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

On Wed, Jan 10, 2024 at 2:38 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> I don't know. That particular system experienced the infinite loop only once.

While I certainly can't recreate the problem on demand, it has been
seen on this same application far more than once.

> > I'm referring to calls such as the
> > "GetOldestNonRemovableTransactionId(NULL)" and
> > "GlobalVisCheckRemovableFullXid()" calls that take place inside
> > _bt_pendingfsm_finalize(). It's not like we do stuff like that in very
> > many other places.
>
> I see what you mean about the rarity and potential importance of
> "GetOldestNonRemovableTransactionId(NULL)". There's just one other caller,
> vac_update_datfrozenxid(), which calls it for an unrelated cause.

I just noticed another detail that adds significant weight to this
theory: it looks like the problem is hit on the first tuple located on
the first heap page that VACUUM scans *after* it completes its first
round of index vacuuming (I'm inferring this from vacrel state,
particular its lpdead_items instrumentation counter). The dead_items
array is as large as possible here (just under 1 GiB), and
lpdead_items is 178956692 (which uses up all of our dead_items space).
VACUUM scans tens of gigabytes of heap pages before it begins this
initial round of index vacuuming (according to vacrel->scanned_pages).

What are the chances that all of this is just a coincidence? Low, I'd say.

--
Peter Geoghegan