Re: subscription worker signalling wal writer too much

On 2017-06-14 15:08:49 -0700, Jeff Janes wrote:
> On Wed, Jun 14, 2017 at 11:55 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>
> > If I publish a pgbench workload and subscribe to it, the subscription
> > worker is signalling the wal writer thousands of times a second, once for
> > every async commit. This has a noticeable performance cost.
> >
>
> I've used a local variable to avoid waking up the wal writer more than once
> for the same page boundary. This reduces the number of wake-ups by about
> 7/8.

Maybe I'm missing something here, but isn't that going to reduce our
guarantees about when asynchronously committed xacts are flushed out?
You can easily fit a number of commits into the same page... As this
isn't specific to logical-rep, I don't think that's ok.

Have you chased down why there's that many wakeups? Normally I'd have
expected that a number of the SetLatch() calls get consolidated
together, but I guess walwriter is "too quick" in waking up and
resetting the latch?

Greetings,

Andres Freund