snapshot scalability: Move PGXACT->xmin back to PGPROC.

Now that xmin isn't needed for GetSnapshotData() anymore, it leads to
unnecessary cacheline ping-pong to have it in PGXACT, as it is updated
considerably more frequently than the other PGXACT members.

After the changes in dc7420c2c92, this is a very straight-forward change.

For highly concurrent, snapshot acquisition heavy, workloads this change alone
can significantly increase scalability. E.g. plain pgbench on a smaller 2
socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench
when submitting queries in batches of 100, and 2.85x for batches of 100
'SELECT';.  The latter numbers are obviously not to be expected in the
real-world, but micro-benchmark the snapshot computation
scalability (previously spending ~80% of the time in GetSnapshotData()).

Author: Andres Freund <[email protected]>
Reviewed-By: Robert Haas <[email protected]>
Reviewed-By: Thomas Munro <[email protected]>
Reviewed-By: David Rowley <[email protected]>
Discussion: https://postgr.es/m/[email protected]

1 files changed, 1 insertions, 1 deletions

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 7819266a630..254dbcdce52 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1535,7 +1535,7 @@ DefineIndex(Oid relationId,
 	StartTransactionCommand();
 
 	/* We should now definitely not be advertising any xmin. */
-	Assert(MyPgXact->xmin == InvalidTransactionId);
+	Assert(MyProc->xmin == InvalidTransactionId);
 
 	/*
 	 * The index is now valid in the sense that it contains all currently