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Commit667912a

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Improve multixact emergency autovacuum logic.
Previously autovacuum was not necessarily triggered if space in themembers slru got tight. The first problem was that the signalling wastied to values in the offsets slru, but members can advance muchfaster. Thats especially a problem if old sessions had been around thatpreviously prevented the multixact horizon to increase. Secondly theskipping logic doesn't work if the database was restarted afterautovacuum was triggered - that knowledge is not preserved acrossrestart. This is especially a problem because it's a commonpanic-reaction to restart the database if it gets slow toanti-wraparound vacuums.Fix the first problem by separating the logic for members fromoffsets. Trigger autovacuum whenever a multixact crosses a segmentboundary, as the current member offset increases in irregular values, sowe can't use a simple modulo logic as for offsets. Add a stopgap forthe second problem, by signalling autovacuum whenver ERRORing outbecause of boundaries.Discussion: 20150608163707.GD20772@alap3.anarazel.deBackpatch into 9.3, where it became more likely that multixacts wraparound.
1 parent90231cd commit667912a

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+48
-17
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1 file changed

+48
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‎src/backend/access/transam/multixact.c

Lines changed: 48 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -977,10 +977,7 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
977977
* Note these are pretty much the same protections in GetNewTransactionId.
978978
*----------
979979
*/
980-
if (!MultiXactIdPrecedes(result,MultiXactState->multiVacLimit)||
981-
!MultiXactState->oldestOffsetKnown||
982-
(MultiXactState->nextOffset-MultiXactState->oldestOffset
983-
>MULTIXACT_MEMBER_SAFE_THRESHOLD))
980+
if (!MultiXactIdPrecedes(result,MultiXactState->multiVacLimit))
984981
{
985982
/*
986983
* For safety's sake, we release MultiXactGenLock while sending
@@ -996,19 +993,17 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
996993

997994
LWLockRelease(MultiXactGenLock);
998995

999-
/*
1000-
* To avoid swamping the postmaster with signals, we issue the autovac
1001-
* request only once per 64K multis generated. This still gives
1002-
* plenty of chances before we get into real trouble.
1003-
*/
1004-
if (IsUnderPostmaster&& (result %65536)==0)
1005-
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1006-
1007996
if (IsUnderPostmaster&&
1008997
!MultiXactIdPrecedes(result,multiStopLimit))
1009998
{
1010999
char*oldest_datname=get_database_name(oldest_datoid);
10111000

1001+
/*
1002+
* Immediately kick autovacuum into action as we're already
1003+
* in ERROR territory.
1004+
*/
1005+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1006+
10121007
/* complain even if that DB has disappeared */
10131008
if (oldest_datname)
10141009
ereport(ERROR,
@@ -1025,7 +1020,16 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
10251020
errhint("Execute a database-wide VACUUM in that database.\n"
10261021
"You might also need to commit or roll back old prepared transactions.")));
10271022
}
1028-
elseif (!MultiXactIdPrecedes(result,multiWarnLimit))
1023+
1024+
/*
1025+
* To avoid swamping the postmaster with signals, we issue the autovac
1026+
* request only once per 64K multis generated. This still gives
1027+
* plenty of chances before we get into real trouble.
1028+
*/
1029+
if (IsUnderPostmaster&& (result %65536)==0)
1030+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1031+
1032+
if (!MultiXactIdPrecedes(result,multiWarnLimit))
10291033
{
10301034
char*oldest_datname=get_database_name(oldest_datoid);
10311035

@@ -1096,6 +1100,10 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
10961100
if (MultiXactState->offsetStopLimitKnown&&
10971101
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,nextOffset,
10981102
nmembers))
1103+
{
1104+
/* see comment in the corresponding offsets wraparound case */
1105+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1106+
10991107
ereport(ERROR,
11001108
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
11011109
errmsg("multixact \"members\" limit exceeded"),
@@ -1106,10 +1114,33 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
11061114
MultiXactState->offsetStopLimit-nextOffset-1),
11071115
errhint("Execute a database-wide VACUUM in database with OID %u with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.",
11081116
MultiXactState->oldestMultiXactDB)));
1109-
elseif (MultiXactState->offsetStopLimitKnown&&
1110-
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
1111-
nextOffset,
1112-
nmembers+MULTIXACT_MEMBERS_PER_PAGE*SLRU_PAGES_PER_SEGMENT*OFFSET_WARN_SEGMENTS))
1117+
}
1118+
1119+
/*
1120+
* Check whether we should kick autovacuum into action, to prevent members
1121+
* wraparound. NB we use a much larger window to trigger autovacuum than
1122+
* just the warning limit. The warning is just a measure of last resort -
1123+
* this is in line with GetNewTransactionId's behaviour.
1124+
*/
1125+
if (!MultiXactState->oldestOffsetKnown||
1126+
(MultiXactState->nextOffset-MultiXactState->oldestOffset
1127+
>MULTIXACT_MEMBER_SAFE_THRESHOLD))
1128+
{
1129+
/*
1130+
* To avoid swamping the postmaster with signals, we issue the autovac
1131+
* request only when crossing a segment boundary. With default
1132+
* compilation settings that's rougly after 50k members. This still
1133+
* gives plenty of chances before we get into real trouble.
1134+
*/
1135+
if ((MXOffsetToMemberPage(nextOffset) /SLRU_PAGES_PER_SEGMENT)!=
1136+
(MXOffsetToMemberPage(nextOffset+nmembers) /SLRU_PAGES_PER_SEGMENT))
1137+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1138+
}
1139+
1140+
if (MultiXactState->offsetStopLimitKnown&&
1141+
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
1142+
nextOffset,
1143+
nmembers+MULTIXACT_MEMBERS_PER_PAGE*SLRU_PAGES_PER_SEGMENT*OFFSET_WARN_SEGMENTS))
11131144
ereport(WARNING,
11141145
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
11151146
errmsg("database with OID %u must be vacuumed before %d more multixact members are used",

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