Knowledge Base

Linux Out of Memory killer

The Out Of Memory Killer or OOM Killer is a process that the linux kernel employs when the system is critically low on memory. This situation occurs because the linux kernel has over allocated memory to its processes.

When a process starts it requests a block of memory from the kernel. This initial request is usually a large request that the process will not immediately or indeed ever use all of. The kernel, aware of this tendency for processes to request redundant memory, over allocates the system memory. This means that when the system has, for example, 2GB of RAM the kernel may allocate 2.5GB to processes.

Normally, this situation does not cause a problem. However, if enough processes begin to use all of their requested memory blocks then there will not be enough physical memory to support them all. This means that the running processes require more memory than is physically available. This situation is critical and must be resolved immediately.

The solution that the linux kernel employs is to invoke the OOM Killer to review all running processes and kill one or more of them in order to free up system memory and keep the system running.

The OOM Killer will only get invoked when the system is critically low on memory.

Due to its nature, Neo4j will always have a high memory footprint and it’s always likely to be a candidate to be killed when the OOM Killer sweeps the running processes. While what gets killed often seems random or simply the highest memory consumer, the OOM Killer doesn’t operate like that. Instead, it chooses which process to kill based on its oom_score. This is a value controled by the operation system itself based on a number of criteria.

You can check the oom_score of a process by looking at /proc/$PID/oom_score.

While we can’t truly prevent Neo4j from being killed if needed, we can adjust the oom_score to make its process less likely to be terminated by the OOM Killer. To do this, you can edit the following file:


Files in /proc are not actual files, they’re an interface to lower-levels of the operating system. Therefore you cannot edit them using nano or vi. Rather, you can echo the value into the file as such:

$echo -100 > /proc/$PID/oom_score_adj

You can adjust the value on this file, valid values are integers in the range of -1000 to 1000. The lower the value, the lower the chance that it’s going to be killed.

Again this will not prevent Neo4j from being killed, but it will influence the likelihood of it happening.

Be aware that if the process restarts, you will have to set the oom_score_adj again. If you want to implement this as a more permanent solution it’s recommended that you automate this adjustment whenever Neo4j starts, for its new PID.