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Commitbe9788e

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Avoid unnecessary acquisition of SyncRepLock in transaction commit time.
In SyncRepWaitForLSN() routine called in transaction commit time,SyncRepLock is necessary to atomically both check the sharedsync_standbys_defined flag and operate the sync replication wait-queue.On the other hand, when the flag is false, the lock is not necessarybecause the wait-queue is not touched. But due to the changes bycommit48c9f49, previously the lock was taken whatever the flag was.This could cause unnecessary performance overhead in every transactioncommit time. Therefore this commit avoids that unnecessary aquisitionof SyncRepLock.Author: Fujii MasaoReviewed-by: Asim Praveen, Masahiko Sawada,Discussion:https://postgr.es/m/20200406050332.nsscfqjzk2d57zyx@alap3.anarazel.de
1 parent05c16b8 commitbe9788e

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‎src/backend/replication/syncrep.c‎

Lines changed: 18 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -158,18 +158,30 @@ SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
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*/
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Assert(InterruptHoldoffCount>0);
160160

161+
/*
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* Fast exit if user has not requested sync replication, or there are no
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* sync replication standby names defined.
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*
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* Since this routine gets called every commit time, it's important to
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* exit quickly if sync replication is not requested. So we check
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* WalSndCtl->sync_standbys_defined flag without the lock and exit
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* immediately if it's false. If it's true, we need to check it again later
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* while holding the lock, to check the flag and operate the sync rep
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* queue atomically. This is necessary to avoid the race condition
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* described in SyncRepUpdateSyncStandbysDefined(). On the other
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* hand, if it's false, the lock is not necessary because we don't touch
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* the queue.
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*/
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if (!SyncRepRequested()||
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!((volatileWalSndCtlData*)WalSndCtl)->sync_standbys_defined)
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return;
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/* Cap the level for anything other than commit to remote flush only. */
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if (commit)
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mode=SyncRepWaitMode;
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else
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mode=Min(SyncRepWaitMode,SYNC_REP_WAIT_FLUSH);
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167-
/*
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* Fast exit if user has not requested sync replication.
169-
*/
170-
if (!SyncRepRequested())
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return;
172-
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Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
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Assert(WalSndCtl!=NULL);
175187

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