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Rust Community · Interview notes: how I answered a question about the Rust concurrency model

31
Rsr/rust·posted by dev_zhou·5 minutes agoInternals

Interview notes: how I answered a question about the Rust concurrency model

Short version: Rust 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.

On trade-offs, my view is this: if nobody on the team owns this area long-term, do not introduce a second mechanism. With two coexistence you first have to work out which one is even in play when things break, and that costs far more than the performance you saved.

Worth noting: the official docs do cover this, just in a very inconspicuous spot. I only found it reading the source comments, where the author explains the reasoning — roughly "so that it degrades into predictable behaviour in extreme cases".

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15 comments

15 comments

M
Rran_bo·3 minutes ago

Has anyone run a controlled experiment? I did, reducing it to a single variable, and the difference was 4% — within noise. So I suspect the main cause is something else.

361
Bbob_chen·12 minutes ago

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

354
NnikicOP·12 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.

2
Nnikic·12 minutes ago

We have run this in production for two years without hitting it. That said, we never reached this scale, so our experience is not really evidence here.

241
Rrase·2 hours ago

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

248
Kkite·just now

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.

172
NnikicOP·3 minutes ago

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

181
Kkite·3 minutes 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.

260
Aalice_devOP·just now

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

4
Mmike_xu·yesterday

I see point 3 differently. The trade-off depends on your read/write ratio: read-heavy with little writing means caching actually widens the inconsistency window.

12
Hhuang_keMod·just now

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

44
Rrase·5 hours agoedited

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

203
Rrase·3 minutes agoedited

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.

10
Nnikic·12 minutes ago

Saved. I am reworking this area this week — this saves a lot of wrong turns.

3
Cchen_dev·5 hours ago

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

1

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