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 dc7420c2c9, 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 <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

src/backend/access/gist/gistxlog.c

@@ -389,7 +389,7 @@ gistRedoPageReuse(XLogReaderState *record)
 	 *
 	 * latestRemovedXid was the page's deleteXid.  The
 	 * GlobalVisIsRemovableFullXid(deleteXid) test in gistPageRecyclable()
-	 * conceptually mirrors the pgxact->xmin > limitXmin test in
+	 * conceptually mirrors the PGPROC->xmin > limitXmin test in
 	 * GetConflictingVirtualXIDs().  Consequently, one XID value achieves the
 	 * same exclusion effect on primary and standby.
 	 */

src/backend/access/nbtree/nbtpage.c

@@ -2317,7 +2317,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 	 * we're in VACUUM and would not otherwise have an XID.  Having already
 	 * updated links to the target, ReadNewTransactionId() suffices as an
 	 * upper bound.  Any scan having retained a now-stale link is advertising
-	 * in its PGXACT an xmin less than or equal to the value we read here.  It
+	 * in its PGPROC an xmin less than or equal to the value we read here.  It
 	 * will continue to do so, holding back the xmin horizon, for the duration
 	 * of that scan.
 	 */

src/backend/access/transam/README

@@ -296,7 +296,7 @@ ensure that the C compiler does exactly what you tell it to.)
 Another important activity that uses the shared ProcArray is
 ComputeXidHorizons, which must determine a lower bound for the oldest xmin
 of any active MVCC snapshot, system-wide.  Each individual backend
-advertises the smallest xmin of its own snapshots in MyPgXact->xmin, or zero
+advertises the smallest xmin of its own snapshots in MyProc->xmin, or zero
 if it currently has no live snapshots (eg, if it's between transactions or
 hasn't yet set a snapshot for a new transaction).  ComputeXidHorizons takes
 the MIN() of the valid xmin fields.  It does this with only shared lock on
@@ -331,7 +331,7 @@ necessary.
 Note that while it is certain that two concurrent executions of
 GetSnapshotData will compute the same xmin for their own snapshots, there is
 no such guarantee for the horizons computed by ComputeXidHorizons.  This is
-because we allow XID-less transactions to clear their MyPgXact->xmin
+because we allow XID-less transactions to clear their MyProc->xmin
 asynchronously (without taking ProcArrayLock), so one execution might see
 what had been the oldest xmin, and another not.  This is OK since the
 thresholds need only be a valid lower bound.  As noted above, we are already

src/backend/access/transam/twophase.c

@@ -464,7 +464,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
 	/* We set up the gxact's VXID as InvalidBackendId/XID */
 	proc->lxid = (LocalTransactionId) xid;
 	pgxact->xid = xid;
-	pgxact->xmin = InvalidTransactionId;
+	Assert(proc->xmin == InvalidTransactionId);
 	proc->delayChkpt = false;
 	pgxact->vacuumFlags = 0;
 	proc->pid = 0;