earlyoom - The Early OOM Daemon
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.
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.
What does it do
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
The percentage value is configurable via command line
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
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 (
- 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.
Why not trigger the kernel oom killer?
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
How much memory does earlyoom use?
2 MiB (
VmRSS), though only
220 kiB is private memory (
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.
Download and compile
Compiling yourself is easy:
git clone https://github.com/rfjakob/earlyoom.git cd earlyoom make
Optional: Run the integrated self-tests:
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:
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 earlyoom v1.4-6-ga4021ae mem total: 9823 MiB, swap total: 9823 MiB sending SIGTERM when mem <= 10 % and swap <= 10 %, SIGKILL when mem <= 5 % and swap <= 5 % Could not lock memory - continuing anyway: Cannot allocate memory mem avail: 5091 of 9823 MiB (51 %), swap free: 9823 of 9823 MiB (100 %) mem avail: 5084 of 9823 MiB (51 %), swap free: 9823 of 9823 MiB (100 %) mem avail: 5086 of 9823 MiB (51 %), swap free: 9823 of 9823 MiB (100 %) [...]
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:
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: sending SIGTERM to process 7378 uid 1000 "tail": badness 156, VmRSS 4962 MiB
For older versions of
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_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;
processes to avoid killing. Processes is specified by a POSIX regular expression.
For instance, to avoid having
bar be killed:
earlyoom --avoid '^(foo|bar)$'
The regex is matched against the basename of the process as shown
If you are running earlyoom as a system service (through systemd or init.d), you can adjust its configuration via the file provided in
/etc/default/earlyoom. The file already contains some examples in the comments, which you can use to build your own set of configuration based on the supported command line options, for example:
EARLYOOM_ARGS="-m 5 -r 60 --avoid '(^|/)(init|Xorg|ssh)$' --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.
Command line options
earlyoom v1.6.2-34-g75a8852-dirty 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 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 --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) -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
vNEXT, in progress
- Make victim selection logic 50% faster by lazy-loading process attributes
- Log the user id
uidof 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 (
- Add unit-test benchmarks (
- Drop root privileges in systemd unit file
- Fix spurious testsuite failure on systems with a lot of RAM (issue #156)
- 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
- 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)
- Wait for processes to actually exit when sending a signal
- 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)
- 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)
--helpin addition to
- 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
- Add support for GUI notifications, add options
-Soptions (@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
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
v0.6: Add command-line options
v0.5: Add swap support
v0.2: Add systemd unit file
v0.1: Initial release