LLM Community · Six rules for error handling in llm that I settled on

174
LLr/llm·posted by kernel_panic·6 hours agoTranslation

Six rules for error handling in llm that I settled on

It took me two weeks of on-and-off digging and plenty of wrong turns. Writing the process down as it happened so the next person spends less time.

What genuinely surprised me was the tail. The average looked great while P99 jumped by an order of magnitude past some threshold. The cause was not llm itself but our upstream connection reuse — the load test traffic was too clean and hid the long-tail requests.

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.

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".

28 comments

28 comments

M
Kkernel_panic·5 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.

451
Ddev_zhou·1 hour ago

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

438
Aalice_dev·just now

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

134
Mmike_xu·yesterdayedited

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.

97
Oops_wang·28 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.

222
Kkite·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.

87
Zzhu_zong·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.

54
Sslow_query·28 minutes ago

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

18
Kkite·2 days ago

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

76
Sslow_query·28 minutes ago

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

29
Mmike_xu·28 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.

162
Rran_bo·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.

96
Llinlin·5 hours ago

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

11
Kkernel_panicMod·5 hours 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.

302
Rran_bo·12 minutes ago

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

2
Kkite·just nowedited

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.

15
Oops_wang·12 minutes ago

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

13
Bbob_chen·2 days 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.

195
Sslow_queryMod·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.

518
Sswoole_leeOPMod·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.

59
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.

79
Wwinter·1 hour ago

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

10
Rran_bo·3 minutes ago

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

36
Aalice_dev·2 days ago

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

244
Zzhou_yi·2 days agoedited

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

45
Sslow_query·12 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.

1
Ddev_zhou·2 hours ago

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

1
Kkernel_panicOP·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.

12

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