In our role as Outsourced Tech Support for web hosts, VPS customers hosted in OpenVZ or Virtuozzo servers often report their sites crashing with the error OOM killed process httpd.

It often happens with high traffic websites or with sites hosted in starter plans. Today we’ll discuss what this error means and how we fix it for our customers.

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What is meant by OOM errors?

OOM is short for Out Of Memory manager. It is a kernel mechanism in server virtualization systems to save the VPS when the available memory becomes too low.

Linux kernel by default gives every httpd process whatever memory it asks for, with the assumption that not all processes are going to use it. However, in high traffic websites, the number of such processes becomes too high and uses up almost all the memory in the VPS.

Then the OOM manager activates and kills off processes with a high oom_score value (calculated based on the process’s memory usage and its priority).

Now, let’s map this to some actual VPS settings. If the VPS uses SLM memory, the guaranteed amount of RAM for the container is set by the “soft limit” memory setting and the burstable RAM is set by the “hard limit” memory setting.

Let’s say the VPS has the following memory setting.

soft limit = 512
hard limit = 1024

This means the kernel will lay low until the actual memory usage exceeds 512 MB. After that limit, as the memory usage approaches 1024 MB, OOM gets activated, and starts killing the httpd processes.

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Resolving OOM errors

Un-optimized websites are the top reason for this error. Website issues could be caused by poorly coded plugins or un-optimized custom website code. Another reason is a surge in website traffic, either due to a marketing campaign, or due to DoS attacks.

If the usage is legitimate, the solution is to increase the memory. If it is not, the solution is to implement Web Application Firewalls, or remove un-optimized sections of the website.

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