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NixOS · Toolchain · Choosing nixos-tools: I turned 5 options into a side-by-side comparison

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NIr/nixos-tools·posted by kite·3 days agoInternals

Choosing nixos-tools: I turned 5 options into a side-by-side comparison

Short version: nixos-tools needs almost no tuning at small and medium scale — the point where it starts to hurt is much further out than most people assume. Full measurements below.

One last trap: in container environments remember to adjust the memory-related parameters in step. Otherwise the host limit and the process expectation disagree, and the symptom is intermittent, unreproducible failure.

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11 comments

11 comments

M
Lli_ming·2 hours ago

Agreeing with the above. One addition: with this option enabled the GC count in your metrics doubles, so adjust the alert threshold at the same time or it will keep firing.

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Ddev_zhou·12 minutes ago

I just read the nixos-tools source — the author actually explains the reasoning in a comment, roughly "so that it degrades into predictable behaviour in extreme cases".

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Lli_mingMod·just now

One counter-example: below nixos-tools 7.4 the semantics of that code are different, so do not copy it verbatim. We got burned in staging and rolled back once.

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Rran_bo·1 hour agoedited

A question: what changes in a container with a 512Mi memory limit? That is how we run it in production.

427
Ddev_zhou·5 hours ago

Worth learning from this debugging approach. We went straight at the logs and took a much longer route.

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Rrase·1 hour ago

Thanks for sharing real numbers — far more useful than the articles that only cover concepts.

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Nnikic·3 minutes ago

Sharing our numbers, 8 cores 16GB, same scenario:

| Concurrency | P50 | P99 |
|---|---|---|
| 200 | 12ms | 88ms |
| 500 | 31ms | 340ms |

P99 clearly collapses at 500 concurrency, which lines up with your knee point.

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Ddev_zhouOP·28 minutes ago

There is actually a simpler fix that needs no architecture change: move this check up to the gateway and the problem disappears. The cost is one extra lookup at the gateway.

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Aalice_dev·28 minutes ago

This matches what we see in production. We only hit it past 3k QPS; the earlier load tests showed nothing — the test traffic was too clean, with no long-tail requests.

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Nnikic·12 minutes ago

This is not a nixos-tools problem, it is a usage problem. The docs say this API is not thread-safe and you must lock around it yourself.

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Zzhu_zongOP·5 hours ago

Can you give a minimal reproduction? I ran it locally for ten minutes and could not reproduce on macOS with the latest version.

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