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Be aware about the out-of-memory killer

This article was originally posted on the Shelly Cloud blog, our Ruby platform as a service that we have decided to shut down on October 2015.

One of the most important things for applications is stability. There are various hosting platforms that give you virtual servers, where you can run multiple services. There is no limit to the number of processes so it is up to you how much of their resources will be used. However, exceeding all available RAM memory can result in poor stability or even a server crash. This article gives you some basic knowledge about the out-of-memory system state.

OOM is a state where there is no memory available for the system. In such a situation, Kernel will invoke oom-killer to choose and sacrifice some processes, to free up the memory. It may force-kill your puma webservers, sidekiq workers or database.

New Reilc

The server stopped reporting stats to NewRelic at around 11:41. Memory usage (RAM and SWAP) exceeded all server resources and oom-killer killed NewRelic daemon, and other processes, to free up the memory.

[email protected][staging]:~# grep -i kill /var/log/syslog
Jun 17 11:41:37 i-10-255-3-105 kernel: [54493.290148] ruby invoked oom-killer: gfp_mask=0x201da, order=0, oom_score_adj=0
[...]
Jun 17 11:41:41 i-10-255-3-105 kernel: [54498.831237] Out of memory: Kill process 32236 (ruby) score 912 or sacrifice child
Jun 17 11:41:41 i-10-255-3-105 kernel: [54498.831894] Killed process 32236 (ruby) total-vm:209732kB, anon-rss:921340kB, file-rss:0kB
[...]

OOM Score

There are various factors taken into consideration for calculating the priority for killing a given process. One of them is oom score adjustment. The user can adjust the priority by using a special file placed in process information pseudo-filesystem called /proc

# set the highest priority
echo 1000 > /proc/[PID]/oom_score_adj

# set the lowest priority
echo -1000 > /proc/[PID]/oom_score_adj

The highest priority means that the particular process will always be marked for killing in the case of an out-of-memory state. However, the process with the lowest priority will only be killed if there are no other processes with a higher score. oom_score_adj can be used as one factor for calculating the oom_score, which is used by oom-killer as a final score.

Shelly Cloud sets the lowest priority for critical services such as sshd, monitoring or databases and the highest priority for application services.

Monitoring

Monitoring is a key factor in such situations. If a process was killed by kernel there should be a service that will bring it back to action. Otherwise the server may become unresponsive and only a complete restart will help.

Each virtual server on Shelly Cloud has its own monitoring.

Conclusion

You should always keep an eye on the available resources to avoid dangerous situations. We recommend NewRelic and their free plan for monitoring server resources. It allows you to set up an alert if RAM usage is too high. Graphs can help recognize if there is a memory leak, or if your application needs more resources.

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