Re: New strategies for freezing, advancing relfrozenxid early

On Thu, Jan 26, 2023 at 8:35 AM Andres Freund <andres@anarazel.de> wrote:
> I think it's probably ok, but perhaps deserves a bit more thought about when
> to "opportunistically" freeze. Perhaps to make it *more* aggressive than it's
> now.
>
> With "opportunistic freezing" I mean freezing the page, even though we don't
> *have* to freeze any of the tuples.
>
> The overall condition gating freezing is:
>         if (pagefrz.freeze_required || tuples_frozen == 0 ||
>                 (prunestate->all_visible && prunestate->all_frozen &&
>                  fpi_before != pgWalUsage.wal_fpi))
>
> fpi_before is set before the heap_page_prune() call.

Have you considered page-level checksums, and how the impact on hint
bits needs to be accounted for here?

All RDS customers use page-level checksums. And I've noticed that it's
very common for the number of FPIs to only be very slightly less than
the number of pages dirtied. Much of which is just hint bits. The
"fpi_before != pgWalUsage.wal_fpi" test catches that.

> To me a condition that checked if the buffer is already dirty and if another
> XLogInsert() would be likely to generate an FPI would make more sense. The
> rare race case of a checkpoint starting concurrently doesn't matter IMO.

That's going to be very significantly more aggressive. For example
it'll impact small tables very differently.

--
Peter Geoghegan