Re: [PATCH v3 0/8] sched/fair: rework the CFS load balance

[Vincent Guittot]

Hi Phil,

On Tue, 8 Oct 2019 at 16:33, Phil Auld <pauld@xxxxxxxxxx> wrote:
>
> Hi Vincent,
>
> On Thu, Sep 19, 2019 at 09:33:31AM +0200 Vincent Guittot wrote:
> > Several wrong task placement have been raised with the current load
> > balance algorithm but their fixes are not always straight forward and
> > end up with using biased values to force migrations. A cleanup and rework
> > of the load balance will help to handle such UCs and enable to fine grain
> > the behavior of the scheduler for other cases.
> >

[...]

> >
>
> We've been testing v3 and for the most part everything looks good. The
> group imbalance issues are fixed on all of our test systems except one.
>
> The one is an 8-node intel system with 160 cpus. I'll put the system
> details at the end.
>
> This shows the average number of benchmark threads running on each node
> through the run. That is, not including the 2 stress jobs. The end
> results are a 4x slow down in the cgroup case versus not. The 152 and
> 156 are the number of LU threads in the run. In all cases there are 2
> stress CPU threads running either in their own cgroups (GROUP) or
> everything is in one cgroup (NORMAL).  The normal case is pretty well
> balanced with only a few >= 20 and those that are are only a little
> over. In the GROUP cases things are not so good. There are some > 30
> for example, and others < 10.
>
>
> lu.C.x_152_GROUP_1   17.52  16.86  17.90  18.52  20.00  19.00  22.00  20.19
> lu.C.x_152_GROUP_2   15.70  15.04  15.65  15.72  23.30  28.98  20.09  17.52
> lu.C.x_152_GROUP_3   27.72  32.79  22.89  22.62  11.01  12.90  12.14  9.93
> lu.C.x_152_GROUP_4   18.13  18.87  18.40  17.87  18.80  19.93  20.40  19.60
> lu.C.x_152_GROUP_5   24.14  26.46  20.92  21.43  14.70  16.05  15.14  13.16
> lu.C.x_152_NORMAL_1  21.03  22.43  20.27  19.97  18.37  18.80  16.27  14.87
> lu.C.x_152_NORMAL_2  19.24  18.29  18.41  17.41  19.71  19.00  20.29  19.65
> lu.C.x_152_NORMAL_3  19.43  20.00  19.05  20.24  18.76  17.38  18.52  18.62
> lu.C.x_152_NORMAL_4  17.19  18.25  17.81  18.69  20.44  19.75  20.12  19.75
> lu.C.x_152_NORMAL_5  19.25  19.56  19.12  19.56  19.38  19.38  18.12  17.62
>
> lu.C.x_156_GROUP_1   18.62  19.31  18.38  18.77  19.88  21.35  19.35  20.35
> lu.C.x_156_GROUP_2   15.58  12.72  14.96  14.83  20.59  19.35  29.75  28.22
> lu.C.x_156_GROUP_3   20.05  18.74  19.63  18.32  20.26  20.89  19.53  18.58
> lu.C.x_156_GROUP_4   14.77  11.42  13.01  10.09  27.05  33.52  23.16  22.98
> lu.C.x_156_GROUP_5   14.94  11.45  12.77  10.52  28.01  33.88  22.37  22.05
> lu.C.x_156_NORMAL_1  20.00  20.58  18.47  18.68  19.47  19.74  19.42  19.63
> lu.C.x_156_NORMAL_2  18.52  18.48  18.83  18.43  20.57  20.48  20.61  20.09
> lu.C.x_156_NORMAL_3  20.27  20.00  20.05  21.18  19.55  19.00  18.59  17.36
> lu.C.x_156_NORMAL_4  19.65  19.60  20.25  20.75  19.35  20.10  19.00  17.30
> lu.C.x_156_NORMAL_5  19.79  19.67  20.62  22.42  18.42  18.00  17.67  19.42
>
>
> From what I can see this was better but not perfect in v1.  It was closer and
> so the end results (LU reported times and op/s) were close enough. But looking
> closer at it there are still some issues. (NORMAL is comparable to above)
>
>
> lu.C.x_152_GROUP_1   18.08  18.17  19.58  19.29  19.25  17.50  21.46  18.67
> lu.C.x_152_GROUP_2   17.12  17.48  17.88  17.62  19.57  17.31  23.00  22.02
> lu.C.x_152_GROUP_3   17.82  17.97  18.12  18.18  24.55  22.18  16.97  16.21
> lu.C.x_152_GROUP_4   18.47  19.08  18.50  18.66  21.45  25.00  15.47  15.37
> lu.C.x_152_GROUP_5   20.46  20.71  27.38  24.75  17.06  16.65  12.81  12.19
>
> lu.C.x_156_GROUP_1   18.70  18.80  20.25  19.50  20.45  20.30  19.55  18.45
> lu.C.x_156_GROUP_2   19.29  19.90  17.71  18.10  20.76  21.57  19.81  18.86
> lu.C.x_156_GROUP_3   25.09  29.19  21.83  21.33  18.67  18.57  11.03  10.29
> lu.C.x_156_GROUP_4   18.60  19.10  19.20  18.70  20.30  20.00  19.70  20.40
> lu.C.x_156_GROUP_5   18.58  18.9   18.63  18.16  17.32  19.37  23.92  21.08
>
> There is high variance so it may not be anything specific between v1 and v3 here.

While preparing v4, I have noticed that I have probably oversimplified
the end of find_idlest_group() in patch "sched/fair: optimize
find_idlest_group" when it compares local vs the idlest other group.
Especially, there were a NUMA specific test that I removed in v3 and
re added in v4.

Then, I'm also preparing a full rework that find_idlest_group() which
will behave more closely to load_balance; I mean : collect statistics,
classify group then selects the idlest

What is the behavior of lu.C Thread ? are they waking up a lot  ?and
could trigger the slow wake path ?

>
> The initial fixes I made for this issue did not exhibit this behavior. They
> would have had other issues dealing with overload cases though. In this case
> however there are only 154 or 158 threads on 160 CPUs so not overloaded.
>
> I'll try to get my hands on this system and poke into it. I just wanted to get
> your thoughts and let you know where we are.

Thanks for testing

Vincent

>
>
>
> Thanks,
> Phil
>
>
> System details:
>
> Architecture:        x86_64
> CPU op-mode(s):      32-bit, 64-bit
> Byte Order:          Little Endian
> CPU(s):              160
> On-line CPU(s) list: 0-159
> Thread(s) per core:  2
> Core(s) per socket:  10
> Socket(s):           8
> NUMA node(s):        8
> Vendor ID:           GenuineIntel
> CPU family:          6
> Model:               47
> Model name:          Intel(R) Xeon(R) CPU E7- 4870  @ 2.40GHz
> Stepping:            2
> CPU MHz:             1063.934
> BogoMIPS:            4787.73
> Virtualization:      VT-x
> L1d cache:           32K
> L1i cache:           32K
> L2 cache:            256K
> L3 cache:            30720K
> NUMA node0 CPU(s):   0-9,80-89
> NUMA node1 CPU(s):   10-19,90-99
> NUMA node2 CPU(s):   20-29,100-109
> NUMA node3 CPU(s):   30-39,110-119
> NUMA node4 CPU(s):   40-49,120-129
> NUMA node5 CPU(s):   50-59,130-139
> NUMA node6 CPU(s):   60-69,140-149
> NUMA node7 CPU(s):   70-79,150-159
> Flags:               fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 popcnt aes lahf_lm epb pti tpr_shadow vnmi flexpriority ept vpid dtherm ida arat
>
> $ numactl --hardware
> available: 8 nodes (0-7)
> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 80 81 82 83 84 85 86 87 88 89
> node 0 size: 64177 MB
> node 0 free: 60866 MB
> node 1 cpus: 10 11 12 13 14 15 16 17 18 19 90 91 92 93 94 95 96 97 98 99
> node 1 size: 64507 MB
> node 1 free: 61167 MB
> node 2 cpus: 20 21 22 23 24 25 26 27 28 29 100 101 102 103 104 105 106 107 108
> 109
> node 2 size: 64507 MB
> node 2 free: 61250 MB
> node 3 cpus: 30 31 32 33 34 35 36 37 38 39 110 111 112 113 114 115 116 117 118
> 119
> node 3 size: 64507 MB
> node 3 free: 61327 MB
> node 4 cpus: 40 41 42 43 44 45 46 47 48 49 120 121 122 123 124 125 126 127 128
> 129
> node 4 size: 64507 MB
> node 4 free: 60993 MB
> node 5 cpus: 50 51 52 53 54 55 56 57 58 59 130 131 132 133 134 135 136 137 138
> 139
> node 5 size: 64507 MB
> node 5 free: 60892 MB
> node 6 cpus: 60 61 62 63 64 65 66 67 68 69 140 141 142 143 144 145 146 147 148
> 149
> node 6 size: 64507 MB
> node 6 free: 61139 MB
> node 7 cpus: 70 71 72 73 74 75 76 77 78 79 150 151 152 153 154 155 156 157 158
> 159
> node 7 size: 64480 MB
> node 7 free: 61188 MB
> node distances:
> node   0   1   2   3   4   5   6   7
>  0:  10  12  17  17  19  19  19  19
>  1:  12  10  17  17  19  19  19  19
>  2:  17  17  10  12  19  19  19  19
>  3:  17  17  12  10  19  19  19  19
>  4:  19  19  19  19  10  12  17  17
>  5:  19  19  19  19  12  10  17  17
>  6:  19  19  19  19  17  17  10  12
>  7:  19  19  19  19  17  17  12  10
>
>
>
> --