Nix Evaluation Profiling with `eval-profiler`

# Using the `eval-profiler` Nix evaluator supports [evaluation](@docroot@/language/evaluation.md) [profiling](<https://en.wikipedia.org/wiki/Profiling_(computer_programming)>) compatible with `flamegraph.pl`. The profiler samples the nix function call stack at regular intervals. It can be enabled with the [`eval-profiler`](@docroot@/command-ref/conf-file.md#conf-eval-profiler) setting: ```console $ nix-instantiate "<nixpkgs>" -A hello --eval-profiler flamegraph ``` Stack sampling frequency and the output file path can be configured with [`eval-profile-file`](@docroot@/command-ref/conf-file.md#conf-eval-profile-file) and [`eval-profiler-frequency`](@docroot@/command-ref/conf-file.md#conf-eval-profiler-frequency). By default the collected profile is saved to `nix.profile` file in the current working directory. The collected profile can be directly consumed by `flamegraph.pl`: ```console $ flamegraph.pl nix.profile > flamegraph.svg ``` The line information in the profile contains the location of the [call site](https://en.wikipedia.org/wiki/Call_site) position and the name of the function being called (when available). For example: ``` /nix/store/x9wnkly3k1gkq580m90jjn32q9f05q2v-source/pkgs/top-level/default.nix:167:5:primop import ``` Here `import` primop is called at `/nix/store/x9wnkly3k1gkq580m90jjn32q9f05q2v-source/pkgs/top-level/default.nix:167:5`.

This chunk describes how to use the `eval-profiler` in Nix for evaluation profiling, which is compatible with `flamegraph.pl`. The profiler samples the Nix function call stack and can be enabled with the `--eval-profiler flamegraph` flag. Users can configure the output file path using `eval-profile-file` (defaulting to `nix.profile`) and the sampling frequency with `eval-profiler-frequency`. The collected profile can then be processed by `flamegraph.pl` to generate an SVG visualization. Profile entries include call site location and the name of the function being called.