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How to know which process is going to be the “next”

Posted by Marius Voila on September 29, 2010 in London, U.K . — 0 comments This post contains 391 words

If the memory runs out the kernel just use an agent which will try to free some memory by killing those processes which are both old and fat (i.e. use a lot of memory for too long). Every process comes with a score which is constantly updated by the kernel. You can view the score by lurking in /proc/$PID/oom_score.

This article briefly describes how you can find out which process has the biggest change to being killed in a “out of memory” event and how to protect those which are required (e.g. ssh daemon for a remote machine) .

When the OOM killer is called, the process which will most likely be the first victim, is the one with the top score. Here is a little bash script that will show you the running processes sorted by score in ascending order:

for file in /proc/[0-9]*/oom_score; do
	pid=$(echo $file | sed 's/^[^0-9]*\([0-9]\+\).*$/\1/');
	echo "$(cat $file) $(ps h -p $pid -ocmd)";
done | sort -n

If you want to protect a process, you just have to change the value in another special file called oom_adj which resides in the same directory of the proc filesystem. While the oom_score contains the current score (more means better chance to be killed by oom_killer), oom_adj is an adjustment that we can use to control the “final chance”; the precise formula is used by OOM killer is: chance to be killed = oom_score * 2^(oom_adj) So, if we want to get half the current score we just have to store “-1? in oom_adj with: $ echo "-1" >/proc/$PID/oom_adj Other examples: oom_adj = -1 => final_chance = oom_score /2 oom_adj = -2 => final_chance = oom_score /4 oom_adj = -3 => final_chance = oom_score /8 etc. In addition, a value of -17 makes the oom killer ignore that process when seeking for victims.

Nevertheless, please notice that the event in which both memory and swap space are used-up is tragic and should be avoided at any cost. Just add a memory slot or upgrade the one/ones installed and live without the sword of Damocles hanging over your head