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From: Jonathan Rudenberg <jonathan@titanous.com>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Andreas Seltenreich <seltenreich@gmx.de>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [sqlsmith] Unpinning error in parallel worker
Date: Fri, 20 Apr 2018 18:24:54 -0400
Message-ID: <1524263094.362491.1345450880.691137B5@webmail.messagingengine.com> (raw)
In-Reply-To: <CAEepm=0awU3+3mdpBTREjSFg1Km1Lx3ikO5NF-fML4tirQMBmw@mail.gmail.com>
References: <87h92g83t3.fsf@credativ.de>
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On Fri, Apr 20, 2018, at 00:42, Thomas Munro wrote:
> On Wed, Apr 18, 2018 at 11:43 AM, Jonathan Rudenberg
> <jonathan@titanous.com> wrote:
> > On Tue, Apr 17, 2018, at 19:31, Thomas Munro wrote:
> >> On Wed, Apr 18, 2018 at 11:01 AM, Jonathan Rudenberg
> >> <jonathan@titanous.com> wrote:
> >> > Yep, I think I know approximately what it looked like, I've attached a lightly redacted plan. All of the hung queries were running some variant of this plan as far as I can tell.
> >>
> >> Hmm, that isn't a parallel query.  I was expecting to see "Gather" and
> >> "Parallel" in there.
> >
> > Oops, I'm really sorry about that. I only have the first part of the hung queries, and there are a few variants. Here's one that's parallel.
> 
> I spent some time trying to reproduce this failure without any luck,
> using query plans similar to your Gather plan fragment, and using some
> test harness code for the allocator stuff in isolation.
> 
> I had an idea that (1) freeing a large object that releases and unpins
> a segment in one backend and then (2) freeing it again in another
> backend (illegally) might produce this effect with sufficiently bad
> luck.  I'm still trying to reproduce that without any success, but I
> get other kinds of failures which I think you'd be seeing too if that
> hunch were right.  Still looking...

Thanks for investigating! We have monitoring in place and a plan to grab stack traces if this happens again. Is there anything else that would be useful for us to try to get in addition to a stack trace from the process processing the stuck query?




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