From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Wed, 3 Jul 2024 23:05:37 +0000 (+0300)
Subject: Optimize memory access in GetRunningTransactionData()
X-Git-Tag: REL_18_BETA1~2479
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=6897f0ec024582a570868939d3f34a6853374723;p=postgresql.git

Optimize memory access in GetRunningTransactionData()

e85662df44 made GetRunningTransactionData() calculate the oldest running
transaction id within the current database.  This commit optimized this
calculation by performing a cheap transaction id comparison before fetching
the process database id, while the latter could cause extra cache misses.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240630231816.bf.nmisch%40google.com
---

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 9fc930e98f8..16b5803d388 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2753,8 +2753,6 @@ GetRunningTransactionData(void)
 	 */
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
-		int			pgprocno = arrayP->pgprocnos[index];
-		PGPROC	   *proc = &allProcs[pgprocno];
 		TransactionId xid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
@@ -2776,11 +2774,18 @@ GetRunningTransactionData(void)
 			oldestRunningXid = xid;
 
 		/*
-		 * Also, update the oldest running xid within the current database.
+		 * Also, update the oldest running xid within the current database. As
+		 * fetching pgprocno and PGPROC could cause cache misses, we do cheap
+		 * TransactionId comparison first.
 		 */
-		if (proc->databaseId == MyDatabaseId &&
-			TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
-			oldestDatabaseRunningXid = xid;
+		if (TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
+		{
+			int			pgprocno = arrayP->pgprocnos[index];
+			PGPROC	   *proc = &allProcs[pgprocno];
+
+			if (proc->databaseId == MyDatabaseId)
+				oldestDatabaseRunningXid = xid;
+		}
 
 		if (ProcGlobal->subxidStates[index].overflowed)
 			suboverflowed = true;