more flexibility about its actions and the
	consequences.

	earlyoom GitHub project page

	# How it works

	earlyoom is simple in that it's a daemon running as root, using nearly
	no memory, that will regularly poll for remaining swap memory and RAM
	memory, if the current level are below the threshold of both, actions
	will be taken.

	What's cool is you can tell it to prefer some processes to terminate
	first, and some processes to avoid as much as possible. For some
	people, it may be preferable to terminate a web browser first and
	instant messaging than their development software.

	I use it with the following parameters:

	```shell
	earlyoom -m 2 -s 2 -r 3600 -g --avoid '^(X\|plasma.*\|konsole\|kwin)$' --prefer '^…
	```

	The command line above means that if my system has less than 2% of its
	RAM and less than 2% of its swap available, earlyoom will try to
	terminate existing program whose binary matches
	electron/libreoffice/gimp etc.... and avoid programs named
	X/Plasma.*/konsole/kwin.

	For configuring it properly as a service, explanations can be found in
	the project README file.

	# NixOS setup

	On NixOS, there is a module for earlyoom, to configure it like in the
	example above:

	```nix
	{
	services.earlyoom = {
	enable = true;
	freeSwapThreshold = 2;
	freeMemThreshold = 2;
	extraArgs = [
	"-g" "--avoid '^(X\|plasma.*\|konsole\|kwin)$'"
	"--prefer '^(electron\|libreoffice\|gimp)$'"
	];
	};
	}
	```

	# Conclusion

	This program is a pleasant surprise to me, I often run out of memory on
	my laptop because I'm running some software requiring a lot of memory
	for good reasons, and while the laptop has barely enough memory to run
	them, I should have most of the other software close to make it fit in.
	However, when I forget to close them, the system would just lock up
	for a while, which most often require a hard reboot. Being able to
	avoid this situation is a big plus for me. Of course, adding some swap
	space would help, but I prefer to avoid adding more swap as it's
	terribly inefficient and only postpone the problem.