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Rust Community · The Rust execution flow in one diagram (with sequence chart)

124
Rsr/rust·posted by huang_ke·1 hour agoTooling

The Rust execution flow in one diagram (with sequence chart)

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.

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".

# Load test: do not jump straight to the max concurrency.
# Ramp it up, otherwise you miss the knee.
for c in 50 100 200 400 800; do
  wrk -t8 -c$c -d60s --latency http://127.0.0.1:8080/api/feed
  sleep 20
done

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.

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.

We also fixed monitoring along the way: replaced average-based alerts with percentiles and split them per endpoint. False alerts dropped by about seventy percent and the on-call rotation visibly cheered up.

47 comments

47 comments

M
Nnikic·2 days ago

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

513
Bbob_chen·1 hour 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.

485
Cchen_dev·just now

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

268
Ttang_hao·28 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.

167
LlinlinOP·2 days agoedited

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.

353
Ddev_zhou·2 days agoedited

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.

373
Oops_wang·yesterday

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

367
Bbob_chen·just now

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.

230
Cchen_dev·2 days 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.

1
Ttang_hao·2 days 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.

125
Aalice_devOPMod·2 days ago

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.

11
Ttang_hao·12 minutes ago

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

21
Kkernel_panic·yesterday

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

65
Ddev_zhou·1 hour ago

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

15
WwinterMod·just now

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.

20
Zzhou_yi·2 days agoedited

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.

138
Rrase·1 hour ago

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

1
Ddev_zhou·2 days 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.

1
Lli_ming·2 days agoedited

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

372
Mmike_xu·2 days 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.

178
Zzhou_yi·2 days agoedited

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.

264
Kkite·2 days 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.

215
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.

262
Wwinter·just nowedited

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

251
Hhuang_ke·2 days 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.

241
LlinlinOP·yesterday

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

134
WwinterOP·yesterday

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.

121
Wwinter·2 days ago

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

170
Sswoole_lee·12 minutes ago

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

101
Ttang_hao·28 minutes ago

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.

290
Sswoole_leeOP·yesterday

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.

161
Mmike_xuOP·1 hour 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.

8
Rran_bo·yesterday

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.

22
Zzhu_zong·2 days ago

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.

454
Oops_wang·2 days agoLevel 6

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.

18
Kkite·2 days 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".

1
Rran_bo·2 days 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.

460
Zzhou_yi·2 days agoeditedLevel 6

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

35
Mmike_xu·1 hour ago

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

22
Oops_wang·yesterdayedited

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.

303
Aalice_dev·2 days 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.

47
Aalice_dev·just now

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

39
RraseOP·2 days 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".

32
Oops_wangOP·28 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".

1
Cchen_dev·just nowedited

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.

60
Kkernel_panic·2 days agoedited

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.

50
Oops_wang·2 days ago

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

5

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