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Commit2031931

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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 parent0f65b6c commit2031931

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+48
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+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
@@ -982,10 +982,7 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
982982
* Note these are pretty much the same protections in GetNewTransactionId.
983983
*----------
984984
*/
985-
if (!MultiXactIdPrecedes(result,MultiXactState->multiVacLimit)||
986-
!MultiXactState->oldestOffsetKnown||
987-
(MultiXactState->nextOffset-MultiXactState->oldestOffset
988-
>MULTIXACT_MEMBER_SAFE_THRESHOLD))
985+
if (!MultiXactIdPrecedes(result,MultiXactState->multiVacLimit))
989986
{
990987
/*
991988
* For safety's sake, we release MultiXactGenLock while sending
@@ -1001,19 +998,17 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
1001998

1002999
LWLockRelease(MultiXactGenLock);
10031000

1004-
/*
1005-
* To avoid swamping the postmaster with signals, we issue the autovac
1006-
* request only once per 64K multis generated. This still gives
1007-
* plenty of chances before we get into real trouble.
1008-
*/
1009-
if (IsUnderPostmaster&& (result %65536)==0)
1010-
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1011-
10121001
if (IsUnderPostmaster&&
10131002
!MultiXactIdPrecedes(result,multiStopLimit))
10141003
{
10151004
char*oldest_datname=get_database_name(oldest_datoid);
10161005

1006+
/*
1007+
* Immediately kick autovacuum into action as we're already
1008+
* in ERROR territory.
1009+
*/
1010+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1011+
10171012
/* complain even if that DB has disappeared */
10181013
if (oldest_datname)
10191014
ereport(ERROR,
@@ -1030,7 +1025,16 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
10301025
errhint("Execute a database-wide VACUUM in that database.\n"
10311026
"You might also need to commit or roll back old prepared transactions.")));
10321027
}
1033-
elseif (!MultiXactIdPrecedes(result,multiWarnLimit))
1028+
1029+
/*
1030+
* To avoid swamping the postmaster with signals, we issue the autovac
1031+
* request only once per 64K multis generated. This still gives
1032+
* plenty of chances before we get into real trouble.
1033+
*/
1034+
if (IsUnderPostmaster&& (result %65536)==0)
1035+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1036+
1037+
if (!MultiXactIdPrecedes(result,multiWarnLimit))
10341038
{
10351039
char*oldest_datname=get_database_name(oldest_datoid);
10361040

@@ -1101,6 +1105,10 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
11011105
if (MultiXactState->offsetStopLimitKnown&&
11021106
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,nextOffset,
11031107
nmembers))
1108+
{
1109+
/* see comment in the corresponding offsets wraparound case */
1110+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1111+
11041112
ereport(ERROR,
11051113
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
11061114
errmsg("multixact \"members\" limit exceeded"),
@@ -1111,10 +1119,33 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
11111119
MultiXactState->offsetStopLimit-nextOffset-1),
11121120
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.",
11131121
MultiXactState->oldestMultiXactDB)));
1114-
elseif (MultiXactState->offsetStopLimitKnown&&
1115-
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
1116-
nextOffset,
1117-
nmembers+MULTIXACT_MEMBERS_PER_PAGE*SLRU_PAGES_PER_SEGMENT*OFFSET_WARN_SEGMENTS))
1122+
}
1123+
1124+
/*
1125+
* Check whether we should kick autovacuum into action, to prevent members
1126+
* wraparound. NB we use a much larger window to trigger autovacuum than
1127+
* just the warning limit. The warning is just a measure of last resort -
1128+
* this is in line with GetNewTransactionId's behaviour.
1129+
*/
1130+
if (!MultiXactState->oldestOffsetKnown||
1131+
(MultiXactState->nextOffset-MultiXactState->oldestOffset
1132+
>MULTIXACT_MEMBER_SAFE_THRESHOLD))
1133+
{
1134+
/*
1135+
* To avoid swamping the postmaster with signals, we issue the autovac
1136+
* request only when crossing a segment boundary. With default
1137+
* compilation settings that's rougly after 50k members. This still
1138+
* gives plenty of chances before we get into real trouble.
1139+
*/
1140+
if ((MXOffsetToMemberPage(nextOffset) /SLRU_PAGES_PER_SEGMENT)!=
1141+
(MXOffsetToMemberPage(nextOffset+nmembers) /SLRU_PAGES_PER_SEGMENT))
1142+
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1143+
}
1144+
1145+
if (MultiXactState->offsetStopLimitKnown&&
1146+
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
1147+
nextOffset,
1148+
nmembers+MULTIXACT_MEMBERS_PER_PAGE*SLRU_PAGES_PER_SEGMENT*OFFSET_WARN_SEGMENTS))
11181149
ereport(WARNING,
11191150
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
11201151
errmsg("database with OID %u must be vacuumed before %d more multixact members are used",

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