On Mon, Dec 18, 2023 at 11:00 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Mon, Dec 18, 2023 at 12:04 PM Robert Haas <robertmhaas@gmail.com> wrote:
> > certain sense they are competing for the same job. However, they do
> > aim to alleviate different TYPES of contention: the group XID update
> > stuff should be most valuable when lots of processes are trying to
> > update the same page, and the banks should be most valuable when there
> > is simultaneous access to a bunch of different pages. So I'm not
> > convinced that this patch is a reason to remove the group XID update
> > mechanism, but someone might argue otherwise.
>
> Hmm, but, on the other hand:
>
> Currently all readers and writers are competing for the same LWLock.
> But with this change, the readers will (mostly) no longer be competing
> with the writers. So, in theory, that might reduce lock contention
> enough to make the group update mechanism pointless.
Thanks for your feedback, I agree that with a bank-wise lock, we might
not need group updates for some of the use cases as you said where
readers and writers are contenting on the centralized lock because, in
most of the cases, readers will be distributed across different banks.
OTOH there are use cases where the writer commit is the bottleneck (on
SLRU lock) like pgbench simple-update or TPC-B then we will still
benefit by group update. During group update testing we have seen
benefits with such a scenario[1] with high client counts. So as per
my understanding by distributing the SLRU locks there are scenarios
where we will not need group update anymore but OTOH there are also
scenarios where we will still benefit from the group update.
[1] https://www.postgresql.org/message-id/CAFiTN-u-XEzhd%3DhNGW586fmQwdTy6Qy6_SXe09tNB%3DgBcVzZ_A%40mail.gmail.com
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Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com