Horton

Re: Snapshot related assert failure on skink

Thomas Munro <thomas.munro@gmail.com>

View thread View on postgr.es
From: Thomas Munro <thomas.munro@gmail.com>
To: Tomas Vondra <tomas@vondra.me>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>
Date: 2025-03-17T12:18:37Z
Lists: pgsql-hackers 
On Tue, Mar 18, 2025 at 12:59 AM Tomas Vondra <tomas@vondra.me> wrote:
> On 3/17/25 12:36, Tomas Vondra wrote:
> > I'm still fiddling with the script, trying to increase the probability
> > of the (apparent) race condition. On one machine (old Xeon) I can hit it
> > very easily/reliably, while on a different machine (new Ryzen) it's very
> > rare. I don't know if that's due to difference in speed of the CPU, or
> > fewer cores, ... I guess it changes the timing just enough.
> >
> > I've also tried running the stress test on PG17, and I'm yet to see a
> > single failure there. Not even on the xeon machine, that hits it
> > reliably on 18. So this seems to be a PG18-only issue.
> >
>
> And of course, the moment I sent this, I got a failure on 17 too. But
> it's seems much harder to hit (compared to 18).

Could there be a connection to this commit?

commit 119c23eb9819213551cbe7e7665c8b493c59ceee
Author: Nathan Bossart <nathan@postgresql.org>
Date:   Tue Sep 5 13:59:06 2023 -0700

    Replace known_assigned_xids_lck with memory barriers.

Commits

  1. Fix rare assertion failure in standby, if primary is restarted

  2. Remove unnecessary GetTransactionSnapshot() calls

  3. Update TransactionXmin when MyProc->xmin is updated

  4. Replace known_assigned_xids_lck with memory barriers.

  5. snapshot scalability: cache snapshots using a xact completion counter.