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tmux · Advanced · Interview notes: how I answered a question about the tmux-pro concurrency model

30
TMr/tmux-pro·posted by kite·1 hour agoPostmortem

Interview notes: how I answered a question about the tmux-pro concurrency model

Most tmux-pro articles stop at "how to use it" and never cover "when not to use it". This is an attempt at the second half.

The first thing was to collapse the variables. We were changing config and upgrading the version at the same time, and afterwards nobody could say which change caused what. We rolled back to moving one variable at a time, re-ran three times, and only then did the curve settle. Tedious, but not skippable.

# 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

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".

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.

16 comments

16 comments

M
Sswoole_lee·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.

261
Mmike_xu·just now

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

177
Llinlin·3 minutes agoedited

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

169
Zzhu_zong·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.

70
Aalice_dev·2 hours ago

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

336
Llinlin·3 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.

114
Bbob_chen·2 hours 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.

55
Aalice_dev·just now

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.

312
Ttang_hao·5 hours ago

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

11
Sslow_query·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.

48
Cchen_dev·2 days ago

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

10
Kkernel_panic·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.

139
Bbob_chen·12 minutes ago

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

92
Kkernel_panic·3 minutes ago

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

20
Bbob_chen·1 hour 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.

3
Rrase·2 hours ago

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

149

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