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Commit7e3da1c

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Mitigate "snapshot too old" performance regression on NUMA
Limit maintenance of time to xid mapping to once per minute. Atleast in the tested case this brings performance within 5% of whenthe feature is off, compared to several times slower without thispatch.While there, fix comments and whitespace.Ants Aasma, with cosmetic adjustments suggested by Andres FreundReviewed by Kevin Grittner and Andres Freund
1 parenteb7de00 commit7e3da1c

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1 file changed

+62
-23
lines changed

1 file changed

+62
-23
lines changed

‎src/backend/utils/time/snapmgr.c

Lines changed: 62 additions & 23 deletions
Original file line numberDiff line numberDiff line change
@@ -78,10 +78,13 @@ typedef struct OldSnapshotControlData
7878
* Variables for old snapshot handling are shared among processes and are
7979
* only allowed to move forward.
8080
*/
81-
slock_tmutex_current;/* protectcurrent timestamp */
81+
slock_tmutex_current;/* protectcurrent_timestamp */
8282
int64current_timestamp;/* latest snapshot timestamp */
83-
slock_tmutex_latest_xmin;/* protect latest snapshot xmin */
83+
slock_tmutex_latest_xmin;/* protect latest_xmin
84+
* and next_map_update
85+
*/
8486
TransactionIdlatest_xmin;/* latest snapshot xmin */
87+
int64next_map_update;/* latest snapshot valid up to */
8588
slock_tmutex_threshold;/* protect threshold fields */
8689
int64threshold_timestamp;/* earlier snapshot is old */
8790
TransactionIdthreshold_xid;/* earlier xid may be gone */
@@ -95,7 +98,10 @@ typedef struct OldSnapshotControlData
9598
* count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
9699
* is full and the head must be advanced to add new entries. Use
97100
* timestamps aligned to minute boundaries, since that seems less
98-
* surprising than aligning based on the first usage timestamp.
101+
* surprising than aligning based on the first usage timestamp. The
102+
* latest bucket is effectively stored within latest_xmin. The circular
103+
* buffer is updated when we get a new xmin value that doesn't fall into
104+
* the same interval.
99105
*
100106
* It is OK if the xid for a given time slot is from earlier than
101107
* calculated by adding the number of minutes corresponding to the
@@ -269,6 +275,7 @@ SnapMgrInit(void)
269275
oldSnapshotControl->current_timestamp=0;
270276
SpinLockInit(&oldSnapshotControl->mutex_latest_xmin);
271277
oldSnapshotControl->latest_xmin=InvalidTransactionId;
278+
oldSnapshotControl->next_map_update=0;
272279
SpinLockInit(&oldSnapshotControl->mutex_threshold);
273280
oldSnapshotControl->threshold_timestamp=0;
274281
oldSnapshotControl->threshold_xid=InvalidTransactionId;
@@ -1595,9 +1602,15 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
15951602
{
15961603
int64ts=GetSnapshotCurrentTimestamp();
15971604
TransactionIdxlimit=recentXmin;
1598-
TransactionIdlatest_xmin=oldSnapshotControl->latest_xmin;
1605+
TransactionIdlatest_xmin;
1606+
int64update_ts;
15991607
boolsame_ts_as_threshold= false;
16001608

1609+
SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
1610+
latest_xmin=oldSnapshotControl->latest_xmin;
1611+
update_ts=oldSnapshotControl->next_map_update;
1612+
SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
1613+
16011614
/*
16021615
* Zero threshold always overrides to latest xmin, if valid. Without
16031616
* some heuristic it will find its own snapshot too old on, for
@@ -1632,26 +1645,35 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
16321645

16331646
if (!same_ts_as_threshold)
16341647
{
1635-
LWLockAcquire(OldSnapshotTimeMapLock,LW_SHARED);
1636-
1637-
if (oldSnapshotControl->count_used>0
1638-
&&ts >=oldSnapshotControl->head_timestamp)
1648+
if (ts==update_ts)
16391649
{
1640-
intoffset;
1641-
1642-
offset= ((ts-oldSnapshotControl->head_timestamp)
1643-
/USECS_PER_MINUTE);
1644-
if (offset>oldSnapshotControl->count_used-1)
1645-
offset=oldSnapshotControl->count_used-1;
1646-
offset= (oldSnapshotControl->head_offset+offset)
1647-
%OLD_SNAPSHOT_TIME_MAP_ENTRIES;
1648-
xlimit=oldSnapshotControl->xid_by_minute[offset];
1649-
1650+
xlimit=latest_xmin;
16501651
if (NormalTransactionIdFollows(xlimit,recentXmin))
16511652
SetOldSnapshotThresholdTimestamp(ts,xlimit);
16521653
}
1654+
else
1655+
{
1656+
LWLockAcquire(OldSnapshotTimeMapLock,LW_SHARED);
16531657

1654-
LWLockRelease(OldSnapshotTimeMapLock);
1658+
if (oldSnapshotControl->count_used>0
1659+
&&ts >=oldSnapshotControl->head_timestamp)
1660+
{
1661+
intoffset;
1662+
1663+
offset= ((ts-oldSnapshotControl->head_timestamp)
1664+
/USECS_PER_MINUTE);
1665+
if (offset>oldSnapshotControl->count_used-1)
1666+
offset=oldSnapshotControl->count_used-1;
1667+
offset= (oldSnapshotControl->head_offset+offset)
1668+
%OLD_SNAPSHOT_TIME_MAP_ENTRIES;
1669+
xlimit=oldSnapshotControl->xid_by_minute[offset];
1670+
1671+
if (NormalTransactionIdFollows(xlimit,recentXmin))
1672+
SetOldSnapshotThresholdTimestamp(ts,xlimit);
1673+
}
1674+
1675+
LWLockRelease(OldSnapshotTimeMapLock);
1676+
}
16551677
}
16561678

16571679
/*
@@ -1681,16 +1703,35 @@ void
16811703
MaintainOldSnapshotTimeMapping(int64whenTaken,TransactionIdxmin)
16821704
{
16831705
int64ts;
1706+
TransactionIdlatest_xmin;
1707+
int64update_ts;
1708+
boolmap_update_required= false;
16841709

16851710
/* Never call this function when old snapshot checking is disabled. */
16861711
Assert(old_snapshot_threshold >=0);
16871712

1688-
/* Keep track of the latest xmin seen by any process. */
1713+
ts=AlignTimestampToMinuteBoundary(whenTaken);
1714+
1715+
/*
1716+
* Keep track of the latest xmin seen by any process. Update mapping
1717+
* with a new value when we have crossed a bucket boundary.
1718+
*/
16891719
SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
1690-
if (TransactionIdFollows(xmin,oldSnapshotControl->latest_xmin))
1720+
latest_xmin=oldSnapshotControl->latest_xmin;
1721+
update_ts=oldSnapshotControl->next_map_update;
1722+
if (ts>update_ts)
1723+
{
1724+
oldSnapshotControl->next_map_update=ts;
1725+
map_update_required= true;
1726+
}
1727+
if (TransactionIdFollows(xmin,latest_xmin))
16911728
oldSnapshotControl->latest_xmin=xmin;
16921729
SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
16931730

1731+
/* We only needed to update the most recent xmin value. */
1732+
if (!map_update_required)
1733+
return;
1734+
16941735
/* No further tracking needed for 0 (used for testing). */
16951736
if (old_snapshot_threshold==0)
16961737
return;
@@ -1716,8 +1757,6 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17161757
return;
17171758
}
17181759

1719-
ts=AlignTimestampToMinuteBoundary(whenTaken);
1720-
17211760
LWLockAcquire(OldSnapshotTimeMapLock,LW_EXCLUSIVE);
17221761

17231762
Assert(oldSnapshotControl->head_offset >=0);

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