nix-eval-jobs

This project evaluates nix attributes sets in parallel with streamable json output. This is useful for time and memory intensive evaluations such as NixOS machines, i.e. in a CI context. The evaluation is done with a controllable number of threads that are restarted when their memory consumption exceeds a certain threshold.

To facilitate integration, nix-eval-jobs creates garbage collection roots for each evaluated derivation (drv file, not the build) within the provided attribute. This prevents race conditions between the nix garbage collection service and user-started nix builds processes.

Why using nix-eval-jobs?

• Faster evaluation by using threads
• Memory used for evaluation is reclaimed after nix-eval-jobs finish, so that the build can use it.
• Evaluation of jobs can fail individually

Example

In the following example we evaluate the hydraJobs attribute of the patchelf flake:

$ nix-eval-jobs --gc-roots-dir $(pwd)/gcroot --flake 'github:NixOS/patchelf#hydraJobs'
{"attr":"build-sanitized-clang.x86_64-linux","drvPath":"/nix/store/igmkq61cwys8nj34yqvnpdg921h0i0mp-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/nwwgff1fwkws4wxv7k7cfvvin8ab9gbh-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"build-sanitized.aarch64-linux","drvPath":"/nix/store/d8ma8d7gjwx6ix4ibs910z9fkm3hwdvz-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/6j26m4sznwdyfk4sbmnls3sk0lxm38ih-patchelf-0.14.3"},"system":"aarch64-linux"}
{"attr":"build-sanitized.i686-linux","drvPath":"/nix/store/87rwijvfqqs7dw9lbmckmz4nbryvjaq3-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/za5w0gzf97na44fza9sdys15qnjqayd7-patchelf-0.14.3"},"system":"i686-linux"}
{"attr":"build-sanitized.x86_64-linux","drvPath":"/nix/store/nmx50wly2qvd00svx0vqsjfh0jv7q3kl-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/38d6bhz3a5jq48gm1diji0rjfcm5vi9n-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"build.aarch64-linux","drvPath":"/nix/store/yjz9msbr6pl8mj7im5kiyhk7wwkvxywa-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/as9xhcfwnhfy5x30kxh7lfgla1qrk182-patchelf-0.14.3"},"system":"aarch64-linux"}
{"attr":"build.i686-linux","drvPath":"/nix/store/nwcmdcimnaci0knri5ga019lgbvc4am4-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/64x12dmbscnnl42r4y2av52y55ksphhk-patchelf-0.14.3"},"system":"i686-linux"}
{"attr":"build.x86_64-linux","drvPath":"/nix/store/k6p4qnjryr2l1lz31pf085ay9bd7j8gj-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/h9a779ghpibfqkkdchx6s08bb3v3i8vy-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"coverage","drvPath":"/nix/store/lsrg05dx3hyi5b6ak99pn9g1rn8xwx39-patchelf-coverage-0.14.3.drv","name":"patchelf-coverage-0.14.3","outputs":{"out":"/nix/store/6h4l5axy5lvxzq662yw47y9r60mxw3zz-patchelf-coverage-0.14.3"},"system":"x86_64-linux"}
{"attr":"release","drvPath":"/nix/store/dgn5gy64pjskfnv7vqh0s86nb998f8sq-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/nn05yaznr5af8g8mpgd82yx16pvfzjcy-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"tarball","drvPath":"/nix/store/5ajrgfd5nx29ykgg942k154mcaqfbhxd-patchelf-tarball-0.14.3.drv","name":"patchelf-tarball-0.14.3","outputs":{"out":"/nix/store/5cli6rh0h32yhfcgjkgbplcc73cqvplv-patchelf-tarball-0.14.3"},"system":"x86_64-linux"}

The output here is newline-seperated json according to https://jsonlines.org.

The code is derived from hydra's eval-jobs executable.

Further options

$ nix-eval-jobs --help
USAGE: nix-eval-jobs [options] expr

  --arg                  Pass the value *expr* as the argument *name* to Nix functions.
  --argstr               Pass the string *string* as the argument *name* to Nix functions.
  --debug                Set the logging verbosity level to 'debug'.
  --eval-store           The Nix store to use for evaluations.
  --flake                build a flake
  --gc-roots-dir         garbage collector roots directory
  --help                 show usage information
  --impure               set evaluation mode
  --include              Add *path* to the list of locations used to look up `<...>` file names.
  --log-format           Set the format of log output; one of `raw`, `internal-json`, `bar` or `bar-with-logs`.
  --max-memory-size      maximum evaluation memory size
  --meta                 include derivation meta field in output
  --option               Set the Nix configuration setting *name* to *value* (overriding `nix.conf`).
  --override-flake       Override the flake registries, redirecting *original-ref* to *resolved-ref*.
  --quiet                Decrease the logging verbosity level.
  --verbose              Increase the logging verbosity level.
  --workers              number of evaluate workers

Potential use-cases for the tool

Faster evaluator in deployment tools. When evaluating NixOS machines, evaluation can take several minutes when run on a single core. This limits scalability for large deployments with deployment tools such as NixOps.

Faster evaluator in CIs. In addition to evaluation speed for CIs, it is also useful if evaluation of individual jobs in CIs can fail, as opposed to failing the entire jobset. For CIs that allow dynamic build steps to be created, one can also take advantage of the fact that nix-eval-jobs outputs the derivation path separately. This allows separate logs and success status per job instead of a single large log file.

Projects using nix-eval-jobs

• colmena - A simple, stateless NixOS deployment tool
• robotnix - Build Android (AOSP) using Nix, used in their CI