Introduce some jitter to the timing of the samples that determine

the system load average. Previously, the load average measurement
was susceptible to synchronisation with processes that run at
regular intervals such as the system bufdaemon process.

Each interval is now chosen at random within the range of 4 to 6
seconds. This large variation is chosen so that over the shorter
5-minute load average timescale there is a good dispersion of
samples across the 5-second sample period (the time to perform 60
5-second samples now has a standard deviation of approx 4.5 seconds).

1 files changed, 15 insertions, 4 deletions

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index db5d51a..348e57a 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -74,6 +74,7 @@ int	hogticks;
 int	lbolt;
 int	sched_quantum;		/* Roundrobin scheduling quantum in ticks. */
 
+static struct callout loadav_callout;
 static struct callout schedcpu_callout;
 static struct callout roundrobin_callout;
 
@@ -90,7 +91,7 @@ static fixpt_t cexp[3] = {
 };
 
 static void	endtsleep __P((void *));
-static void	loadav __P((struct loadavg *));
+static void	loadav __P((void *arg));
 static void	roundrobin __P((void *arg));
 static void	schedcpu __P((void *arg));
 
@@ -339,8 +340,6 @@ schedcpu(arg)
 		mtx_unlock_spin(&sched_lock);
 	} /* end of process loop */
 	sx_sunlock(&allproc_lock);
-	if (time_second % 5 == 0)
-		loadav(&averunnable);
 	wakeup((caddr_t)&lbolt);
 	callout_reset(&schedcpu_callout, hz, schedcpu, NULL);
 }
@@ -863,12 +862,14 @@ resetpriority(kg)
  * Completely Bogus.. only works with 1:1 (but compiles ok now :-)
  */
 static void
-loadav(struct loadavg *avg)
+loadav(void *arg)
 {
 	int i, nrun;
+	struct loadavg *avg;
 	struct proc *p;
 	struct ksegrp *kg;
 
+	avg = &averunnable;
 	sx_slock(&allproc_lock);
 	nrun = 0;
 	FOREACH_PROC_IN_SYSTEM(p) {
@@ -888,6 +889,14 @@ nextproc:
 	for (i = 0; i < 3; i++)
 		avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
 		    nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
+
+	/*
+	 * Schedule the next update to occur after 5 seconds, but add a
+	 * random variation to avoid synchronisation with processes that
+	 * run at regular intervals.
+	 */
+	callout_reset(&loadav_callout, hz * 4 + (int)(random() % (hz * 2 + 1)),
+	    loadav, NULL);
 }
 
 /* ARGSUSED */
@@ -898,10 +907,12 @@ sched_setup(dummy)
 
 	callout_init(&schedcpu_callout, 1);
 	callout_init(&roundrobin_callout, 0);
+	callout_init(&loadav_callout, 0);
 
 	/* Kick off timeout driven events by calling first time. */
 	roundrobin(NULL);
 	schedcpu(NULL);
+	loadav(NULL);
 }
 
 /*