From af1e73116f105cb3d2cdc4da2f9289c30707fc39 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Sun, 30 Aug 2020 23:22:19 +1200
Subject: [PATCH] Optimize setitimer() usage.

Don't call setitimer() so often.  Instead, let a preexisting alarm expire
and then request a new one as requried.  It's better to take a few extra
spurious alarm interrupts at a rate that's a function of the timeout
setting than to have a system call rate that's a function of statement
execution frequency.

XXX:  Need to review this carefully for races, perhaps involving
reentrancy.
---
 src/backend/utils/misc/timeout.c | 42 ++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/src/backend/utils/misc/timeout.c b/src/backend/utils/misc/timeout.c
index f1c9518b0c..b5027815db 100644
--- a/src/backend/utils/misc/timeout.c
+++ b/src/backend/utils/misc/timeout.c
@@ -51,6 +51,13 @@ static bool all_timeouts_initialized = false;
 static volatile int num_active_timeouts = 0;
 static timeout_params *volatile active_timeouts[MAX_TIMEOUTS];
 
+/*
+ * State used to avoid installing a new timer interrupt when the previous one
+ * hasn't fired yet, but isn't too late.
+ */
+static TimestampTz sigalrm_due_at = PG_INT64_MAX;
+static volatile sig_atomic_t sigalrm_delivered = false;
+
 /*
  * Flag controlling whether the signal handler is allowed to do anything.
  * We leave this "false" when we're not expecting interrupts, just in case.
@@ -195,12 +202,13 @@ schedule_alarm(TimestampTz now)
 		struct itimerval timeval;
 		long		secs;
 		int			usecs;
+		TimestampTz	nearest_timeout;
 
 		MemSet(&timeval, 0, sizeof(struct itimerval));
 
 		/* Get the time remaining till the nearest pending timeout */
-		TimestampDifference(now, active_timeouts[0]->fin_time,
-							&secs, &usecs);
+		nearest_timeout = active_timeouts[0]->fin_time;
+		TimestampDifference(now, nearest_timeout, &secs, &usecs);
 
 		/*
 		 * It's possible that the difference is less than a microsecond;
@@ -244,9 +252,18 @@ schedule_alarm(TimestampTz now)
 		 */
 		enable_alarm();
 
+		/*
+		 * Try to avoid having to set the interval timer, if we already know
+		 * that there is an undelivered signal due at the same time or sooner.
+		 */
+		if (nearest_timeout >= sigalrm_due_at && !sigalrm_delivered)
+			return;
+
 		/* Set the alarm timer */
 		if (setitimer(ITIMER_REAL, &timeval, NULL) != 0)
 			elog(FATAL, "could not enable SIGALRM timer: %m");
+		sigalrm_due_at = nearest_timeout;
+		sigalrm_delivered = false;
 	}
 }
 
@@ -266,6 +283,8 @@ handle_sig_alarm(SIGNAL_ARGS)
 {
 	int			save_errno = errno;
 
+	sigalrm_delivered = true;
+
 	/*
 	 * Bump the holdoff counter, to make sure nothing we call will process
 	 * interrupts directly. No timeout handler should do that, but these
@@ -591,8 +610,9 @@ disable_timeouts(const DisableTimeoutParams *timeouts, int count)
 }
 
 /*
- * Disable SIGALRM and remove all timeouts from the active list,
- * and optionally reset their timeout indicators.
+ * Remove all timeouts from the active list, and optionally reset their timeout
+ * indicators.  Leave any existing itimer installed, because it may allow us to
+ * avoid having to set it again soon.
  */
 void
 disable_all_timeouts(bool keep_indicators)
@@ -601,20 +621,6 @@ disable_all_timeouts(bool keep_indicators)
 
 	disable_alarm();
 
-	/*
-	 * Only bother to reset the timer if we think it's active.  We could just
-	 * let the interrupt happen anyway, but it's probably a bit cheaper to do
-	 * setitimer() than to let the useless interrupt happen.
-	 */
-	if (num_active_timeouts > 0)
-	{
-		struct itimerval timeval;
-
-		MemSet(&timeval, 0, sizeof(struct itimerval));
-		if (setitimer(ITIMER_REAL, &timeval, NULL) != 0)
-			elog(FATAL, "could not disable SIGALRM timer: %m");
-	}
-
 	num_active_timeouts = 0;
 
 	for (i = 0; i < MAX_TIMEOUTS; i++)
-- 
2.20.1