Make zipAttrsWith more parallel-eval friendly#535
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The previous implementation used a std::map with traceable_allocator, which costs one GC_MALLOC_UNCOLLECTABLE() and one GC_FREE() per distinct attribute name. Uncollectable allocations always take the global GC allocation lock, making this map a major source of mutex contention during parallel evaluation (~12% of all futex calls in a 16-core 'nix search' run, plus the corresponding GC_free traffic). Instead, collect the (name, value) pairs into a plain std::vector and group them by stable-sorting on the name, which yields the same attribute order and per-attribute value order as the map. The vector doesn't need to be visible to the GC since the values it points to are kept alive by the attrsets in args[1] for the duration of the call. At 16 eval cores (with GC disabled via GC_INITIAL_HEAP_SIZE=40G), this cuts kernel time from ~21s to ~8-9s, context switches from 2.6M to 1.5M, and elapsed time from ~6.2s to ~4.9s for 'nix search nixpkgs --no-eval-cache fizzbuzz'. Single-core performance is unchanged. Assisted-by: Claude Fable 5 <noreply@anthropic.com>
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📝 WalkthroughWalkthroughThe implementation of ChangeszipAttrsWith Refactor
Estimated code review effort: 2 (Simple) | ~10 minutes 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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cole-h
approved these changes
Jul 6, 2026
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Motivation
The previous implementation used an
std::mapwithtraceable_allocatorto collect the attributes, which is extremely expensive in parallel eval because every uncollectable alloc/free takes the global GC lock. The new implementation collects all the attributes in a vector (without theListBuilderthat was the reason why we neededtraceable_allocator), sorts them by name, and then performs a single sweep through the vector to build one list at a time.While this is mostly beneficial for parallel eval, it also seems to give a modest speedup for single-threaded eval (e.g.
nix search nixpkgs fizzbuzz --no-eval-cachewent from 28.9s to 28.5s).Assisted-by: Claude Fable 5 noreply@anthropic.com
Taken from #534.
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