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

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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
This commit is contained in:
Andres Freund
2020-08-13 16:25:21 -07:00
parent a811ea5bde
commit 1f51c17c68
12 changed files with 52 additions and 56 deletions
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6
src/backend/replication/logical/snapbuild.c
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@@ -553,8 +553,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore");
/* so we don't overwrite the existing value */
if (TransactionIdIsValid(MyPgXact->xmin))
elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid");
if (TransactionIdIsValid(MyProc->xmin))
elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid");
snap = SnapBuildBuildSnapshot(builder);
@@ -575,7 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
}
#endif
MyPgXact->xmin = snap->xmin;
MyProc->xmin = snap->xmin;
/* allocate in transaction context */
newxip = (TransactionId *)