Number of cpu cores being different on different system seems like an easy way to ruin Nix derivation determinism. I’m wonder if Nix sandbox is doing any effort to pretend the number of CPUs is always the same. Should it?
This isn’t controlled as it’d be impractical to build everything with limited parallelism.
Should individual packages have bugs it can be disabled via enableParallelBuilding = false.
Also, with the exception of some builders, parallelism in a single derivation is off by default.
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Although quite a few derivation use those “some builders”. I’d wager most of them even; anything cmake or meson for instance which are quite a few packages and most of the ecosystem-specific builders which, by raw numbers, make up the majority of Nixpkgs’ packages.