How does the Linux kernel device process to kill when their system goes out of memory?

Picture by Sai Kiran Anagani

Linux kernel has a feature or application called OOM Killer. It kills the process whenever it is out of memory. OOM decides which process to kill based on a score that it calculates which is called oom_score.

You can find oom_score for each process in /proc/PID/oom_score

That rings the bell but not sure on what basis it gives a score to each process, how does it know which process have less/more?

Well, It assigns metrics based on a process like whether it is a system process, daemon process, user spawned process, and prioritizes them, checks for a child process that it spawned, and their impact.

Suppose if the process was started by privileged users then it has higher OOM than a process spawned by Non-privilege users.

Mathematically in the Linux kernel website, it writes it is calculated as (10 X percentage of memory used) i.e if a process is using 100% memory then its OOM score is 10 X 100% = 1000. But other factors like process nice value, process type might comes into play as well.

According to Linux kernel.org “OOM_killer selects a process that is using a large amount of memory but is not that long-lived. Processes that have been running a long time are unlikely to be the cause of memory shortage so this calculation is likely to select a process that uses a lot of memory but has not been running long. If the process is a root processor that has CAP_SYS_ADMIN capabilities, the points are divided by four as it is assumed that root privilege processes are well behaved.

Similarly, if it has CAP_SYS_RAWIO capabilities (access to raw devices) privileges, the points are further divided by 4 as it is undesirable to kill a process that has direct access to hardware.”

Hope you get some idea of how Linux kills old processes. Happy reading.

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Prabesh Thapa

Prabesh Thapa

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DevOps / SRE Engineer. Blog: 99devops.com System admin turned SRE. I love Linux.