[llvm-dev] buildbot failure in LLVM on clang-native-arm-cortex-a9

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 26 09:21:45 PDT 2015

On Wed, Aug 26, 2015 at 9:01 AM, Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > On Aug 26, 2015, at 8:21 AM, Renato Golin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > On 26 August 2015 at 15:44, Tobias Grosser <tobias at grosser.es> wrote:
> >> What time-line do you have in mind for this fix? If you are in charge
> >> and can make this happen within a day, giving cmake + ninja a chance
> seems
> >> OK.
> >
> > It's not my bot. All my bots are CMake+Ninja based and are stable enough.
> >
> >
> >> However, if the owner of the buildbot is not known or the fix can not
> come
> >> soon, I am in favor of disabling the noise and (re)enabling it when
> someone
> >> found time to address the problem and verify the solution.
> >
> > That's up to Galina. We haven't had any action against unstable bots
> > so far, and this is not the only one. There are lots of Windows and
> > sanitizer bots that break randomly and provide little information, are
> > we going to disable them all? How about the perf bots that still fail
> > occasionally and we haven't managed to fix the root cause, are we
> > going to disable then, too?
> >
> > You're asking to reduce considerably the quality of testing on some
> > areas so that you can reduce the time spent looking at spurious
> > failures. I don't agree with that in principle.
> That’s not how I understand his point. In my opinion, he is asking to
> increase the quality of testing. You just happen to disagree on his
> solution :)
> The situation does not seem that black and white to me here. In the end,
> it seems to me that is is about a threshold: if a bot is crashing 90% of
> the time, does it really contributes to increase the quality of testing or
> on the opposite it is just adding noise? Same question with 20%, 40%, 60%,
> …  We may all have a different answer, but I’m pretty sure we could reach
> an agreement on what seems appropriate
> Another way of considering in general the impact of a bot on the quality
> is: “how many legit failures were found by this bot in the last x years
> that weren’t covered by another bot”.

Even that doesn't really capture it - if the bot has enough false
positives, or spends long periods being red, even those legit failures will
be lost in the noise & the cost to the whole project (not only in ignoring
that bot, but in reducing confidence in the bots in general (which is
pretty low generally because of this kind of situation)) may outweigh the
value of those bugs being found.

If a bot is of low enough quality that most engineers ignore it due to
false positives, long periods of broken-ness, then it makes sense to me to
remove it from the main buildbot view and from sending email. The owner can
monitor the bot and, once they triage a failure, manually reach out to
those who might be to blame.

(oh, and add long cycle times to the list of issues - people do have a
tendency to ignore bots that come back with giant blame lists & no obvious
determination as to who's patch caused the problem, if any)

- David

> Because sometimes you may just having a HW lab stress rack, without
> providing any increased coverage for the software.
> Cheers,
>> Mehdi
> > There were other
> > threads focusing on how to make them less spurious, more stable, less
> > noisy, and some work is being done on the GreenDragon bot structure.
> > But killing everything that looks suspicious now will reduce our
> > ability to validate LLVM on the range of configurations that we do
> > today, and that, for me, is a lot worse than a few minutes' worth of
> > some engineers.
> >
> >
> >> The cost of
> >> buildbot noise is very high, both in terms of developer time spent, but
> >> more importantly due to people starting to ignore them when monitoring
> them
> >> becomes costly.
> >
> > I think you're overestimating the cost.
> >
> > When I get bot emails, I click on the link and if it was timeout, I
> > always ignore it. If I can't make heads or tails (like the sanitizer
> > ones), I ignore it temporarily, then look again next day.
> >
> > My assumption is that the bot owner will make me aware if the reason
> > is not obvious, as I do with my bots. I always wait for people to
> > realise, and fix. But if they can't, either because the bot was
> > already broken, or because the breakage isn't clear, I let people know
> > where to search for the information in the bot itself. This is my
> > responsibility as a bot owner.
> >
> > I appreciate the benefit of having green / red bots, but you also have
> > to appreciate that hardware is not perfect, and they will invariably
> > fail once in a while. I had some Polly bots failing randomly and it
> > took me only a couple of seconds to infer so. I'm not asking to remove
> > them, even those that fail more than pass throughout the year. I
> > assume that, if they're still there, it provides *some* value to
> > someone.
> >
> > cheers,
> > --renato
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