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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
Download Patch File Download Diff File Expand all files Collapse all files

6
src/backend/replication/logical/snapbuild.c
View File

@@ -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 *)

10
src/backend/replication/walsender.c
View File

@@ -1964,7 +1964,7 @@ PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbac
ReplicationSlot *slot = MyReplicationSlot;
SpinLockAcquire(&slot->mutex);
MyPgXact->xmin = InvalidTransactionId;
MyProc->xmin = InvalidTransactionId;
/*
* For physical replication we don't need the interlock provided by xmin
@@ -2093,7 +2093,7 @@ ProcessStandbyHSFeedbackMessage(void)
if (!TransactionIdIsNormal(feedbackXmin)
&& !TransactionIdIsNormal(feedbackCatalogXmin))
{
MyPgXact->xmin = InvalidTransactionId;
MyProc->xmin = InvalidTransactionId;
if (MyReplicationSlot != NULL)
PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
return;
@@ -2135,7 +2135,7 @@ ProcessStandbyHSFeedbackMessage(void)
* risk already since a VACUUM could already have determined the horizon.)
*
* If we're using a replication slot we reserve the xmin via that,
* otherwise via the walsender's PGXACT entry. We can only track the
* otherwise via the walsender's PGPROC entry. We can only track the
* catalog xmin separately when using a slot, so we store the least of the
* two provided when not using a slot.
*
@@ -2148,9 +2148,9 @@ ProcessStandbyHSFeedbackMessage(void)
{
if (TransactionIdIsNormal(feedbackCatalogXmin)
&& TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin))
MyPgXact->xmin = feedbackCatalogXmin;
MyProc->xmin = feedbackCatalogXmin;
else
MyPgXact->xmin = feedbackXmin;
MyProc->xmin = feedbackXmin;
}
}