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Message-ID: <d9ada2d4-1967-7b80-68e0-ef7052c41e0f@hisilicon.com>
Date: Tue, 14 Sep 2021 20:08:38 +0800
From: Yicong Yang <yangyicong@...ilicon.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
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 Vincent,
thanks for the reply!
On 2021/9/14 17:04, Vincent Guittot wrote:
> 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
>
In load_balance(), it's used to judge whether a task is hot in task_hot(). so I think it
participates in the periodic 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 ?
>
I use stress-ng to generate the load. Since it's a 128core server, `stress-ng -c 64` for
50% load, and `stress-ng -c 128` for 100% load. This is not measured during the benchmarks'
process.
> Also, do you have more details about the topology and the number of
> sched domain ?
>
sure. for `numactl -H`:
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
node 0 size: 257149 MB
node 0 free: 253518 MB
node 1 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
node 1 size: 193531 MB
node 1 free: 192916 MB
node 2 cpus: 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
node 2 size: 96763 MB
node 2 free: 92654 MB
node 3 cpus: 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
node 3 size: 127668 MB
node 3 free: 125846 MB
node distances:
node 0 1 2 3
0: 10 12 20 22
1: 12 10 22 24
2: 20 22 10 12
3: 22 24 12 10
Kunpeng 920 is non-smt. There're 4 level domains and below is part of the /proc/schedstat:
[...]
cpu0
domain0 00000000,00000000,00000000,ffffffff
domain1 00000000,00000000,ffffffff,ffffffff
domain2 00000000,ffffffff,ffffffff,ffffffff
domain3 ffffffff,ffffffff,ffffffff,ffffffff
[...]
cpu32
domain0 00000000,00000000,ffffffff,00000000
domain1 00000000,00000000,ffffffff,ffffffff
domain2 00000000,ffffffff,ffffffff,ffffffff
domain3 ffffffff,ffffffff,ffffffff,ffffffff
[...]
cpu64
domain0 00000000,ffffffff,00000000,00000000
domain1 ffffffff,ffffffff,00000000,00000000
domain2 ffffffff,ffffffff,00000000,ffffffff
domain3 ffffffff,ffffffff,ffffffff,ffffffff
[...]
cpu96
domain0 ffffffff,00000000,00000000,00000000
domain1 ffffffff,ffffffff,00000000,00000000
domain2 ffffffff,ffffffff,00000000,ffffffff
domain3 ffffffff,ffffffff,ffffffff,ffffffff
[...]
> Are you using cgroup hierarchy ?
>
No cgroup hierarchy during the test.
>>
>> 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
>
yes it's 500us. sorry for the wrong unit.
>>
>> 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
>
thanks for the explanation. I agree that it should be tuned per platform, and maybe also
per workload. Current default value seems to have be well tuned on x86 but not on the some
arm64 platforms.
Thanks.
>> 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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