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Date: Tue, 14 Sep 2021 11:04:03 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Yicong Yang <yangyicong@...ilicon.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
21cnbao@...il.com,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
prime.zeng@...wei.com,
"guodong.xu@...aro.org" <guodong.xu@...aro.org>
Subject: Re: [RFC] Perfomance varies according to sysctl_sched_migration_cost
Hi Yicong,
On Tue, 14 Sept 2021 at 09:27, Yicong Yang <yangyicong@...ilicon.com> wrote:
>
> Hi all,
>
> I noticed that some benchmark performance varies after tunning the sysctl_sched_migration_cost
> through /sys/kernel/debug/sched/migration_cost_ns on arm64. The default value is 500000, and
> I tried 10000, 100000, 1000000. Below are some results from mmtests, based on 5.14-release.
>
> On Kunpeng920 (128cores, 4numa, 2socket):
>
> pgbench (config-db-pgbench-timed-ro-medium)
> mig-cost-500000 mig-cost-100000 mig-cost-10000 mig-cost-1000000
> Hmean 1 9558.99 ( 0.00%) 9735.31 * 1.84%* 9410.84 * -1.55%* 9602.47 * 0.45%*
> Hmean 8 17615.90 ( 0.00%) 17439.78 * -1.00%* 18056.44 * 2.50%* 19222.18 * 9.12%*
> Hmean 12 25228.38 ( 0.00%) 25592.69 * 1.44%* 26739.06 * 5.99%* 27575.48 * 9.30%*
> Hmean 24 46623.27 ( 0.00%) 48853.30 * 4.78%* 47386.02 * 1.64%* 48542.94 * 4.12%*
> Hmean 32 60578.78 ( 0.00%) 62116.81 * 2.54%* 59961.36 * -1.02%* 58681.07 * -3.13%*
> Hmean 48 68159.12 ( 0.00%) 67867.90 ( -0.43%) 65631.79 * -3.71%* 66487.16 * -2.45%*
> Hmean 80 66894.87 ( 0.00%) 73440.92 * 9.79%* 68751.63 * 2.78%* 67326.70 ( 0.65%)
> Hmean 112 68582.27 ( 0.00%) 65339.90 * -4.73%* 68454.99 ( -0.19%) 67211.66 * -2.00%*
> Hmean 144 76290.98 ( 0.00%) 70455.65 * -7.65%* 64851.23 * -14.99%* 64940.61 * -14.88%*
> Hmean 172 63245.68 ( 0.00%) 68790.24 * 8.77%* 66246.46 * 4.74%* 69536.96 * 9.95%*
> Hmean 204 61793.47 ( 0.00%) 63711.62 * 3.10%* 66055.64 * 6.90%* 58023.20 * -6.10%*
> Hmean 236 61486.75 ( 0.00%) 68404.44 * 11.25%* 70499.70 * 14.66%* 58285.67 * -5.21%*
> Hmean 256 57476.13 ( 0.00%) 65645.83 * 14.21%* 69437.05 * 20.81%* 60518.05 * 5.29%*
>
> tbench (config-network-tbench)
> mig-cost-500000 mig-cost-100000 mig-cost-10000 mig-cost-1000000
> Hmean 1 333.12 ( 0.00%) 332.93 ( -0.06%) 335.34 * 0.67%* 334.36 * 0.37%*
> Hmean 2 665.88 ( 0.00%) 667.19 * 0.20%* 666.47 * 0.09%* 667.02 * 0.17%*
> Hmean 4 1324.10 ( 0.00%) 1312.23 * -0.90%* 1313.07 * -0.83%* 1315.13 * -0.68%*
> Hmean 8 2618.85 ( 0.00%) 2602.00 * -0.64%* 2577.49 * -1.58%* 2600.48 * -0.70%*
> Hmean 16 5100.74 ( 0.00%) 5068.80 * -0.63%* 5041.34 * -1.16%* 5069.78 * -0.61%*
> Hmean 32 8157.22 ( 0.00%) 8163.50 ( 0.08%) 7936.25 * -2.71%* 8329.18 * 2.11%*
> Hmean 64 4824.56 ( 0.00%) 4890.81 * 1.37%* 5319.97 * 10.27%* 4830.68 * 0.13%*
> Hmean 128 4635.17 ( 0.00%) 6810.90 * 46.94%* 5304.36 * 14.44%* 4516.06 * -2.57%*
> Hmean 256 8816.62 ( 0.00%) 8851.28 * 0.39%* 8448.76 * -4.17%* 6840.12 * -22.42%*
> Hmean 512 7825.56 ( 0.00%) 8538.04 * 9.10%* 8002.77 * 2.26%* 7946.54 * 1.55%*
>
> Also on Raspberrypi 4B:
>
> pgbench (config-db-pgbench-timed-ro-medium)
> mig-cost-500000 mig-cost-100000
> Hmean 1 1651.41 ( 0.00%) 3444.27 * 108.56%*
> Hmean 4 4015.83 ( 0.00%) 6883.21 * 71.40%*
> Hmean 7 4161.45 ( 0.00%) 6646.18 * 59.71%*
> Hmean 8 4277.28 ( 0.00%) 6764.60 * 58.15%*
>
> For tbench on Raspberrypi 4B and both pgbench and tbench on x86, tuning sysctl_sched_migration_cost
> doesn't have such huge difference and will have some degradations (max -8% on x86 for pgbench) in some cases.
>
> The sysctl_sched_migration_cost will affects the frequency of load balance. It will affect
So it doesn't affect the periodic load but only the newly idle load balance
> directly in task_hot() and newidle_balance() to decide whether we can do a migration or load
> balance. And affects other parameters like rq->avg_idle, rq->max_idle_balance_cost and
> sd->max_newidle_lb_cost to indirectly affect the load balance process. These parameters record
> the load_balance() cost and will be limited up to sysctl_sched_migration_cost, so I measure
> the average cost of load_balance() on Kunpeng920 with bcc tools(./funclantency load_balance -d 10):
>
> system status idle 50%load 100%load
> avg cost 3160ns 4790ns 7563ns
What is the setup of your test ? has this been measured during the
benchmarks above ?
Also, do you have more details about the topology and the number of
sched domain ?
Are you using cgroup hierarchy ?
>
> The average cost of load balance seems quite smaller than the default sysctl_sched_migration_cost
> which is 500000(500ms).
AFAICT, it is 500us not 500ms
>
> So I have some RFC questions:
> 1. how is the default 500000 (500ms) migration cost is measured or caculated?
500us not ms
I would say that it's a heuristic value that works for most of system
but it should probably be tuned per platform. But also note that it's
quite difficult to get a correct value
> The value has never changed in the past decade. I dig into the git commits and find it was introduced
> in da84d9617672 ("sched: reintroduce cache-hot affinity"). But it didn't explain how did this value come.
> 2. The ABI now has been removed from sysctl and moved to debugfs. As tuning this can improve the performance
> of some workloads on some platforms, maybe it's better to make it a formal sysctl again with docs?
>
> I'll be appreciated for any comments and replies!
>
> Thanks,
> Yicong
>
>
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