lkml.org 
[lkml]   [2022]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v4] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest_group
On Tue, Feb 22, 2022 at 11:30:48AM +0530, K Prateek Nayak wrote:
> Hello Mel,
>
> On 2/17/2022 6:45 PM, Mel Gorman wrote:
> > I don't object to the change but I would wonder if it's measurable for
> > anything other than a fork-intensive microbenchmark given it's one branch
> > in a relatively heavy operation.
>
> I used stress-ng to see if we get any measurable difference from the
> optimizations in a fork intensive scenario. I'm measuring time in ns
> between the events sched_process_fork and sched_wakeup_new for the
> stress-ng processes.
>
> Following are the results from testing:
>
> - Un-affined runs:
> Command: stress-ng -t 30s --exec <Worker>
>
> Kernel versions:
> - balance-wake - This patch
> - branch - This patch + Mel's suggested branch
> - branch-unlikely - This patch + Mel's suggested branch + unlikely
>
> Result format: Amean in ns [Co-eff of Var] (% Improvement)
>
> Workers balance-wake branch branch-unlikely
> 1 18613.20 [0.01] (0.00 pct) 18348.00 [0.04] (1.42 pct) 18299.20 [0.02] (1.69 pct)
> 2 18634.40 [0.03] (0.00 pct) 18163.80 [0.04] (2.53 pct) 19037.80 [0.05] (-2.16 pct)
> 4 20997.40 [0.02] (0.00 pct) 20980.80 [0.02] (0.08 pct) 21527.40 [0.02] (-2.52 pct)
> 8 20890.20 [0.01] (0.00 pct) 19714.60 [0.07] (5.63 pct) 20021.40 [0.05] (4.16 pct)
> 16 21200.20 [0.02] (0.00 pct) 20564.40 [0.00] (3.00 pct) 20676.00 [0.01] (2.47 pct)
> 32 21301.80 [0.02] (0.00 pct) 20767.40 [0.02] (2.51 pct) 20945.00 [0.01] (1.67 pct)
> 64 22772.40 [0.01] (0.00 pct) 22505.00 [0.01] (1.17 pct) 22629.40 [0.00] (0.63 pct)
> 128 25843.00 [0.01] (0.00 pct) 25124.80 [0.00] (2.78 pct) 25377.40 [0.00] (1.80 pct)
> 256 18691.00 [0.02] (0.00 pct) 19086.40 [0.05] (-2.12 pct) 18013.00 [0.04] (3.63 pct)
> 512 19658.40 [0.03] (0.00 pct) 19568.80 [0.01] (0.46 pct) 18972.00 [0.02] (3.49 pct)
> 1024 19126.80 [0.04] (0.00 pct) 18762.80 [0.02] (1.90 pct) 18878.20 [0.04] (1.30 pct)
>

Co-eff of variance looks low but for the lower counts before the machine
is saturated (>=256?) it does not look like it helps and if anything,
it hurts. A branch mispredict profile might reveal more but I doubt
it's worth the effort at this point.

> - Affined runs:
> Command: taskset -c 0-254 stress-ng -t 30s --exec <Worker>
>
> Kernel versions:
> - balance-wake-affine - This patch + affined run
> - branch-affine - This patch + Mel's suggested branch + affined run
> - branch-unlikely-affine - This patch + Mel's suggested branch + unlikely + affined run
>
> Result format: Amean in ns [Co-eff of Var] (% Improvement)
>
> Workers balance-wake-affine branch-affine branch-unlikely-affine
> 1 18515.00 [0.01] (0.00 pct) 18538.00 [0.02] (-0.12 pct) 18568.40 [0.01] (-0.29 pct)
> 2 17882.80 [0.01] (0.00 pct) 19627.80 [0.09] (-9.76 pct) 18790.40 [0.01] (-5.08 pct)
> 4 21204.20 [0.01] (0.00 pct) 21410.60 [0.04] (-0.97 pct) 21715.20 [0.03] (-2.41 pct)
> 8 20840.20 [0.01] (0.00 pct) 19684.60 [0.07] (5.55 pct) 21074.20 [0.02] (-1.12 pct)
> 16 21115.20 [0.02] (0.00 pct) 20823.00 [0.01] (1.38 pct) 20719.80 [0.00] (1.87 pct)
> 32 21159.00 [0.02] (0.00 pct) 21371.20 [0.01] (-1.00 pct) 21253.20 [0.01] (-0.45 pct)
> 64 22768.20 [0.01] (0.00 pct) 22816.80 [0.00] (-0.21 pct) 22662.00 [0.00] (0.47 pct)
> 128 25671.80 [0.00] (0.00 pct) 25528.20 [0.00] (0.56 pct) 25404.00 [0.00] (1.04 pct)
> 256 27209.00 [0.01] (0.00 pct) 26751.00 [0.01] (1.68 pct) 26733.20 [0.00] (1.75 pct)
> 512 20241.00 [0.03] (0.00 pct) 19378.60 [0.03] (4.26 pct) 19671.40 [0.00] (2.81 pct)
> 1024 19380.80 [0.05] (0.00 pct) 18940.40 [0.02] (2.27 pct) 19071.80 [0.00] (1.59 pct)

Same here, the cpumask check obviously hurts but it does not look like
the unlikely helps.

> With or without the unlikely, adding the check before doing the
> cpumask operation benefits most cases of un-affined tasks.
>

I think repost the patch with the num_online_cpus check added in. Yes,
it hurts a bit for the pure fork case when the cpus_ptr is contrained by
a scheduler policy but at least it makes sense.

--
Mel Gorman
SUSE Labs

\
 
 \ /
  Last update: 2022-02-22 09:46    [W:0.080 / U:0.352 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site