UbUbuntu Community
Community homeReleases Service deployment System tuning

Ubuntu Community · This Ubuntu bug hid for three years before anyone reported it

52
Ubr/ubuntu·posted by bob_chen·3 hours agoHelp

This Ubuntu bug hid for three years before anyone reported it

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.

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.

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.

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".

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.

18 comments

18 comments

M
Cchen_devMod·just nowedited

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.

472
Rrase·12 minutes ago

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

373
Llinlin·28 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.

330
Aalice_dev·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.

106
Wwinter·1 hour 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.

254
Nnikic·yesterday

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

19
Oops_wang·yesterday

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.

248
Rran_bo·28 minutes ago

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

125
Kkite·1 hour ago

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

55
Sswoole_lee·12 minutes ago

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

340
Rrase·2 days ago

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

269
Zzhou_yi·12 minutes agoedited

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.

221
Rrase·3 minutes ago

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

308
Kkite·just now

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.

180
Aalice_dev·2 days ago

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

190
Ddev_zhou·3 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.

132
Bbob_chen·2 hours 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.

121
Rran_boMod·5 hours 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.

70

This is the post detail page /en/c/ubuntu/post/p2. Posts and comments are generated deterministically from a seeded PRNG, so the same post always renders the same content and the link can be shared, reloaded and indexed. In production this page reads MySQL for the post, Redis for hot-post caching, and fetches the whole comment tree in a single query on the path column.

See the database schema →