OOMkiller

OOMKiller, short for Out-Of-Memory killer, is a component of the Linux kernel that activates when the system is under severe memory pressure. Its role is to reclaim memory by terminating one or more processes to restore system stability and prevent a complete crash. The mechanism is invoked when available memory is critically low and the kernel cannot reclaim enough memory through standard means, such as reclaiming page cache or freeing unused memory.

When invoked, the OOM killer evaluates processes to identify a victim. Each process is assigned an OOM score based on factors including memory usage, process age, and an adjustable parameter called oom_score_adj. The process with the highest OOM score is selected for termination, typically receiving a SIGKILL signal to promptly free memory. Kernel threads usually receive a very low probability of being killed, and processes with negative oom_score_adj values are protected to some extent. Administrators can inspect a process’s scores via /proc/[pid]/oom_score and /proc/[pid]/oom_score_adj.

Impact and management: The action of the OOM killer is abrupt and can result in loss of unsaved work in the terminated process. It is possible to mitigate OOM killer triggers through memory management practices such as configuring memory limits with cgroups, tuning overcommit and swappiness settings (vm.overcommit_memory, vm.overcommit_ratio, vm.swappiness), and ensuring adequate swap space. Observability tools include dmesg or journal logs that record OOM events, often with the killed process’s name and memory usage details.