By default, Nix sets the "cores" setting to the number of CPUs which are
physically present on the machine. If cgroups are used to limit the CPU
and memory consumption of a large Nix build, the OOM killer may be
invoked.
For example, consider a GitLab CI pipeline which builds a large software
package. The GitLab runner spawns a container whose CPU is limited to 4
cores and whose memory is limited to 16 GiB. If the underlying machine
has 64 cores, Nix will invoke the build with -j64. In many cases, that
level of parallelism will invoke the OOM killer and the build will
completely fail.
This change sets the default value of "cores" to be
ceil(cpu_quota / cpu_period), with a fallback to
std:🧵:hardware_concurrency() if cgroups v2 is not detected.
`--override-input` id snarky because it takes two arguments, so it
doesn't play well when completed in the middle of the CLI (since the
argument just after gets interpreted as its second argument). So use
`--update-input` instead
I recently got fairly confused why the following expression didn't have
any effect
{
description = "Foobar";
inputs.sops-nix = {
url = github:mic92/sops-nix;
inputs.nixpkgs_22_05.follows = "nixpkgs";
};
}
until I found out that the input was called `nixpkgs-22_05` (please note
the dash vs. underscore).
IMHO it's not a good idea to not throw an error in that case and
probably leave end-users rather confused, so I implemented a small check
for that which basically checks whether `follows`-declaration from
overrides actually have corresponding inputs in the transitive flake.
In fact this was done by accident already in our own test-suite where
the removal of a `follows` was apparently forgotten[1].
Since the key of the `std::map` that holds the `overrides` is a vector
and we have to find the last element of each vector (i.e. the override)
this has to be done with a for loop in O(n) complexity with `n` being
the total amount of overrides (which shouldn't be that large though).
Please note that this doesn't work with nested expressions, i.e.
inputs.fenix.inputs.nixpkgs.follows = "...";
which is a known problem[2].
For the expression demonstrated above, an error like this will be
thrown:
error: sops-nix has a `follows'-declaration for a non-existant input nixpkgs_22_05!
[1] 2664a216e5
[2] https://github.com/NixOS/nix/issues/5790
- Don't use `printf` for the expected result, but just use bash's `$' '`
litteral strings
- Quote the `nix` call result
- Invert the order in the comparisons (just because it feels more
natural)
