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