On Fri, Jan 12, 2007 at 09:59:40AM +0000, 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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