Ubuntu is quickly running out of RAM, and my computer is starting to freeze. What command will solve this?

It happens pretty often to me when I am compiling software in the background and suddenly everything starts to slow down and eventually freeze up [if I do nothing], as I have run out of both RAM and swap space.

This question assumes that I have enough time and resources to open up Gnome Terminal, search through my history, and execute one sudo command.

What command can save me from having to do a hard reboot, or any reboot at all?

Solution 1:

In my experience Firefox and Chrome use more RAM than my first 7 computers combined. Probably more than that but I'm getting away from my point. The very first thing you should do is close your browser. A command?

killall -9 firefox google-chrome google-chrome-stable chromium-browser

I've tied the most popular browsers together into one command there but obviously if you're running something else (or know you aren't using one of these) just modify the command. The killall -9 ... is the important bit. People do get iffy about SIGKILL (signal number 9) but browsers are extremely resilient. More than that, terminating slowly via SIGTERM will mean the browser does a load of cleanup rubbish —which requires a burst of additional RAM— and that's something you can't afford in this situation.

If you can't get that into an already-running terminal or an Alt+F2 dialogue, consider switching to a TTY. Control + Alt + F2 will get you to TTY2 which should allow you to login (though it might be slow) and should even let you use something like htop to debug the issue. I don't think I've ever run out of RAM to the point I couldn't get htop up.

The long term solution involves either buying more RAM, renting it via a remote computer, or not doing what you're currently doing. I'll leave the intricate economic arguments up to you but generally speaking, RAM is cheap to buy, but if you only need a burst amount, a VPS server billed per minute, or hour is a fine choice.

Solution 2:

On a system with the Magic System Request Key enabled, pressing Alt + System Request + f (if not marked on your keyboard, System Request is often on the Print Screen key) will manually invoke the kernel's out of memory killer (oomkiller), which tries to pick the worst offending process for memory usage and kill it. You can do this if you have perhaps less time than you've described and the system is just about to start (or maybe has already started) thrashing - in which case you probably don't care exactly what gets killed, just that you end up with a usable system. Sometimes this can end up killing X, but most of the time these days it's a lot better at picking a bad process than it used to be.

Solution 3:

Contrary to other answers, I suggest that you disable swap while you are doing this. While swap keeps your system running in a predictable manner, and is often used to increase the throughput of applications accessing the disk (by evicting unused pages to allow room for the disk cache), in this case it sounds like your system is being slowed down to unusable levels because too much actively used memory is being forcibly evicted to swap.

I would recommend disabling swap altogether while doing this task, so that the out-of-memory killer will act as soon as the RAM fills up.

Alternative solutions:

• Increase the read speed of swap by putting your swap partition in RAID1
  • Or RAID0 if you're feeling risky but that will bring down a large number of running programs if any of your disks malfunction.
• Decrease the number of concurrent build jobs ("more cores = more speed", we all say, forgetting that it takes a linear toll on RAM)
• This could go both ways, but try enabling zswap in the kernel. This compresses pages before they are sent to swap, which may provide just enough wiggle room to speed your machine up. On the other hand, it could just end up being a hindrance with the extra compression/decompression it does.
• Turn down optimisations or use a different compiler. Optimising code can sometimes take up several gigabytes of memory. If you have LTO turned on, you're going to use a lot of RAM at the link stage too. If all else fails, you can try compiling your project with a lighter-weight compiler (e.g. tcc), at the expense of a slight runtime performance hit to the compiled product. (This is usually acceptable if you're doing this for development/debugging purposes.)

Solution 4:

You can use the following command (repeatedly if needed) to kill the process using the most RAM on your system:

ps -eo pid --no-headers --sort=-%mem | head -1 | xargs kill -9

With:

• ps -eo pid --no-headers --sort=-%mem: display the process ids of all running processes, sorted by memory usage
• head -1: only keep the first line (process using the most memory)
• xargs kill -9: kill the process

Edit after Dmitry's accurate comment:

This is a quick and dirty solution that should be executed when there are no sensitive tasks running (tasks that you don't want to kill -9).