Backport `nix-eval-jobs --no-instantiate` #987

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opened 2025-09-05 18:24:11 +00:00 by wolfgangwalther · 4 comments

wolfgangwalther commented

2025-09-05 18:24:11 +00:00

Using nix-eval-jobs for pure Eval often has a bottleneck with writing .drv files to disk. This considerably slows down the whole thing. This was the main reason why nix-eval-jobs wasn't used in GitHub Actions as a replacement for OfBorg, but why we ended up with the current "chunked evaluation" strategy.

nix-eval-jobs introduced a --no-instantiate flag recently in https://github.com/nix-community/nix-eval-jobs/pull/379 and the follow up https://github.com/nix-community/nix-eval-jobs/pull/380. My tests in https://github.com/nix-community/nix-eval-jobs/pull/379#issuecomment-3244806892 show that this can even speed up current Evaluation in Nixpkgs.

In https://github.com/NixOS/nixpkgs/pull/433057#issuecomment-3180220683 I outlined a plan to run both Lix and Nix for Eval in Nixpkgs on each PR without needing more resources. To be able to use nix-eval-jobs, this would need to be supported by both.

Describe the solution you'd like

Please backport the two PRs mentioned above / add the same feature to Lix' nix-eval-jobs.

Describe alternatives you've considered

Keeping two different Eval mechanisms around in Nixpkgs forever and not being able to run Lix in the Merge Queue.

Additional context

Introducing nix-eval-jobs to Nixpkgs CI also has additional benefits on top of performance: It will allow us to catch evaluation errors much better, because these are streamed as json output as well, from what I can tell. That means we can treat some of these as "expected", for example "broken", "unsupported", "unfree" etc. - but still report others like failing asserts etc. as actual eval failures. This is currently not consistently possibly in Nixpkgs, not with a lot of work and introducing more complex logic than we'd want in a lot of places (my first step towards that was in https://github.com/NixOS/nixpkgs/pull/426629, but that's only a fraction of what's needed with the current nix-env based approach).

## Is your feature request related to a problem? Please describe. Using `nix-eval-jobs` for pure Eval often has a bottleneck with writing `.drv` files to disk. This considerably slows down the whole thing. This was the main reason why `nix-eval-jobs` wasn't used in GitHub Actions as a replacement for OfBorg, but why we ended up with the current "chunked evaluation" strategy. `nix-eval-jobs` introduced a `--no-instantiate` flag recently in https://github.com/nix-community/nix-eval-jobs/pull/379 and the follow up https://github.com/nix-community/nix-eval-jobs/pull/380. My tests in https://github.com/nix-community/nix-eval-jobs/pull/379#issuecomment-3244806892 show that this can even speed up current Evaluation in Nixpkgs. In https://github.com/NixOS/nixpkgs/pull/433057#issuecomment-3180220683 I outlined a plan to run both Lix and Nix for Eval in Nixpkgs on each PR without needing more resources. To be able to use `nix-eval-jobs`, this would need to be supported by both. ## Describe the solution you'd like Please backport the two PRs mentioned above / add the same feature to Lix' `nix-eval-jobs`. ## Describe alternatives you've considered Keeping two different Eval mechanisms around in Nixpkgs forever and not being able to run Lix in the Merge Queue. ## Additional context Introducing `nix-eval-jobs` to Nixpkgs CI also has additional benefits on top of performance: It will allow us to catch evaluation errors much better, because these are streamed as json output as well, from what I can tell. That means we can treat some of these as "expected", for example "broken", "unsupported", "unfree" etc. - but *still* report others like failing asserts etc. as actual eval failures. This is currently not consistently possibly in Nixpkgs, not with a lot of work and introducing more complex logic than we'd want in a lot of places (my first step towards that was in https://github.com/NixOS/nixpkgs/pull/426629, but that's only a fraction of what's needed with the current `nix-env` based approach).

raito commented

2025-09-08 14:42:33 +00:00

Owner

@ma27 Can you work on a port of this?

ma27 self-assigned this

2025-09-08 19:56:47 +00:00

ma27 commented

2025-09-08 19:56:53 +00:00

Member

Sure.

❤️ 1

qyriad added the

Area/nix-eval-jobs

label

2025-09-11 18:04:47 +00:00

lix-bot commented

2025-09-14 20:58:44 +00:00

Member

This issue was mentioned on Gerrit on the following CLs:

commit message in cl/4163 ("nix-eval-jobs: add --no-instantiate flag")

This issue was mentioned on Gerrit on the following CLs: * commit message in [cl/4163](https://gerrit.lix.systems/c/lix/+/4163) ("nix-eval-jobs: add `--no-instantiate` flag")