[patch] sched: avoid div in rebalance_tick

[patch] sched: avoid div in rebalance_tick

[Nick Piggin]

Just noticed this while looking at a bug.

--

Avoid an expensive integer divide 3 times per CPU per tick.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2887,13 +2887,14 @@ static void active_load_balance(struct r
 static void update_load(struct rq *this_rq)
 {
 	unsigned long this_load;
-	int i, scale;
+	int i;
 
 	this_load = this_rq->raw_weighted_load;
 
 	/* Update our load: */
-	for (i = 0, scale = 1; i < 3; i++, scale <<= 1) {
+	for (i = 0; i < 3; i++) {
 		unsigned long old_load, new_load;
+		int scale;
 
 		old_load = this_rq->cpu_load[i];
 		new_load = this_load;
@@ -2902,9 +2903,11 @@ static void update_load(struct rq *this_
 		 * prevents us from getting stuck on 9 if the load is 10, for
 		 * example.
 		 */
+		scale = 1 << i;
 		if (new_load > old_load)
 			new_load += scale-1;
-		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) / scale;
+		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load)
+					>> i; /* (divide by 'scale') */
 	}
 }
 

Re: [patch] sched: avoid div in rebalance_tick

[Alan]

On Fri, 12 Jan 2007 07:02:13 +0100
Nick Piggin <npiggin@suse.de> wrote:

> Just noticed this while looking at a bug.
> Avoid an expensive integer divide 3 times per CPU per tick.

Integer divide is cheap on some modern processors, and multibit shift
isn't on all embedded ones.

How about putting back scale = 1 and using

scale += scale;

instead of the shift and getting what ought to be even better results

Re: [patch] sched: avoid div in rebalance_tick

[Nick Piggin]

On Fri, Jan 12, 2007 at 09:59:40AM +0000, Alan wrote:
> On Fri, 12 Jan 2007 07:02:13 +0100
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > Just noticed this while looking at a bug.
> > Avoid an expensive integer divide 3 times per CPU per tick.
> 
> Integer divide is cheap on some modern processors, and multibit shift
> isn't on all embedded ones.

Well integer divide unit is non-pipelined on P4 K8 Core2 and probably
most processors, AFAIK. So the 3 divs would take 240 cycles on a P4,
perhaps.

> How about putting back scale = 1 and using
> 
> scale += scale;
> 
> instead of the shift and getting what ought to be even better results

Yes I gues we ccan do this as well, good idea. I'll make a
quick userspace benchmark and post some numbers with my next
submission.

Re: [patch] sched: avoid div in rebalance_tick

[Nick Piggin]

On Fri, Jan 12, 2007 at 09:59:40AM +0000, Alan wrote:
> On Fri, 12 Jan 2007 07:02:13 +0100
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > Just noticed this while looking at a bug.
> > Avoid an expensive integer divide 3 times per CPU per tick.
> 
> Integer divide is cheap on some modern processors, and multibit shift
> isn't on all embedded ones.
> 
> How about putting back scale = 1 and using
> 
> scale += scale;
> 
> instead of the shift and getting what ought to be even better results

OK, how about this? It only works out to be around 0.01% of my P3's CPU time
at 1000HZ, but it also did make the x86 code 16 bytes smaller.


--
Avoid expensive integer divide 3 times per CPU per tick.

A userspace test of this loop went from 26ns, down to 19ns on a G5; and
from 123ns down to 28ns on a P3.

(Also avoid a variable bit shift, as suggested by Alan. The effect
of this wasn't noticable on the CPUs I tested with).

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2887,14 +2887,16 @@ static void active_load_balance(struct r
 static void update_load(struct rq *this_rq)
 {
 	unsigned long this_load;
-	int i, scale;
+	unsigned int i, scale;
 
 	this_load = this_rq->raw_weighted_load;
 
 	/* Update our load: */
-	for (i = 0, scale = 1; i < 3; i++, scale <<= 1) {
+	for (i = 0, scale = 1; i < 3; i++, scale += scale) {
 		unsigned long old_load, new_load;
 
+		/* scale is effectively 1 << i now, and >> i divides by scale */
+
 		old_load = this_rq->cpu_load[i];
 		new_load = this_load;
 		/*
@@ -2904,7 +2906,7 @@ static void update_load(struct rq *this_
 		 */
 		if (new_load > old_load)
 			new_load += scale-1;
-		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) / scale;
+		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
 	}
 }