Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()

Peter Geoghegan <pg@bowt.ie>

From: Peter Geoghegan <pg@bowt.ie>

To: Matthias van de Meent <boekewurm+postgres@gmail.com>

Cc: Alexander Lakhin <exclusion@gmail.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>, Andres Freund <andres@anarazel.de>

Date: 2021-11-03T16:21:20Z

Lists: pgsql-bugs

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Ensure vacuum removes all visibly dead tuples older than OldestXmin
- 06bf404cd07b 16.4 landed
- 45ce054c02b8 14.13 landed
- dc6354c67017 15.8 landed
Combine freezing and pruning steps in VACUUM
- 6dbb490261a6 17.0 cited
Handle non-chain tuples outside of heap_prune_chain()
- 6f47f6883151 17.0 cited
Fix false reports in pg_visibility
- e85662df44ff 17.0 cited
Remove retry loop in heap_page_prune().
- 1ccc1e05ae8f 17.0 cited
vacuumlazy.c: document vistest and OldestXmin.
- 73f6ec3d3c8d 15.0 cited
Deduplicate choice of horizon for a relation procarray.c.
- d9d8aa9bb9aa 15.0 cited
Remove tupgone special case from vacuumlazy.c.
- 8523492d4e34 14.0 cited
Simplify state managed by VACUUM.
- b4af70cb2103 14.0 cited
Recycle nbtree pages deleted during same VACUUM.
- 9dd963ae2534 14.0 cited
snapshot scalability: Don't compute global horizons while building snapshots.
- dc7420c2c927 14.0 cited
Raise error when affecting tuple moved into different partition.
- f16241bef7cc 11.0 cited

On Wed, Nov 3, 2021 at 8:46 AM Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
> I seem to repeatedly get backends of which the xmin is set from
> InvalidTransactionId to some value < min(ProcGlobal->xids), which then
> result in shared_oldest_nonremovable (and others) being less than the
> value of their previous iteration. This leads to the infinite loop in
> lazy_scan_prune (it stores and uses one value of
> *_oldest_nonremovable, whereas heap_page_prune uses a more up-to-date
> variant).

> I noticed that when this happens, generally a parallel vacuum worker
> is involved.

Hmm. That is plausible. The way that VACUUM (and concurrent index
builds) avoid being seen via the PROC_IN_VACUUM thing is pretty
delicate. Wouldn't surprise me if the parallel VACUUM issue subtly
broke lazy_scan_prune in the way that we see here.

What about testing? Can we find a simple way of reducing this
complicated repro to a less complicated repro with a failing
assertion? Maybe an assertion that we get to keep after the bug is
fixed?

-- 
Peter Geoghegan