Wake up cluster monitor more aggressively when a backend is waiting

When a backend is waiting for another transaction to commit on the GTM, it
wakes up the cluster monitor process before going to sleep. This ensures that
the backend gets to know about transaction completion much sooner. This
mechanism shouldn't be too bad because a backend potentially may wait on
the global commit only after the transaction is locally committed. Hence in
most common cases, the wait shouldn't be too long.

Per report from Hengbing on the last patch.

diff --git a/src/backend/postmaster/clustermon.c b/src/backend/postmaster/clustermon.c
index d881afaba74ce275648f8572f5eb77edcf17ec59..5098ff33a4775963ebce21e79523cbfd99079510 100644 (file)
--- a/src/backend/postmaster/clustermon.c
+++ b/src/backend/postmaster/clustermon.c
@@ -64,7 +64,9 @@ static void ClusterMonitorSetReportingGlobalXmin(GlobalTransactionId xmin);
 /* PID of clustser monitoring process */
 int                    ClusterMonitorPid = 0;
 
-#define CLUSTER_MONITOR_NAPTIME        5
+#define CLUSTER_MONITOR_NAPTIME        5000
+
+long           ClusterMonitorNapTime = CLUSTER_MONITOR_NAPTIME;
 
 /*
  * Report xmin to the GTM and fetch the global xmin information in the
@@ -331,10 +333,10 @@ ClusterMonitorInit(void)
                if (!bootingUp)
                {
                        /*
-                        * Repeat at CLUSTER_MONITOR_NAPTIME seconds interval
+                        * Repeat at ClusterMonitorNapTime interval
                         */
-                       nap.tv_sec = CLUSTER_MONITOR_NAPTIME;
-                       nap.tv_usec = 0;
+                       nap.tv_sec = ClusterMonitorNapTime / 1000;
+                       nap.tv_usec = (ClusterMonitorNapTime % 1000) * 1000;
 
                        /*
                         * Wait until naptime expires or we get some type of signal (all the
@@ -600,7 +602,20 @@ ClusterMonitorWaitForEOFTransaction(GlobalTransactionId gxid)
 {
        ConditionVariablePrepareToSleep(&ClusterMonitorCtl->cv);
        while (ClusterMonitorTransactionIsInProgress(gxid))
+       {
+               /*
+                * Wake up the cluster monitor process before going to sleep. If the
+                * transaction is finishing soon, the cluster monitor will see it
+                * immediately that way.
+                *
+                * This is not very optimal and we might end up triggering cluster
+                * monitor too often. But this is not too bad either because most often
+                * transactions would wait on other transactions using the transaction
+                * locks and this path will be taken only in some corner cases.
+                */
+               ClusterMonitorWakeUp();
                ConditionVariableSleep(&ClusterMonitorCtl->cv,
                                WAIT_EVENT_CLUSTER_MONITOR_MAIN);
+       }
        ConditionVariableCancelSleep();
 }