Re: Lockless exit path for ReplicationOriginExitCleanup

On Wed, Nov 22, 2023 at 3:06 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> Hello,
>
> On 2023-Nov-22, Bharath Rupireddy wrote:
>
> > While looking at the use of session_replication_state, I noticed that
> > ReplicationOriginLock is acquired in ReplicationOriginExitCleanup()
> > even if session_replication_state is reset to NULL by
> > replorigin_session_reset(). Why can't there be a lockless exit path
> > something like [1] similar to
> > replorigin_session_reset() which checks session_replication_state ==
> > NULL without a lock?
>
> I suppose we can do this on consistency grounds -- I'm pretty sure you'd
> have a really hard time proving that this makes a performance difference --

Yes, can't measure the perf gain, however, as said upthread
https://www.postgresql.org/message-id/CALj2ACVVhPn7BVQZLGPVvBrLoDZNRaV0LS9rBpt5y%2Bj%3DxRebWw%40mail.gmail.com,
it avoids unnecessary lock acquisition and release.

> but this patch is incomplete: just two lines below, we're still testing
> session_replication_state for nullness, which would now be dead code.
> Please repair.

Done.

> The comment on session_replication_state is confusing also:
>
> /*
>  * Backend-local, cached element from ReplicationState for use in a backend
>  * replaying remote commits, so we don't have to search ReplicationState for
>  * the backends current RepOriginId.
>  */
>
> My problem with it is that this is not a "cached element", but instead a
> "cached pointer to [shared memory]".  This is what makes testing that
> pointer for null-ness doable, but access to each member therein
> requiring lwlock acquisition.

Right. I've reworded the comment a bit.

PSA v1 patch.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com