database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-11-04 20:11:56 +03:00
Code Activity

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

Browse Source 
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

28
src/backend/utils/time/snapmgr.c
View File

@@ -27,11 +27,11 @@
* their lifetime is managed separately (as they live longer than one xact.c
* transaction).
*
* These arrangements let us reset MyPgXact->xmin when there are no snapshots
* These arrangements let us reset MyProc->xmin when there are no snapshots
* referenced by this transaction, and advance it when the one with oldest
* Xmin is no longer referenced. For simplicity however, only registered
* snapshots not active snapshots participate in tracking which one is oldest;
* we don't try to change MyPgXact->xmin except when the active-snapshot
* we don't try to change MyProc->xmin except when the active-snapshot
* stack is empty.
*
*
@@ -187,7 +187,7 @@ static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
/*
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
* quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
* quickly find the one with lowest xmin, to advance our MyProc->xmin.
*/
static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
void *arg);
@@ -475,7 +475,7 @@ GetNonHistoricCatalogSnapshot(Oid relid)
/*
* Make sure the catalog snapshot will be accounted for in decisions
* about advancing PGXACT->xmin. We could apply RegisterSnapshot, but
* about advancing PGPROC->xmin. We could apply RegisterSnapshot, but
* that would result in making a physical copy, which is overkill; and
* it would also create a dependency on some resource owner, which we
* do not want for reasons explained at the head of this file. Instead
@@ -596,7 +596,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
/* NB: curcid should NOT be copied, it's a local matter */
/*
* Now we have to fix what GetSnapshotData did with MyPgXact->xmin and
* Now we have to fix what GetSnapshotData did with MyProc->xmin and
* TransactionXmin. There is a race condition: to make sure we are not
* causing the global xmin to go backwards, we have to test that the
* source transaction is still running, and that has to be done
@@ -950,13 +950,13 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
/*
* SnapshotResetXmin
*
* If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
* If there are no more snapshots, we can reset our PGPROC->xmin to InvalidXid.
* Note we can do this without locking because we assume that storing an Xid
* is atomic.
*
* Even if there are some remaining snapshots, we may be able to advance our
* PGXACT->xmin to some degree. This typically happens when a portal is
* dropped. For efficiency, we only consider recomputing PGXACT->xmin when
* PGPROC->xmin to some degree. This typically happens when a portal is
* dropped. For efficiency, we only consider recomputing PGPROC->xmin when
* the active snapshot stack is empty; this allows us not to need to track
* which active snapshot is oldest.
*
@@ -977,15 +977,15 @@ SnapshotResetXmin(void)
if (pairingheap_is_empty(&RegisteredSnapshots))
{
MyPgXact->xmin = InvalidTransactionId;
MyProc->xmin = InvalidTransactionId;
return;
}
minSnapshot = pairingheap_container(SnapshotData, ph_node,
pairingheap_first(&RegisteredSnapshots));
if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin))
MyPgXact->xmin = minSnapshot->xmin;
if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
MyProc->xmin = minSnapshot->xmin;
}
/*
@@ -1132,13 +1132,13 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
/*
* During normal commit processing, we call ProcArrayEndTransaction() to
* reset the MyPgXact->xmin. That call happens prior to the call to
* reset the MyProc->xmin. That call happens prior to the call to
* AtEOXact_Snapshot(), so we need not touch xmin here at all.
*/
if (resetXmin)
SnapshotResetXmin();
Assert(resetXmin || MyPgXact->xmin == 0);
Assert(resetXmin || MyProc->xmin == 0);
}
@@ -1830,7 +1830,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
*/
if (old_snapshot_threshold == 0)
{
if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
if (TransactionIdPrecedes(latest_xmin, MyProc->xmin)
&& TransactionIdFollows(latest_xmin, xlimit))
xlimit = latest_xmin;