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Commit e620ee3

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simonat2ndQuadrantsimonat2ndQuadrant
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Optimize commit_siblings in two ways to improve group commit.
First, avoid scanning the whole ProcArray once we know there are at least commit_siblings active; second, skip the check altogether if commit_siblings = 0. Greg Smith
1 parent 5a031a5 commit e620ee3
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‎doc/src/sgml/config.sgml

Copy file name to clipboardExpand all lines: doc/src/sgml/config.sgml
+12-5Lines changed: 12 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1683,17 +1683,24 @@ SET ENABLE_SEQSCAN TO OFF;
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</indexterm>
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<listitem>
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<para>
1686-
Time delay between writing a commit record to the WAL buffer
1687-
and flushing the buffer out to disk, in microseconds. A
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nonzero delay can allow multiple transactions to be committed
1689-
with only one <function>fsync()</function> system call, if
1686+
When the commit data for a transaction is flushed to disk, any
1687+
additional commits ready at that time are also flushed out.
1688+
<varname>commit_delay</varname> adds a time delay, set in
1689+
microseconds, before writing some commit records to the WAL
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buffer and flushing the buffer out to disks. A nonzero delay
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can allow more transactions to be committed with only one call
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to the active <varname>wal_sync_method</varname>, if
16901693
system load is high enough that additional transactions become
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ready to commit within the given interval. But the delay is
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just wasted if no other transactions become ready to
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commit. Therefore, the delay is only performed if at least
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<varname>commit_siblings</varname> other transactions are
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active at the instant that a server process has written its
1696-
commit record. The default is zero (no delay).
1699+
commit record. The default is zero (no delay). Since
1700+
all pending commit data flushes are written at every flush
1701+
regardless of this setting, it is rare that adding delay to
1702+
that by increasing this parameter will actually improve commit
1703+
performance.
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</para>
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</listitem>
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</varlistentry>

‎src/backend/access/transam/xact.c

Copy file name to clipboardExpand all lines: src/backend/access/transam/xact.c
+1-1Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1052,7 +1052,7 @@ RecordTransactionCommit(void)
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* fewer than CommitSiblings other backends with active transactions.
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*/
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if (CommitDelay > 0 && enableFsync &&
1055-
CountActiveBackends() >= CommitSiblings)
1055+
MinimumActiveBackends(CommitSiblings))
10561056
pg_usleep(CommitDelay);
10571057

10581058
XLogFlush(XactLastRecEnd);

‎src/backend/storage/ipc/procarray.c

Copy file name to clipboardExpand all lines: src/backend/storage/ipc/procarray.c
+12-5Lines changed: 12 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1886,20 +1886,25 @@ CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode)
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}
18871887

18881888
/*
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* CountActiveBackends --- count backends (other than myself) that are in
1890-
* active transactions. This is used as a heuristic to decide if
1889+
* MinimumActiveBackends --- count backends (other than myself) that are
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* in active transactions. Return true if the count exceeds the
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* minimum threshold passed. This is used as a heuristic to decide if
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* a pre-XLOG-flush delay is worthwhile during commit.
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*
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* Do not count backends that are blocked waiting for locks, since they are
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* not going to get to run until someone else commits.
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*/
1896-
int
1897-
CountActiveBackends(void)
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bool
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MinimumActiveBackends(int min)
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{
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ProcArrayStruct *arrayP = procArray;
19001901
int count = 0;
19011902
int index;
19021903

1904+
/* Quick short-circuit if no minimum is specified */
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if (min == 0)
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return true;
1907+
19031908
/*
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* Note: for speed, we don't acquire ProcArrayLock. This is a little bit
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* bogus, but since we are only testing fields for zero or nonzero, it
@@ -1932,9 +1937,11 @@ CountActiveBackends(void)
19321937
if (proc->waitLock != NULL)
19331938
continue; /* do not count if blocked on a lock */
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count++;
1940+
if (count >= min)
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break;
19351942
}
19361943

1937-
return count;
1944+
return count >= min;
19381945
}
19391946

19401947
/*

‎src/backend/utils/misc/guc.c

Copy file name to clipboardExpand all lines: src/backend/utils/misc/guc.c
+1-1Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1816,7 +1816,7 @@ static struct config_int ConfigureNamesInt[] =
18161816
NULL
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},
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&CommitSiblings,
1819-
5, 1, 1000, NULL, NULL
1819+
5, 0, 1000, NULL, NULL
18201820
},
18211821

18221822
{

‎src/include/storage/procarray.h

Copy file name to clipboardExpand all lines: src/include/storage/procarray.h
+1-1Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -60,7 +60,7 @@ extern VirtualTransactionId *GetCurrentVirtualXIDs(TransactionId limitXmin,
6060
extern VirtualTransactionId *GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid);
6161
extern pid_t CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode);
6262

63-
extern int CountActiveBackends(void);
63+
extern bool MinimumActiveBackends(int min);
6464
extern int CountDBBackends(Oid databaseid);
6565
extern void CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conflictPending);
6666
extern int CountUserBackends(Oid roleid);

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