Re: -mm seems significanty slower than mainline on kernbench

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Attached is a new patch to fix the excessive idle problem. This patch takes a new approach to the problem as it was becoming obvious that trying to alter the load balancing code to cope with biased load was harder than it seemed.

This approach reverts to the old load values but weights them according to tasks' bias_prio values. This means that any assumptions by the load balancing code that the load generated by a single task is SCHED_LOAD_SCALE will still hold. Then, in find_busiest_group(), the imbalance is scaled back up to bias_prio scale so that move_tasks() can move biased load rather than tasks.

OK, this one seems to fix the issue that I had, AFAICS. Congrats, and thanks,

M.

One advantage of this is that when there are no non zero niced tasks the processing will be mathematically the same as the original code. Kernbench results from a 2 CPU Celeron 550Mhz system are:

Average Optimal -j 8 Load Run:
Elapsed Time 1056.16 (0.831102)
User Time 1906.54 (1.38447)
System Time 182.086 (0.973386)
Percent CPU 197 (0)
Context Switches 48727.2 (249.351)
Sleeps 27623.4 (413.913)

This indicates that, on average, 98.9% of the total available CPU was used by the build.

Signed-off-by: Peter Williams <[email protected]>

BTW I think that we need to think about a slightly more complex nice to bias mapping function. The current one gives a nice==19 1/20 of the bias of a nice=0 task but only gives nice=-20 tasks twice the bias of a nice=0 task. I don't think this is a big problem as the majority of non nice==0 tasks will have positive nice but should be looked at for a future enhancement.

Peter

------------------------------------------------------------------------

Index: MM-2.6.X/kernel/sched.c
===================================================================
--- MM-2.6.X.orig/kernel/sched.c	2006-01-13 14:53:34.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-01-13 15:11:19.000000000 +1100
@@ -1042,7 +1042,8 @@ void kick_process(task_t *p)
static unsigned long source_load(int cpu, int type)
{
	runqueue_t *rq = cpu_rq(cpu);
-	unsigned long load_now = rq->prio_bias * SCHED_LOAD_SCALE;
+	unsigned long load_now = (rq->prio_bias * SCHED_LOAD_SCALE) /
+		NICE_TO_BIAS_PRIO(0);

	if (type == 0)
		return load_now;
@@ -1056,7 +1057,8 @@ static unsigned long source_load(int cpu
static inline unsigned long target_load(int cpu, int type)
{
	runqueue_t *rq = cpu_rq(cpu);
-	unsigned long load_now = rq->prio_bias * SCHED_LOAD_SCALE;
+	unsigned long load_now = (rq->prio_bias * SCHED_LOAD_SCALE) /
+		NICE_TO_BIAS_PRIO(0);

	if (type == 0)
		return load_now;
@@ -1322,7 +1324,8 @@ static int try_to_wake_up(task_t *p, uns
			 * of the current CPU:
			 */
			if (sync)
-				tl -= p->bias_prio * SCHED_LOAD_SCALE;
+				tl -= (p->bias_prio * SCHED_LOAD_SCALE) /
+					NICE_TO_BIAS_PRIO(0);

			if ((tl <= load &&
				tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
@@ -2159,7 +2162,7 @@ find_busiest_group(struct sched_domain *
	}

	/* Get rid of the scaling factor, rounding down as we divide */
-	*imbalance = *imbalance / SCHED_LOAD_SCALE;
+	*imbalance = (*imbalance * NICE_TO_BIAS_PRIO(0)) / SCHED_LOAD_SCALE;
	return busiest;

out_balanced:
@@ -2472,7 +2475,8 @@ static void rebalance_tick(int this_cpu,
	struct sched_domain *sd;
	int i;

-	this_load = this_rq->prio_bias * SCHED_LOAD_SCALE;
+	this_load = (this_rq->prio_bias * SCHED_LOAD_SCALE) /
+		NICE_TO_BIAS_PRIO(0);
	/* Update our load */
	for (i = 0; i < 3; i++) {
		unsigned long new_load = this_load;

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