Re: [patch] sched: avoid div in rebalance_tick
On Fri, Jan 12, 2007 at 09:59:40AM +, Alan wrote:
> On Fri, 12 Jan 2007 07:02:13 +0100
> Nick Piggin <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
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;
}
}
-
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Re: [patch] sched: avoid div in rebalance_tick
On Fri, Jan 12, 2007 at 09:59:40AM +, Alan wrote: > On Fri, 12 Jan 2007 07:02:13 +0100 > Nick Piggin <[EMAIL PROTECTED]> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch] sched: avoid div in rebalance_tick
On Fri, 12 Jan 2007 07:02:13 +0100 Nick Piggin <[EMAIL PROTECTED]> 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch] sched: avoid div in rebalance_tick
On Fri, 12 Jan 2007 07:02:13 +0100 Nick Piggin [EMAIL PROTECTED] 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 - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch] sched: avoid div in rebalance_tick
On Fri, Jan 12, 2007 at 09:59:40AM +, Alan wrote: On Fri, 12 Jan 2007 07:02:13 +0100 Nick Piggin [EMAIL PROTECTED] 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. - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch] sched: avoid div in rebalance_tick
On Fri, Jan 12, 2007 at 09:59:40AM +, Alan wrote:
On Fri, 12 Jan 2007 07:02:13 +0100
Nick Piggin [EMAIL PROTECTED] 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 [EMAIL PROTECTED]
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;
}
}
-
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[patch] sched: avoid div in rebalance_tick
Just noticed this while looking at a bug.
--
Avoid an expensive integer divide 3 times per CPU per tick.
Signed-off-by: Nick Piggin <[EMAIL PROTECTED]>
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') */
}
}
-
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More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[patch] sched: avoid div in rebalance_tick
Just noticed this while looking at a bug.
--
Avoid an expensive integer divide 3 times per CPU per tick.
Signed-off-by: Nick Piggin [EMAIL PROTECTED]
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') */
}
}
-
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