Re: [PATCH] Improve performance of NOTIFY over many databases (v2)

Hoi Tom,

On Mon, 16 Sep 2019 at 15:33, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Martijn van Oosterhout <kleptog(at)gmail(dot)com> writes:
> > I think I like the idea of having SignalBackend do the waking up a
> > slow backend but I'm not enthused by the "lets wake up (at once)
> > everyone that is behind". That's one of the issues I was explicitly
> > trying to solve. If there are any significant number of "slow"
> > backends then we get the "thundering herd" again.
>
> But do we care? With asyncQueueAdvanceTail gone from the listeners,
> there's no longer an exclusive lock for them to contend on. And,
> again, I failed to see any significant contention even in HEAD as it
> stands; so I'm unconvinced that you're solving a live problem.

You're right, they only acquire a shared lock which is much less of a
problem. And I forgot that we're still reducing the load from a few
hundred signals and exclusive locks per NOTIFY to perhaps a dozen
shared locks every thousand messages. You'd be hard pressed to
demonstrate there's a real problem here.

So I think your patch is fine as is.

Looking at the release cycle it looks like the earliest either of
these patches will appear in a release is PG13, right?

Thanks again.
--
Martijn van Oosterhout <kleptog(at)gmail(dot)com> http://svana.org/kleptog/