nixos-install/nixos-rebuild fails with "Out of memory" on systems with 512MB RAM

[sasoiliev]

Describe the bug
512MB of RAM seem to be insufficient for a successful NixOS installation via the installer. Bumping the memory size of the VM from the steps to reproduce below ends up in a successful installation, but bringing the memory back down to 512MB renders the VM unable to do nixos-rebuild switch (fails with error: unable to fork: Cannot allocate memory).

Even though 512MB is quite low by today's standards, there are still use cases where you don't really need more than that (e.g., I discovered this while building a VM that will act as a Wireguard VPN endpoint for a couple of boxes doing just routing between them).

I didn't find any requirements in the NixOS Manual regarding the amount of memory needed. I would suggest adding this in the documentation at minimum. (I can try to submit a PR if the project maintainers agree with my suggestion.)

(Apologies if this is a known problem, I couldn't find an existing issue describing it.)

To Reproduce
Steps to reproduce the behavior:

1. Create a qemu disk image: qemu-img create /tmp/nixos.img 8G
2. Start a qemu VM with 512MB RAM and the NixOS minimal ISO: qemu-kvm -m 512M -drive file=/tmp/nixos.img -cdrom ~/Downloads/nixos-minimal-19.09.1957.57c35c63004-x86_64-linux.iso
3. Create a Linux partition through sudo fdisk
4. Create a filesystem: sudo mkfs.ext4 /dev/sda1
5. Mount the partition under /mnt: sudo mount /dev/sda1 /mnt
6. Generate the NixOS config: sudo nixos-generate-config --root /mnt
7. Set the GRUB boot device - in /mnt/etc/nixos/configuration.nix uncomment the boot.loader.grub.device setting
8. Run the installer: sudo nixos-install --root /mnt

Actual behavior
The installation fails with:

building the configuration in /mnt/etc/nixos/configuration.nix...
[  103.481160] Out of memory: Kill process 891 (nix) score 749 or sacrifice child
[  103.481676] Killed process 891 (nix) total-vm:494588kB, anon-rss:371860kB, file-rss:0kB, shmem-rss:8kB
/run/current-system/sw/bin/nixos-install: line 99:   891 Killed                       ...

Expected behavior
The NixOS installer completes successfully.

Metadata

- system: `"x86_64-linux"`
- host os: `Linux 4.19.98, NixOS, 19.09.1957.57c35c63004 (Loris)`
- multi-user: `yes`
- sandbox: `yes`
- version: `nix-env (Nix) 2.3.2`
- channels(root): `"nixos-19.09.1957.57c35c63004"`
- nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[eadwu]

Have you tried using swap?

[sasoiliev]

@eadwu, fair enough. Documenting the minimum requirements for building a NixOS configuration is still worthwhile IMO.

[ghost]

Thanks for the detailed description. I agree we should have some documentation on the minimum system requirements for evaluating a normal NixOS config.

However there are ways around this. You can actually install NixOS without running a rebuild on the target.

• Boot the NixOS ISO or kexec a NixOS netboot image
• Set up ssh access
• Do partitioning, formatting and mounting
• On another NixOS computer:
  • ssh root@target "nixos-generate-config --root /mnt --show-hardware-config" > ./hardware-configuration.nix
  • Write a configuration for the new machine in ./configuration.nix and make sure it imports ./hardware-configuration.nix
  • nixos-rebuild build -I nixos-config=./configuration.nix
  • nix-store --export $(nix-store -qR result) | ssh root@target "nix-store --import --store /mnt"
  • ssh root@target "nixos-install --system $(readlink result)"
• reboot the target machine
• profit

Future rebuilds can be done with nixos-rebuild switch --target-host root@target --build-host localhost -I nixos-config=./configuration.

[lheckemann]

https://discourse.nixos.org/t/adding-nixos-entry-to-libosinfo-recommended-resources/6441/ somewhat related to system requirements documentation

[stale]

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[raidenovich]

I'm having this issue with 16 gb of ram, probably a bug

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/building-nixos-on-lower-end-systems/51914/1

[eclairevoyant]

I'm having this issue with 16 gb of ram, probably a bug

Depends what you're building and what your nix settings are.
This is either an RTM moment or the same as #320528, depending on what your scenario is.
Closing this as the main discussion is happening on the other issue.