Re: Lockless exit path for ReplicationOriginExitCleanup

On Wed, Nov 22, 2023 at 2:28 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Nov 22, 2023 at 2:12 PM Bharath Rupireddy
> <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> 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 don't see any problem with such a check but not sure of the benefit
> of doing so either.

It avoids an unnecessary lock acquisition and release when
session_replication_state is already reset by
replorigin_session_reset() before reaching
ReplicationOriginExitCleanup(). A patch something like [1] and a run
of subscription tests shows that 153 times the lock acquisition and
release can be avoided.

ubuntu:~/postgres/src/test/subscription$ grep -ir "with
session_replication_state NULL" . | wc -l
153

ubuntu:~/postgres/src/test/subscription$ grep -ir "with
session_replication_state not NULL" . | wc -l
174

[1]
diff --git a/src/backend/replication/logical/origin.c
b/src/backend/replication/logical/origin.c
index 460e3dcc38..dd3824bd27 100644
--- a/src/backend/replication/logical/origin.c
+++ b/src/backend/replication/logical/origin.c
@@ -1056,6 +1056,11 @@ ReplicationOriginExitCleanup(int code, Datum arg)
{
ConditionVariable *cv = NULL;

+ if (session_replication_state == NULL)
+ elog(LOG, "In ReplicationOriginExitCleanup() with
session_replication_state NULL");
+ else
+ elog(LOG, "In ReplicationOriginExitCleanup() with
session_replication_state not NULL");
+
LWLockAcquire(ReplicationOriginLock, LW_EXCLUSIVE);

if (session_replication_state != NULL &&

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