We could instead set sudo to true, so that machine would have 7G of ram: https://github.com/NixOS/nixpkgs/blob/master/.travis.yml#L5

I don't understand why @matthewbauer switched to container based infrastructure in 95b28da, but it did decrease ram significantly. Maybe that should be noted as a comment.

travis docs about this: https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments

@domenkozar, added this:

# 'sudo: false' == containers that start fast, but only get 4G ram;
# 'sudo: true'  == VMs that start slow, but with 8G
# ..as per:  https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments
# Nixpkgs PR tests OOM with 4G: https://github.com/NixOS/nixpkgs/issues/24200

Line 11 should be sudo: required so PR additional line is not needed.

Citing @edolstra wrt memory usage here: NixOS/nix#421 (comment)

You can tell Nix to garbage-collect more aggressively:

GC_INITIAL_HEAP_SIZE=128k nix-env -qa

With this, nix-env -qa takes 235 MB on my laptop. (Without it, 471 MB.)

You can also do:

echo 1 > /proc/sys/vm/overcommit_memory

That should make the fork error go away.

It seems to work again:

error: undefined variable ‘fetchPypi’ at /home/travis/build/NixOS/nixpkgs/pkgs/development/python-modules/btrees/default.nix:10:9
(use ‘--show-trace’ to show detailed location information)