Re: [PATCH 2/2 v2] sched: use load_avg for selecting idlest group

[Matt Fleming]

On Mon, 05 Dec, at 01:35:46PM, Matt Fleming wrote:
> On Mon, 05 Dec, at 10:27:36AM, Vincent Guittot wrote:
> > 
> > Hi Matt,
> > 
> > Thanks for the results.
> > 
> > During the review, it has been pointed out by Morten that the test condition
> > (100*this_avg_load < imbalance_scale*min_avg_load) makes more sense than
> > (100*min_avg_load > imbalance_scale*this_avg_load). But i see lower
> > performances with this change. Coud you run tests with the change below on
> > top of the patchset ?
> > 
> > ---
> >  kernel/sched/fair.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e8d1ae7..0129fbb 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5514,7 +5514,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
> >  	if (!idlest ||
> >  	    (min_runnable_load > (this_runnable_load + imbalance)) ||
> >  	    ((this_runnable_load < (min_runnable_load + imbalance)) &&
> > -			(100*min_avg_load > imbalance_scale*this_avg_load)))
> > +			(100*this_avg_load < imbalance_scale*min_avg_load)))
> >  		return NULL;
> >  	return idlest;
> >  }
> 
> Queued for testing.

Most of the machines didn't notice the difference with this new patch.
However, I did come across one test that showed a negative change,


hackbench-thread-pipes
                        4.9.0-rc6             4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                        tip-sched      fix-fig-for-fork               fix-sig   fix-fig-for-fork-v2
Amean    1       0.1266 (  0.00%)      0.1269 ( -0.23%)      0.1287 ( -1.69%)      0.1357 ( -7.22%)
Amean    4       0.4989 (  0.00%)      0.5174 ( -3.72%)      0.5251 ( -5.27%)      0.5117 ( -2.58%)
Amean    7       0.8510 (  0.00%)      0.8517 ( -0.08%)      0.8964 ( -5.34%)      0.8801 ( -3.42%)
Amean    12      1.0699 (  0.00%)      1.0484 (  2.00%)      1.0147 (  5.15%)      1.0759 ( -0.56%)
Amean    21      1.2816 (  0.00%)      1.2140 (  5.27%)      1.1879 (  7.31%)      1.2414 (  3.13%)
Amean    30      1.4440 (  0.00%)      1.4003 (  3.03%)      1.3969 (  3.26%)      1.4057 (  2.65%)
Amean    48      1.5773 (  0.00%)      1.5983 ( -1.33%)      1.3984 ( 11.34%)      1.5624 (  0.94%)
Amean    79      2.2343 (  0.00%)      2.3066 ( -3.24%)      2.0053 ( 10.25%)      2.2630 ( -1.29%)
Amean    96      2.6736 (  0.00%)      2.4313 (  9.06%)      2.4181 (  9.55%)      2.4717 (  7.55%)

           4.9.0-rc6   4.9.0-rc6   4.9.0-rc6   4.9.0-rc6
           tip-schedfix-fig-for-fork     fix-sigfix-fig-for-fork-v2
User          129.53      128.64      127.70      131.00
System       1784.54     1744.21     1654.08     1744.00
Elapsed        92.07       90.44       86.95       91.00

Looking at the 48 and 79 groups rows for mean there's a noticeable
drop off in performance of ~10%, which should be outside of the noise
for this test. This is a 2 socket, 4 NUMA node (yes, really), 24 cpus
AMD opteron circa 2010.

Given the age of this machine, I don't think it's worth holding up the
patch.