Re: logical decoding and replication of sequences, take 2

On Tue, Dec 5, 2023 at 10:23 PM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> On 12/5/23 13:17, Amit Kapila wrote:
>
> > (b) for transactional
> > cases, we see overhead due to traversing all the top-level txns and
> > check the hash table for each one to find whether change is
> > transactional.
> >
>
> Not really, no. As I explained in my preceding e-mail, this check makes
> almost no difference - I did expect it to matter, but it doesn't. And I
> was a bit disappointed the global hash table didn't move the needle.
>
> Most of the time is spent in
>
> 78.81% 0.00% postgres postgres [.] DecodeCommit (inlined)
> |
> ---DecodeCommit (inlined)
> |
> |--72.65%--SnapBuildCommitTxn
> | |
> | --72.61%--SnapBuildBuildSnapshot
> | |
> | --72.09%--pg_qsort
> | |
> | |--66.24%--pg_qsort
> | | |
>
> And there's almost no difference between master and build with sequence
> decoding - see the attached diff-alter-sequence.perf, comparing the two
> branches (perf diff -c delta-abs).
>

I think in this the commit time predominates which hides the overhead.
We didn't investigate in detail if that can be improved but if we see
a similar case of abort [1], it shows the overhead of
ReorderBufferSequenceIsTransactional(). I understand that aborts won't
be frequent and it is sort of unrealistic test but still helps to show
that there is overhead in ReorderBufferSequenceIsTransactional(). Now,
I am not sure if we can ignore that case because theoretically, the
overhead can increase based on the number of top-level transactions.

[1]: https://www.postgresql.org/message-id/TY3PR01MB9889D457278B254CA87D1325F581A%40TY3PR01MB9889.jpnprd01.prod.outlook.com

--
With Regards,
Amit Kapila.