The oom-killer generally has a bad reputation among Linux users. This may be part of the reason Linux invokes it only when it has absolutely no other choice. It will swap out the desktop environment, drop the whole page cache and empty every buffer before it will ultimately kill a process. At least that's what I think that it will do. I have yet to be patient enough to wait for it, sitting in front of an unresponsive system.

This made me and other people wonder if the oom-killer could be configured to step in earlier: reddit r/linux, superuser.com, unix.stackexchange.com.

As it turns out, no, it can't. At least using the in-kernel oom-killer. In the user space, however, we can do whatever we want.

earlyoom wants to be simple and solid. It is written in pure C with no dependencies. An extensive test suite (unit- and integration tests) is written in Go.

earlyoom checks the amount of available memory and free swap up to 10 times a second (less often if there is a lot of free memory). By default if both are below 10%, it will kill the largest process (highest oom_score). The percentage value is configurable via command line arguments.

In the free -m output below, the available memory is 2170 MiB and the free swap is 231 MiB.

              total        used        free      shared  buff/cache   available
Mem:           7842        4523         137         841        3182        2170
Swap:          1023         792         231

Why is "available" memory checked as opposed to "free" memory? On a healthy Linux system, "free" memory is supposed to be close to zero, because Linux uses all available physical memory to cache disk access. These caches can be dropped any time the memory is needed for something else.

The "available" memory accounts for that. It sums up all memory that is unused or can be freed immediately.

Note that you need a recent version of free and Linux kernel 3.14+ to see the "available" column. If you have a recent kernel, but an old version of free, you can get the value from grep MemAvailable /proc/meminfo.

When both your available memory and free swap drop below 10% of the total memory available to userspace processes (=total-shared), it will send the SIGTERM signal to the process that uses the most memory in the opinion of the kernel (/proc/*/oom_score).

• nohang, a similar project like earlyoom, written in Python and with additional features and configuration options.
• facebooks's pressure stall information (psi) kernel patches and the accompanying oomd userspace helper. The patches are merged in Linux 4.20.

earlyoom does not use echo f > /proc/sysrq-trigger because:

In some kernel versions (tested on v4.0.5), triggering the kernel oom killer manually does not work at all. That is, it may only free some graphics memory (that will be allocated immediately again) and not actually kill any process. Here you can see how this looks like on my machine (Intel integrated graphics).

This problem has been fixed in Linux v5.17 (commit f530243a) .

Like the Linux kernel would, earlyoom finds its victim by reading through /proc/*/oom_score.

About 2 MiB (VmRSS), though only 220 kiB is private memory (RssAnon). The rest is the libc library (RssFile) that is shared with other processes. All memory is locked using mlockall() to make sure earlyoom does not slow down in low memory situations.

Compiling yourself is easy:

git clone https://github.com/rfjakob/earlyoom.git
cd earlyoom
make

Optional: Run the integrated self-tests:

make test

Start earlyoom automatically by registering it as a service:

sudo make install              # systemd
sudo make install-initscript   # non-systemd

Note that for systems with SELinux disabled (Ubuntu 19.04, Debian 9 ...) chcon warnings reporting failure to set the context can be safely ignored.

For Debian 10+ and Ubuntu 18.04+, there's a Debian package:

sudo apt install earlyoom

For Fedora and RHEL 8 with EPEL, there's a Fedora package:

sudo dnf install earlyoom
sudo systemctl enable --now earlyoom

For Arch Linux, there's an Arch Linux package:

sudo pacman -S earlyoom
sudo systemctl enable --now earlyoom

Availability in other distributions: see repology page.

Just start the executable you have just compiled:

./earlyoom

It will inform you how much memory and swap you have, what the minimum is, how much memory is available and how much swap is free.

./earlyoom
eearlyoom v1.8
mem total: 23890 MiB, user mem total: 21701 MiB, swap total: 8191 MiB
sending SIGTERM when mem avail <= 10.00% and swap free <= 10.00%,
        SIGKILL when mem avail <=  5.00% and swap free <=  5.00%
mem avail: 20012 of 21701 MiB (92.22%), swap free: 5251 of 8191 MiB (64.11%)
mem avail: 20031 of 21721 MiB (92.22%), swap free: 5251 of 8191 MiB (64.11%)
mem avail: 20033 of 21723 MiB (92.22%), swap free: 5251 of 8191 MiB (64.11%)
[...]

If the values drop below the minimum, processes are killed until it is above the minimum again. Every action is logged to stderr. If you are running earlyoom as a systemd service, you can view the last 10 lines using

systemctl status earlyoom

In order to see earlyoom in action, create/simulate a memory leak and let earlyoom do what it does:

tail /dev/zero

If you need any further actions after a process is killed by earlyoom (such as sending emails), you can parse the logs by:

sudo journalctl -u earlyoom | grep sending

Example output for above test command (tail /dev/zero) will look like:

Feb 20 10:59:34 debian earlyoom[10231]: sending SIGTERM to process 7378 uid 1000 "tail": oom_score 156, VmRSS 4962 MiB

For older versions of earlyoom, use:

sudo journalctl -u earlyoom | grep -iE "(sending|killing)"

Since version 1.6, earlyoom can send notifications about killed processes via the system d-bus. Pass -n to enable them.

To actually see the notifications in your GUI session, you need to have systembus-notify running as your user.

Additionally, earlyoom can execute a script for each process killed, providing information about the process via the EARLYOOM_PID, EARLYOOM_UID and EARLYOOM_NAME environment variables. Pass -N /path/to/script to enable.

Warning: In case of dryrun mode, the script will be executed in rapid succession, ensure you have some sort of rate-limit implemented.

The command-line flag --prefer specifies processes to prefer killing; likewise, --avoid specifies processes to avoid killing. See https://github.com/rfjakob/earlyoom/blob/master/MANPAGE.md#--prefer-regex for details.

If you are running earlyoom as a system service (through systemd or init.d), you can adjust its configuration via the file provided in /etc/default/earlyoom. The file already contains some examples in the comments, which you can use to build your own set of configuration based on the supported command line options, for example:

EARLYOOM_ARGS="-m 5 -r 60 --avoid '(^|/)(init|Xorg|ssh)$' --prefer '(^|/)(java|chromium)$'"

After adjusting the file, simply restart the service to apply the changes. For example, for systemd:

systemctl restart earlyoom

Please note that this configuration file has no effect on earlyoom instances outside of systemd/init.d.

earlyoom v1.8
Usage: ./earlyoom [OPTION]...

  -m PERCENT[,KILL_PERCENT] set available memory minimum to PERCENT of total
                            (default 10 %).
                            earlyoom sends SIGTERM once below PERCENT, then
                            SIGKILL once below KILL_PERCENT (default PERCENT/2).
  -s PERCENT[,KILL_PERCENT] set free swap minimum to PERCENT of total (default
                            10 %).
                            Note: both memory and swap must be below minimum for
                            earlyoom to act.
  -M SIZE[,KILL_SIZE]       set available memory minimum to SIZE KiB
  -S SIZE[,KILL_SIZE]       set free swap minimum to SIZE KiB
  -n                        enable d-bus notifications
  -N /PATH/TO/SCRIPT        call script after oom kill
  -g                        kill all processes within a process group
  -d, --debug               enable debugging messages
  -v                        print version information and exit
  -r INTERVAL               memory report interval in seconds (default 1), set
                            to 0 to disable completely
  -p                        set niceness of earlyoom to -20 and oom_score_adj to
                            -100
  --ignore-root-user        do not kill processes owned by root
  --sort-by-rss             find process with the largest rss (default oom_score)
  --prefer REGEX            prefer to kill processes matching REGEX
  --avoid REGEX             avoid killing processes matching REGEX
  --ignore REGEX            ignore processes matching REGEX
  --dryrun                  dry run (do not kill any processes)
  --syslog                  use syslog instead of std streams
  -h, --help                this help text

See the man page for details.

Bug reports and pull requests are welcome via github. In particular, I am glad to accept

• Use case reports and feedback

• We don't use procps/libproc2 because procps_pids_select(), for some reason, always parses /proc/$pid/status. This is relatively expensive, and we don't need it.

• v1.8.2, 2024-05-07

  • Fixes in earlyoom.service systemd unit file
    • Add process_mrelease to allowed syscalls (commit)
    • Fix IPAddressDeny syntax (commit)
    • Allow -p (commit)

• v1.8.1, 2024-04-17

  • Fix trivial test failures caused by message rewording (commit)

• v1.8, 2024-04-15

  • Introduce user mem total / meminfo_t.UserMemTotal and calculate MemAvailablePercent based on it (commit, more info in man page)
  • Use process_mrelease (#266)
  • Support NO_COLOR (https://no-color.org/)
  • Don't get confused by processes with a zombie main thread (commit)
  • Add --sort-by-rss, thanks @RanHuang! This will select a process to kill acc. to the largest RSS instead of largest oom_score.
  • The Gitlab CI testsuite now also runs on Amazon Linux 2 and Oracle Linux 7.

• v1.7, 2022-03-05

  • Add -N flag to run a script every time a process is killed (commit, man page section)
  • Add -g flag to kill whole process group (#247)
  • Remove -i flag (ignored for compatibility), it does not work properly on Linux kernels 5.9+ (#234)
  • Hardening: Drop ambient capabilities on startup (#234)

• v1.6.2, 2020-10-14

  • Double-check memory situation before killing victim (commit)
  • Never terminate ourselves (#205)
  • Dump buffer on /proc/meminfo conversion error (#214)

• 1.6.1, 2020-07-07

  • Clean up dbus-send zombie processes (#200)
  • Skip processes with oom_score_adj=-1000 (210)

• 1.6, 2020-04-11

  • Replace old notify-send GUI notification logic with dbus-send / systembus-notify (#183)
    • -n/-N now enables the new logic
    • You need to have systembus-notify running in your GUI session for notifications for work
  • Handle /proc mounted with hidepid gracefully (issue #184)

• v1.5, 2020-03-22

  • -p: set oom_score_adj to -100 instead of -1000 (#170)
  • Allow using both -M and -m, and -S and -s. The lower value (converted to percentages) will be used.
  • Set memory report interval in earlyoom.default to 1 hour instead of 1 minute (#177)

• v1.4, 2020-03-01

  • Make victim selection logic 50% faster by lazy-loading process attributes
  • Log the user id uid of killed processes in addition to pid and name
  • Color debug log in light grey
  • Code clean-up
    • Use block-local variables where possible
    • Introduce PATH_LEN to replace several hardcoded buffer lengths
  • Expand testsuite (make test)
  • Run cppcheck when available
  • Add unit-test benchmarks (make bench)
  • Drop root privileges in systemd unit file earlyoom.service

• v1.3.1, 2020-02-27

  • Fix spurious testsuite failure on systems with a lot of RAM (issue #156)

• v1.3, 2019-05-26

  • Wait for processes to actually exit when sending a signal
    • This fixes the problem that earlyoom sometimes kills more than one process when one would be enough (issue #121)
  • Be more liberal in what limits to accepts for SIGTERM and SIGKILL (issue #97)
    • Don't exit with a fatal error if SIGTERM limit < SIGKILL limit
    • Allow zero SIGKILL limit
  • Reformat startup output to make it clear that BOTH swap and mem must be <= limit
  • Add notify_all_users.py helper script
  • Add CODE_OF_CONDUCT.md (Contributor Covenant 1.4) (#102)
  • Fix possibly truncated UTF8 app names in log output (#110)

• v1.2, 2018-10-28

  • Implement adaptive sleep time (= adaptive poll rate) to lower CPU usage further (issue #61)
  • Remove option to use kernel oom-killer (-k, now ignored for compatibility) (issue #80)
  • Gracefully handle the case of swap being added or removed after earlyoom was started (issue 62, commit)
  • Implement staged kill: first SIGTERM, then SIGKILL, with configurable limits (issue #67)

• v1.1, 2018-07-07

  • Fix possible shell code injection through GUI notifications (commit)
  • On failure to kill any process, only sleep 1 second instead of 10 (issue #74)
  • Send the GUI notification after killing, not before (issue #73)
  • Accept --help in addition to -h
  • Fix wrong process name in log and in kill notification (commit 1, commit 2, issue #52, issue #65, issue #194)
  • Fix possible division by zero with -S (commit)

• v1.0, 2018-01-28

  • Add --prefer and --avoid options (@TomJohnZ)
  • Add support for GUI notifications, add options -n and -N

• v0.12: Add -M and -S options (@nailgun); add man page, parameterize Makefile (@yangfl)

• v0.11: Fix undefined behavior in get_entry_fatal (missing return, commit)

• v0.10: Allow to override Makefile's VERSION variable to make packaging easier, add -v command-line option

• v0.9: If oom_score of all processes is 0, use VmRss to find a victim

• v0.8: Use a guesstimate if the kernel does not provide MemAvailable

• v0.7: Select victim by oom_score instead of VmRSS, add options -i and -d

• v0.6: Add command-line options -m, -s, -k

• v0.5: Add swap support

• v0.4: Add SysV init script (thanks @joeytwiddle), use the new MemAvailable from /proc/meminfo (needs Linux 3.14+, commit)

• v0.2: Add systemd unit file

• v0.1: Initial release