Fix legacyPackages performance#423290
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roberth merged 2 commits intoNixOS:masterfrom Jul 13, 2025
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No change in behavior. By factoring this out, we can pass an extra overlay in addition to the impure ones in callers.
This saves the construction of an unused `pkgs` attrset and all that
is required to reach that, which is not a lot, but still significant.
Benchmark 1: nix eval .?ref=42608bc8e65c5bf596d6155ad2f0ded6253b1e69#hello.outPath --no-eval-cache
Time (mean ± σ): 1.062 s ± 0.011 s [User: 0.674 s, System: 0.226 s]
Range (min … max): 1.040 s … 1.080 s 10 runs
Benchmark 2: nix eval .?ref=8a7e18b270cb2256a0526b9cda1d9e410aacd440#hello.outPath --no-eval-cache
Time (mean ± σ): 1.317 s ± 0.033 s [User: 0.835 s, System: 0.270 s]
Range (min … max): 1.282 s … 1.371 s 10 runs
Summary
nix eval .?ref=42608bc8e65c5bf596d6155ad2f0ded6253b1e69#hello.outPath --no-eval-cache ran
1.24 ± 0.03 times faster than nix eval .?ref=8a7e18b270cb2256a0526b9cda1d9e410aacd440#hello.outPath --no-eval-cache
(where 42608bc had the same tree as this commit)
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The eval performance comparison uses the stable CLI, not flakes, so those times won't improve; they're already good in this regard. Also that p value seems way too low, probably because it's measuring the wrong thing. |
arianvp
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Thank you Arian! |
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Remove one
pkgsconstruction.This makes
.#hello.outPath1.24 ± 0.03 times faster to evaluate.Larger closures will see a less dramatic effect. Expect a 100 to 300 ms improvement, depending on hardware etc.
(I don't like the impurities which are now more visible, but not they're not new; just preserved. Presumably they're used with
--impure, and I'd like to call that OT because unchanged. I don't like the alteredlibeither.)Things done
nix.conf? (See Nix manual)sandbox = relaxedsandbox = truenix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage./result/bin/)Add a 👍 reaction to pull requests you find important.