1. Overview

Linux, when used with the default settings, will overcommit memory. On one hand, this improves resource utilization. On the other hand, this is dangerous and cannot be directly controlled by applications. That’s why we need to implement analysis and recovery rather than try to mitigate the error.

2. Did the System Report OOM Error?

We can configure Linux to use different policies to allocate memory. We can find the effective policy by mounting the proc filesystem and reading from /proc/sys/vm/. The files overcommit_ratio and overcommit_memory govern how much memory the system may commit that is not backed by physical memory.

OOM (out of memory) killer will only run if we configure the system to overcommit memory. If, however, we wanted to prevent the system from overcommitting we’d set overcommit_memory to 2. In this case, the system would only overcommit total_memory * overcommit_ratio / 100. In other words, if we set overcommit_memory to 2 and overcommit_ratio to 0, the OOM killer would be disabled.

Unfortunately, we need to rely on logs to find out about OOM errors. Linux has a whole zoo of different logging tools. In this article, we’ll use journald to examine the system logs. journald reads the information about kernel events using syslog interface, typically, from a file in the proc filesystem: /proc/kmsg.

Another popular program that reads this file is dmesg. We’ll use journald because, unlike dmesg, it keeps the information available after the system reboot.

3. Finding the Logs

journald is a daemon responsible for collecting and storing system logs. It has a sister program: journalctl for controlling it. Since journald stores structured logs, journalctl gives us plenty of tools to sift through them quickly.

3.1. Non-Interactive Use

If we wanted to find the process id and the program name of the process that was killed by the OOM killer, we could run:

journalctl --list-boots | \
    awk '{ print $1 }' | \
    xargs -I{} journalctl --utc --no-pager -b {} -kqg 'killed process' -o verbose --output-fields=MESSAGE

And it may output something like:

Sun 2021-06-20 18:49:15.791688 UTC [s=f81666ba69664fb88b545f20e7a3eea6;i=4dc1ac1;b=b7a40e823b4a4bd0bdf9d
    MESSAGE=Killed process 3029 (Web Content) total-vm:10206696kB, anon-rss:6584572kB, file-rss:0kB, shm
Tue 2021-07-06 08:58:35.087345 UTC [s=bc64b16638964d46b0df6e203765c3f7;i=4dd75a3;b=20f10519157b4f1a9b244
    MESSAGE=Killed process 131641 (bash) total-vm:11257404kB, anon-rss:11243152kB, file-rss:12kB, shmem-

Let’s take a closer look at the journalctl command options we used:

  • –utc tells journalctl to convert dates to UTC. This is especially useful when dealing with systemd logs imported from other systems whose clock we cannot access. Otherwise, dealing with geographically diverse systems becomes very difficult
  • -b selects logs associated with a particular system boot. There’s no way to select multiple boots in one call. Unfortunately, a reboot or even a series of reboots will often follow an OOM error
  • -k filters log messages using _TRANSPORT=kernel. In other words, it filters the kernel events
  • -q instructs journalctl to omit technical information related to log display. This is useful in scripts searching for any OOM-related events
  • -g runs regular expression search in a manner similar to how search works in Emacs. If the expression contains no capital letters, it is assumed to be case-insensitive
  • –list-boots prints all the boots for which journalctl has logs
  • -o verbose instructs journalctl to print a filtered list of fields associated with the message. Alternatively, we could’ve used -o json, which will produce JSON output for further processing in a log aggregation system
  • –output-fields is a comma-separated list of fields to be printed. journalctl will print some fields unconditionally. Of particular interest are those appearing in the square brackets after the date. The expression inside the brackets specifies the cursor. We can later supply the cursor with the -c option to open the logs in this location

In addition, we used xargs with the -I option to substitute the build number printed by awk into the final journalctl call.

3.2. Interactive Use

The non-interactive example outputs very little valuable information. It might be used to trigger an alert, but it’s not enough to analyze the incident. Once we can access the system that reported the error, we should examine its log in more detail.

To start examining the OOM log interactively, we may launch journalctl like:

$ journalctl --utc -b -X -ke

X should be an integer, (the minus sign is the part of the literal integer rather than an option syntax). journalctl interprets negative integers as counting from the last boot backward, while positive integers count from the first boot forward.

Navigation commands in interactive mode will use less key bindings.

The -e option will fast-forward the pager to the end of the log. Next, we’ll use one of the available search commands, for example, by typing “?” followed by “Killed”. Once the cursor moves to the error message of interest, we can search again, now for “Mem-info”, which will rewind the log to the summary of the state of the virtual memory at the time OOM killer ran, followed by the list of processes and statistics of their memory usage.

4. journald Alternatives

Some Linux systems don’t use journald. A popular tool to manage system logs on Linux is rsyslogd. This tool has a rich system of modules and a number of configuration languages. But, covering all the functionality of rsyslog requires a substantial effort. The entry point to rsyslog configuration is the file /etc/rsyslog.conf. Furthermore, it is usually accompanied by the /etc/rsyslog.d directory, which adds more configuration files.

4.1. rsyslogd Structure

Conceptually, rsyslog consists of these kinds of modules:

  • inputs (the module name will start with “im”, for example, imflie for reading logs from arbitrary files)
  • outputs (the module name will start with “om”, for example, ommysql for storing logs in MySQL database)
  • parser (the module name will start with “pm”, for example, pmlastmsg for dealing with repeating messages)
  • there are few more module types, serving more technical functions.

4.2. rsyslog Configuration

Let’s take a look at some common patterns in rsyslog configuration, especially those that might help us find OOM-related messages. A configuration rule for matching kernel messages will typically look like this:

kern.*				-/var/log/kern.log

This will instruct rsyslog to store messages coming from the kernel in a file called /var/log/kern.log. If this file is too long, rsyslog will typically rotate and archive it. Subsequently, zgrep will help us to read from these files:

$ find /var/log -name kern* -exec grep -PnHe 'Killed process' {} + 2>

Here’s an example of the output we’ll see:

/dev/null/var/log/kern.log:5207:Jul  6 11:58:34 cheetah kernel: [401912.220043] Killed process 131641 (bash) total-vm:11257404kB, anon-rss:11243152kB, file-rss:12kB, shmem-rss:0kB

And for archived logs:

$ find /var/log -name kern*.gz -exec zgrep -PnHe 'Killed process' {} + 2>/dev/null

We’ll see output like:

/var/log/kern.log.3.gz:1967:Jun 13 14:41:02 cheetah kernel: [347531.586344] Killed process 98254 (chrome) total-vm:889684kB, anon-rss:113068kB, file-rss:0kB, shmem-rss:1784kB
/var/log/kern.log.3.gz:2861:Jun 13 14:41:22 cheetah kernel: [347551.430284] Killed process 102503 (chrome) total-vm:911152kB, anon-rss:104748kB, file-rss:0kB, shmem-rss:1968kB
/var/log/kern.log.3.gz:3781:Jun 13 14:41:52 cheetah kernel: [347582.087504] Killed process 96187 (chrome) total-vm:873296kB, anon-rss:103512kB, file-rss:0kB, shmem-rss:4540kB
/var/log/kern.log.2.gz:1593:Jun 20 21:49:15 cheetah kernel: [630398.445681] Killed process 3029 (Web Content) total-vm:10206696kB, anon-rss:6584572kB, file-rss:0kB, shmem-rss:8732kB

The modular structure of rsyslog is both a curse and a blessing. As a result of great flexibility comes a great number of possible storage locations where our data is sent. It may be sent to a centralized log collector, or to a local database server, or simply to a pseudoterminal, to name a few.

Of course, the choice of loggers is far greater than journald and rsyslogd. To find all the programs potentially logging kernel events, we may try:

$ sudo lsof | grep /dev/kmsg

This will list all the programs that are reading this file, and, potentially, storing its contents somewhere.

5. Conclusion

In this article, we looked at how the operating system stores information about OOM errors and where to look for the information about recent OOM errors.

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