Re: subscription worker signalling wal writer too much

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.

I'm testing it by doing 1e6 transactions over 8 clients while replication
is in effect, then waiting for the logical replica to catch up. This cycle
takes 183.1 seconds in HEAD, and 162.4 seconds with the attached patch.
N=14, p-value for difference of the means 6e-17.

If I suppress all wake-ups just to see what would happen, it further
reduces the runtime to 153.7.

Cheers,

Jeff