Nohang

Nohang is a highly configurable daemon for Linux which is able to correctly prevent out of memory (OOM) and keep system responsiveness in low memory conditions.

What is the problem?

OOM conditions may cause freezes, livelocks, drop caches and processes to be killed (via sending SIGKILL) instead of trying to terminate them correctly (via sending SIGTERM or takes other corrective action). Some applications may crash if it's impossible to allocate memory.

Here are the statements of some users:

"How do I prevent Linux from freezing when out of memory? Today I (accidentally) ran some program on my Linux box that quickly used a lot of memory. My system froze, became unresponsive and thus I was unable to kill the offender. How can I prevent this in the future? Can't it at least keep a responsive core or something running?"

— serverfault

"With or without swap it still freezes before the OOM killer gets run automatically. This is really a kernel bug that should be fixed (i.e. run OOM killer earlier, before dropping all disk cache). Unfortunately kernel developers and a lot of other folk fail to see the problem. Common suggestions such as disable/enable swap, buy more RAM, run less processes, set limits etc. do not address the underlying problem that the kernel's low memory handling sucks camel's balls."

— serverfault

Also look at Why are low memory conditions handled so badly? (discussion with 480+ posts on r/linux).

Solution

• Use of earlyoom. This is a simple and lightweight OOM preventer written in C.
• Use of oomd. This is a userspace OOM killer for linux systems whitten in C++ and developed by Facebook.
• Use of nohang.

Some features

• SIGKILL and SIGTERM as signals that can be sent to the victim
• impact on the badness of processes via matching their names and cmdlines with regular expressions
• possibility of restarting processes via command like systemctl restart something if the process is selected as a victim
• GUI notifications: OOM prevention results and low memory warnings
• zram support (mem_used_total as a trigger)
• customizable intensity of monitoring
• convenient configuration with a well commented config file

Demo

Nohang prevents Out Of Memory with GUI notifications: video

Requirements

For basic usage:

• Linux 3.14+
• Python 3.4+

To show GUI notifications:

• libnotify (Fedora, Arch Linux) or libnotify-bin (Debian GNU/Linux, Ubuntu)
• sudo and procps if nohang is started as root

Memory and CPU usage

• VmRSS is 10 — 14 MiB depending on the settings (about 10 MiB by default)
• CPU usage depends on the level of available memory (the frequency of memory status checks increases as the amount of available memory decreases) and monitoring intensity (can be changed by the user via the config)

Download, install, uninstall

Please use the latest release version. Current version may be more unstable.

$ git clone https://github.com/hakavlad/nohang.git
$ cd nohang

Run without installing (low memory warnings may not work; note that processes with UID != your UID will not receive signals if nohang is started as a regular user):

$ ./nohang

To install:

$ sudo make install

To enable and start on systems with systemd:

$ sudo make systemd

To uninstall:

$ sudo make uninstall

For Arch Linux, there's an AUR package. Use your favorite AUR helper.

Command line options

./nohang -h
usage: nohang [-h] [-c CONFIG]

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG, --config CONFIG
                        path to the config file, default values:
                        ./nohang.conf, /etc/nohang/nohang.conf

How to configure nohang

The program can be configured by editing the config file. The configuration includes the following sections:

1. Memory levels to respond to as an OOM threat
2. The frequency of checking the level of available memory (and CPU usage)
3. The prevention of killing innocent victims
4. Impact on the badness of processes via matching their names and cmdlines with regular expressions
5. The execution of a specific command instead of sending the SIGTERM signal
6. GUI notifications:
   • results of preventing OOM
   • low memory warnings
7. Preventing the slowing down of the program
8. Output verbosity

Just read the description of the parameters and edit the values. Please restart nohang to apply changes. Default path to the config after installing is /etc/nohang/nohang.conf.

oom-sort

oom-sort is an additional diagnostic tool that will be installed with nohang package. It sorts the processes in descending order of their oom_score and also displays oom_score_adj, Uid, Pid, Name, VmRSS, VmSwap and optionally cmdline. Run oom-sort --help for more info.

Usage:

$ oom-sort

Output like follow:

oom_score oom_adj oom_score_adj   Pid Name                 RSS       Swap
--------- ------- ------------- ----- --------------- --------- ---------
      314       5           300  3519 chromium             85 M       0 M
      307       5           300  3581 chromium             44 M       0 M
      215       3           200  3485 chromium             93 M       0 M
      202       3           200  3509 chromium             16 M       0 M
      133       0             0  2696 firefox-esr         782 M       0 M
       74       0             0  3150 VirtualBox          436 M       0 M
       38       0             0  3388 Web Content         223 M       0 M

oom-trigger

Interactive OOM trigger (not implemented)

Logging

If nohang is installed on a system that uses systemd, you can use the following command to view the log:

$ sudo journalctl -eu nohang

See also man journalctl.

Known problems

• Awful documentation

Feedback

Please create issues. Use cases, feature requests and any questions are welcome.

Changelog

• In progress

  • add oom-sort
  • add oom-trigger
  • fix regex matching: replace re.fullmatch() by re.search()
  • add suppot cmdline regex matching
  • improve output: display cmdline and Uid in corrective action reports

• v0.1, 2018-11-23

  • 1st release