[jbossts-issues] [JBoss JIRA] Created: (JBTM-688) TransactionReaper synchronization is a performance bottleneck

[Oleg Nitz (JIRA)]

TransactionReaper synchronization is a performance bottleneck
-------------------------------------------------------------

                 Key: JBTM-688
                 URL: https://jira.jboss.org/jira/browse/JBTM-688
             Project: JBoss Transaction Manager
          Issue Type: Patch
      Security Level: Public (Everyone can see)
          Components: JTS
    Affects Versions:  4.6.1.CP04
         Environment: Linux 32-bit, Sun JDK 6
            Reporter: Oleg Nitz


During stress testing I found that threads often massively block on synchronization on TransactionReaper.class, due to synchronized static methods create() and transactionReaper() in the TransactionReaper class.
I propose to use "double-checked locking idiom" (see http://en.wikipedia.org/wiki/Double-checked_locking ) to avoid unnecesary synchronizations.
My patch follows.

--- TransactionReaper.java.orig	2010-01-04 16:27:21.000000000 +0200
+++ TransactionReaper.java	2010-01-11 17:19:55.000000000 +0200
@@ -1059,7 +1059,7 @@
 	 * Could get priority from environment.
 	 */
 
-	public static synchronized TransactionReaper create(long checkPeriod)
+	public static TransactionReaper create(long checkPeriod)
 	{
 		if (tsLogger.arjLogger.debugAllowed())
 		{
@@ -1070,6 +1070,10 @@
 
 		if (TransactionReaper._theReaper == null)
 		{
+        synchronized (TransactionReaper.class) {
+        if (TransactionReaper._theReaper == null)
+        {
+		    
             // default to dynamic mode
             TransactionReaper._dynamic = true;
 
@@ -1215,6 +1219,8 @@
 
 			_reaperWorkerThread.start();
 		}
+        }
+		}
 
 		return TransactionReaper._theReaper;
 	}
@@ -1233,7 +1239,7 @@
 	 * If parameter is true then do a create.
 	 */
 
-	public static synchronized TransactionReaper transactionReaper(
+	public static TransactionReaper transactionReaper(
 			boolean createReaper)
 	{
 		if (createReaper)
@@ -1315,7 +1321,7 @@
 	 */
 	private int _zombieMax = 0;
 
-	private static TransactionReaper _theReaper = null;
+	private static volatile TransactionReaper _theReaper = null;
 
 	private static ReaperThread _reaperThread = null;
 


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