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Commit7c3e803

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Add a few entries to the tail of time mapping, to see old values.
Without a few entries beyond old_snapshot_threshold, the lookupwould often fail, resulting in the more aggressive pruning orvacuum being skipped often enough to matter. This was very clearlyshown by a python test script posted by Ants Aasma, and was likelya factor in an earlier but somewhat less clear-cut test case postedby Jeff Janes.This patch makes no change to the logic, per se -- it just makesthe array of mapping entries big enough to make lookup misses basedon timing much less likely. An occasional miss is still possibleif a thread stalls for more than 10 minutes, but that does notcreate any problem with correctness of behavior. Besides, ifthings are so busy that a thread is stalling for more than 10minutes, it is probably OK to skip the more aggressive cleanup atthat particular point in time.
1 parentd34e7b2 commit7c3e803

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2 files changed

+26
-13
lines changed

2 files changed

+26
-13
lines changed

‎src/backend/utils/time/snapmgr.c

Lines changed: 13 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -92,10 +92,10 @@ typedef struct OldSnapshotControlData
9292
* Use a circular buffer with a head offset, a count of entries currently
9393
* used, and a timestamp corresponding to the xid at the head offset. A
9494
* count_used value of zero means that there are no times stored; a
95-
* count_used value ofold_snapshot_threshold means that the buffer is
96-
* full and the head must be advanced to add new entries. Use timestamps
97-
* aligned to minute boundaries, since that seems less surprising than
98-
* aligning based on the first usage timestamp.
95+
* count_used value ofOLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
96+
*isfull and the head must be advanced to add new entries. Use
97+
*timestampsaligned to minute boundaries, since that seems less
98+
*surprising thanaligning based on the first usage timestamp.
9999
*
100100
* It is OK if the xid for a given time slot is from earlier than
101101
* calculated by adding the number of minutes corresponding to the
@@ -243,7 +243,7 @@ SnapMgrShmemSize(void)
243243
size= offsetof(OldSnapshotControlData,xid_by_minute);
244244
if (old_snapshot_threshold>0)
245245
size=add_size(size,mul_size(sizeof(TransactionId),
246-
old_snapshot_threshold));
246+
OLD_SNAPSHOT_TIME_MAP_ENTRIES));
247247

248248
returnsize;
249249
}
@@ -1643,7 +1643,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
16431643
if (offset>oldSnapshotControl->count_used-1)
16441644
offset=oldSnapshotControl->count_used-1;
16451645
offset= (oldSnapshotControl->head_offset+offset)
1646-
%old_snapshot_threshold;
1646+
%OLD_SNAPSHOT_TIME_MAP_ENTRIES;
16471647
xlimit=oldSnapshotControl->xid_by_minute[offset];
16481648

16491649
if (NormalTransactionIdFollows(xlimit,recentXmin))
@@ -1720,10 +1720,10 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17201720
LWLockAcquire(OldSnapshotTimeMapLock,LW_EXCLUSIVE);
17211721

17221722
Assert(oldSnapshotControl->head_offset >=0);
1723-
Assert(oldSnapshotControl->head_offset<old_snapshot_threshold);
1723+
Assert(oldSnapshotControl->head_offset<OLD_SNAPSHOT_TIME_MAP_ENTRIES);
17241724
Assert((oldSnapshotControl->head_timestamp %USECS_PER_MINUTE)==0);
17251725
Assert(oldSnapshotControl->count_used >=0);
1726-
Assert(oldSnapshotControl->count_used <=old_snapshot_threshold);
1726+
Assert(oldSnapshotControl->count_used <=OLD_SNAPSHOT_TIME_MAP_ENTRIES);
17271727

17281728
if (oldSnapshotControl->count_used==0)
17291729
{
@@ -1750,7 +1750,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17501750
intbucket= (oldSnapshotControl->head_offset
17511751
+ ((ts-oldSnapshotControl->head_timestamp)
17521752
/USECS_PER_MINUTE))
1753-
%old_snapshot_threshold;
1753+
%OLD_SNAPSHOT_TIME_MAP_ENTRIES;
17541754

17551755
if (TransactionIdPrecedes(oldSnapshotControl->xid_by_minute[bucket],xmin))
17561756
oldSnapshotControl->xid_by_minute[bucket]=xmin;
@@ -1763,7 +1763,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17631763

17641764
oldSnapshotControl->head_timestamp=ts;
17651765

1766-
if (advance >=old_snapshot_threshold)
1766+
if (advance >=OLD_SNAPSHOT_TIME_MAP_ENTRIES)
17671767
{
17681768
/* Advance is so far that all old data is junk; start over. */
17691769
oldSnapshotControl->head_offset=0;
@@ -1777,12 +1777,12 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17771777

17781778
for (i=0;i<advance;i++)
17791779
{
1780-
if (oldSnapshotControl->count_used==old_snapshot_threshold)
1780+
if (oldSnapshotControl->count_used==OLD_SNAPSHOT_TIME_MAP_ENTRIES)
17811781
{
17821782
/* Map full and new value replaces old head. */
17831783
intold_head=oldSnapshotControl->head_offset;
17841784

1785-
if (old_head== (old_snapshot_threshold-1))
1785+
if (old_head== (OLD_SNAPSHOT_TIME_MAP_ENTRIES-1))
17861786
oldSnapshotControl->head_offset=0;
17871787
else
17881788
oldSnapshotControl->head_offset=old_head+1;
@@ -1793,7 +1793,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
17931793
/* Extend map to unused entry. */
17941794
intnew_tail= (oldSnapshotControl->head_offset
17951795
+oldSnapshotControl->count_used)
1796-
%old_snapshot_threshold;
1796+
%OLD_SNAPSHOT_TIME_MAP_ENTRIES;
17971797

17981798
oldSnapshotControl->count_used++;
17991799
oldSnapshotControl->xid_by_minute[new_tail]=xmin;

‎src/include/utils/snapmgr.h

Lines changed: 13 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,19 @@
1919
#include"utils/snapshot.h"
2020

2121

22+
/*
23+
* The structure used to map times to TransactionId values for the "snapshot
24+
* too old" feature must have a few entries at the tail to hold old values;
25+
* otherwise the lookup will often fail and the expected early pruning or
26+
* vacuum will not usually occur. It is best if this padding is for a number
27+
* of minutes greater than a thread would normally be stalled, but it's OK if
28+
* early vacuum opportunities are occasionally missed, so there's no need to
29+
* use an extreme value or get too fancy. 10 minutes seems plenty.
30+
*/
31+
#defineOLD_SNAPSHOT_PADDING_ENTRIES 10
32+
#defineOLD_SNAPSHOT_TIME_MAP_ENTRIES (old_snapshot_threshold + OLD_SNAPSHOT_PADDING_ENTRIES)
33+
34+
2235
/* GUC variables */
2336
externPGDLLIMPORTintold_snapshot_threshold;
2437

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