When an object is being fetched from the backend, other requests for it

are parked not holding a worker thread, until the fetching session lowers
the busy bit on the object.

At that point we would release all the parked requests into the thread
pool to be serviced, causing instant congestion and calls for road-pricing
if the backend were a bit slow on an oft-requested object.

Change the restart policy to be paced exponential:

When we clear the busy bits, we unpark "rush_exponent" requests into
the thread pool to start the show.

Whenever the object is dereferenced, in practice whenever a request
has been serviced, another "rush_exponent" worth of requests are
unparked into the tread pool.

Set the parameter to a conservative 3 until we know more about the
behaviour in practice.

If it is a big object and/or the clients are on slow lines, 3 may
be an order of magnitude to small.

Attempts to fix: #188



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@2294 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_hash.c

@@ -244,11 +244,26 @@ HSH_Lookup(struct sess *sp)
 	return (o);
 }
 
+static void
+hsh_rush(struct object *o)
+{
+	unsigned u;
+	struct sess *sp;
+
+	for (u = 0; u < params->rush_exponent; u++) {
+		sp = VTAILQ_FIRST(&o->waitinglist);
+		if (sp == NULL)
+			return;
+		VTAILQ_REMOVE(&o->waitinglist, sp, list);
+		VSL(SLT_Debug, sp->id, "of waiting list");
+		WRK_QueueSession(sp);
+	}
+}
+
 void
 HSH_Unbusy(struct object *o)
 {
 	struct objhead *oh;
-	struct sess *sp;
 
 	CHECK_OBJ_NOTNULL(o, OBJECT_MAGIC);
 	assert(o->busy);
@@ -261,15 +276,9 @@ HSH_Unbusy(struct object *o)
 		LOCK(&oh->mtx);
 	}
 	o->busy = 0;
+	hsh_rush(o);
 	if (oh != NULL)
 		UNLOCK(&oh->mtx);
-	while (1) {
-		sp = VTAILQ_FIRST(&o->waitinglist);
-		if (sp == NULL)
-			break;
-		VTAILQ_REMOVE(&o->waitinglist, sp, list);
-		WRK_QueueSession(sp);
-	}
 }
 
 void
@@ -305,6 +314,7 @@ HSH_Deref(struct object *o)
 	}
 	assert(o->refcnt > 0);
 	r = --o->refcnt;
+	hsh_rush(o);
 	if (oh != NULL) {
 		if (!r)
 			VTAILQ_REMOVE(&oh->objects, o, list);

bin/varnishd/heritage.h

@@ -131,6 +131,8 @@ struct params {
 	/* Maximum esi:include depth allowed */
 	unsigned		max_esi_includes;
 
+	/* Rush exponent */
+	unsigned		rush_exponent;
 };
 
 extern volatile struct params *params;

bin/varnishd/mgt_param.c

@@ -294,6 +294,17 @@ tweak_overflow_max(struct cli *cli, struct parspec *par, const char *arg)
 
 /*--------------------------------------------------------------------*/
 
+static void
+tweak_rush_exponent(struct cli *cli, struct parspec *par, const char *arg)
+{
+
+	(void)par;
+	tweak_generic_uint(cli, &master.rush_exponent, arg,
+	    2, UINT_MAX);
+}
+
+/*--------------------------------------------------------------------*/
+
 static void
 tweak_http_workspace(struct cli *cli, struct parspec *par, const char *arg)
 {
@@ -642,6 +653,14 @@ static struct parspec parspec[] = {
 		"thread_pool_max parameter.",
 		EXPERIMENTAL,
 		"100", "%" },
+	{ "rush_exponent", tweak_rush_exponent,
+		"How many parked request we start for each completed "
+		"request on the object.\n"
+		"NB: Even with the implict delay of delivery, "
+		"this parameter controls an exponential increase in "
+		"number of worker threads.  ",
+		EXPERIMENTAL,
+		"3", "requests per request" },
 	{ "http_workspace", tweak_http_workspace,
 		"Bytes of HTTP protocol workspace allocated. "
 		"This space must be big enough for the entire HTTP protocol "