Re: [RFC] Perfomance varies according to sysctl_sched_migration_cost

[Vincent Guittot]

On Tue, 14 Sept 2021 at 14:08, Yicong Yang <yangyicong@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> 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.

Not really. The periodic load balance always happens but task_hot is
used to skip task that have recently run on the cpu and select older
tasks instead
At the contrary, sysctl_sched_migration_cost is used to decide if we
should abort newly_idle_load_balance

As a side point, would be good to know if the improvement and
regression seen in your tests are more linked to the task hotness or
for  skipping/aborting 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 ?
> >
>
> 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.

I don't think this is the best benchmark to evaluate the real cost of
load_balance because it create always running task and  you measure
only the periodic load balance and not the newly load balance which is
the one really impacted by sysctl_sched_migration_cost

>
> > 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

Because of the large difference between the number of cpus at 1st and
last level, an average duration of load_balance() is not really
meaningful and we can expect a factor of 4 between smallest and larger
one

> [...]
> 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.

This can slow down a bit the load_balance so might be good to take
that into account

>
> >>
> >> 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.

Adjusting the value based on the platform seems reasonable although
i'm not sure which input should be used (arch type / interconnect
bandwidth / cache size / number of cpu per cache level ...)

>
> 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
> >>
> >>
> >
> > .
> >
>