# Nohang Nohang is a highly configurable daemon for Linux which is able to correctly prevent [out of memory](https://en.wikipedia.org/wiki/Out_of_memory) (OOM) and keep system responsiveness in low memory conditions. ## What is the problem? OOM conditions may cause [freezes](https://en.wikipedia.org/wiki/Hang_(computing)), [livelocks](https://en.wikipedia.org/wiki/Deadlock#Livelock), drop [caches](https://en.wikipedia.org/wiki/Page_cache) and processes to be killed (via sending [SIGKILL](https://en.wikipedia.org/wiki/Signal_(IPC)#SIGKILL)) instead of trying to terminate them correctly (via sending [SIGTERM](https://en.wikipedia.org/wiki/Signal_(IPC)#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](https://serverfault.com/questions/390623/how-do-i-prevent-linux-from-freezing-when-out-of-memory) > "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](https://serverfault.com/questions/390623/how-do-i-prevent-linux-from-freezing-when-out-of-memory#comment417508_390625) Also look at [Why are low memory conditions handled so badly?](https://www.reddit.com/r/linux/comments/56r4xj/why_are_low_memory_conditions_handled_so_badly/) (discussion with 480+ posts on r/linux). ## Solution - Use of [earlyoom](https://github.com/rfjakob/earlyoom). This is a simple and lightweight OOM preventer written in C. - Use of [oomd](https://github.com/facebookincubator/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, cmdlines and UIDs with regular expressions - possibility of restarting processes via command like `systemctl restart something` if the process is selected as a victim (or run any other command) - GUI notifications: - OOM prevention results (displays sended signal and displays PID and name of victim) - Low memory warnings (displays available memory and name of fattest process) - `zram` support (`mem_used_total` as a trigger) - customizable intensity of monitoring - convenient configuration with a well commented [config file](https://github.com/hakavlad/nohang/blob/master/nohang.conf) ## Demo Nohang prevents Out Of Memory with GUI notifications: [video](https://youtu.be/ChTNu9m7uMU) ![pic](https://i.imgur.com/7Fj8nZR.png) ## 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](https://github.com/hakavlad/nohang/releases).** Current version may be more unstable. ```bash $ 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: ```bash $ sudo make install ``` To enable and start on systems with systemd: ```bash $ sudo make systemd ``` To uninstall: ```bash $ sudo make uninstall ``` For Arch Linux, there's an [AUR package](https://aur.archlinux.org/packages/nohang-git/). Use your favorite [AUR helper](https://wiki.archlinux.org/index.php/AUR_helpers). ## 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](https://github.com/hakavlad/nohang/blob/master/nohang.conf). 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, cmdlines and UIDs 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_score_adj Uid Pid Name VmRSS VmSwap cmdline --------- ------------- ----- ----- --------------- -------- -------- ------- 314 300 1000 991 chromium 84 M 0 M /usr/lib/chromium/chromium --type=renderer --field-trial-handle=868244496792098610,5765419126773948943,131072 --service-pipe-token=14782672631740123203 --lang=ru --user-data-dir=/tmp/tmp.TJ91B6F0zB --disable-client-side-phishing-detection --enable-offline-auto-reload --enable-offline-auto-reload-visible-only --num-raster-threads=1 --service-request-channel-token=14782672631740123203 --renderer-client-id=4 --no-v8-untrusted-code-mitigations --shared-files=v8_context_snapshot_data:100,v8_natives_data:101 307 300 1000 1124 chromium 44 M 0 M /usr/lib/chromium/chromium --type=renderer --field-trial-handle=868244496792098610,5765419126773948943,131072 --service-pipe-token=10276223625123198448 --lang=ru --user-data-dir=/tmp/tmp.TJ91B6F0zB --disable-client-side-phishing-detection --enable-offline-auto-reload --enable-offline-auto-reload-visible-only --num-raster-threads=1 --service-request-channel-token=10276223625123198448 --renderer-client-id=6 --no-v8-untrusted-code-mitigations --shared-files=v8_context_snapshot_data:100,v8_natives_data:101 217 200 1000 962 chromium 99 M 0 M /usr/lib/chromium/chromium --type=gpu-process --field-trial-handle=868244496792098610,5765419126773948943,131072 --user-data-dir=/tmp/tmp.TJ91B6F0zB --disable-breakpad --gpu-preferences=KAAAAAAAAACAAABAAQAAAAAAAAAAAGAAAAAAAAEAAAAIAAAAAAAAAAgAAAAAAAAA --user-data-dir=/tmp/tmp.TJ91B6F0zB --service-request-channel-token=2848128951654484113 202 200 1000 1032 chromium 16 M 0 M /usr/lib/chromium/chromium --type=-broker 43 0 1000 736 firefox-esr 251 M 0 M /usr/lib/firefox-esr/firefox-esr 21 0 1000 914 chromium 124 M 0 M /usr/lib/chromium/chromium --show-component-extension-options --ignore-gpu-blacklist --no-default-browser-check --disable-pings --media-router=0 --enable-remote-extensions --user-data-dir=/tmp/tmp.TJ91B6F0zB 17 0 1000 844 Web Content 103 M 0 M /usr/lib/firefox-esr/plugin-container -greomni /usr/lib/firefox-esr/omni.ja -appomni /usr/lib/firefox-esr/browser/omni.ja -appdir /usr/lib/firefox-esr/browser 736 true tab 16 0 1000 31555 dolphin 95 M 0 M dolphin 15 0 0 863 Xorg 92 M 0 M /usr/lib/xorg/Xorg :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch 8 0 110 860 tor 50 M 0 M /usr/bin/tor --defaults-torrc /usr/share/tor/tor-service-defaults-torrc -f /etc/tor/torrc --RunAsDaemon 0 8 0 1000 918 chromium 48 M 0 M /usr/lib/chromium/chromium --type=zygote --user-data-dir=/tmp/tmp.TJ91B6F0zB 7 0 1000 1106 mate-panel 43 M 0 M mate-panel 6 0 1000 1157 wnck-applet 35 M 0 M /usr/lib/mate-panel/wnck-applet ``` Kthreads and Pid 1 will not be displayed. ## 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: ```bash $ sudo journalctl -eu nohang ``` See also `man journalctl`. ## Known problems - Awful documentation - regex matching need fixes ## Feedback Please create [issues](https://github.com/hakavlad/nohang/issues). Use cases, feature requests and any questions are welcome. ## Changelog - In progress - Improve modifing badness by matching with RE pattern: - Add suppot matching `cmdline` and `UID` with regular expressions - Fix: replace `re.fullmatch()` by `re.search()` - Validation RE patterns at startup - Improve output: - Display `UID`, `VmSize`, `RssAnon`, `RssFile`, `RssShmem` and `cmdline` of the victim in corrective action reports - Print in terminal with colors - Optimize limiting `oom_score_adj`: now it can works without UID=0 - Optimize GUI warnings: find env without run `ps` and `env` (partially implemented) - Fix conf parsing: use of `line.partition('=')` instead of `line.split('=')` - Add `oom-sort` - Add `oom-trigger` (partially implemented) - [v0.1](https://github.com/hakavlad/nohang/releases/tag/v0.1), 2018-11-23 - 1st release