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Subject[RESEND RFC/RFT V2 PATCH 0/5] Improve scheduler scalability for fast path
Date
Current select_idle_sibling first tries to find a fully idle core using
select_idle_core which can potentially search all cores and if it fails it
finds any idle cpu using select_idle_cpu. select_idle_cpu can potentially
search all cpus in the llc domain. This doesn't scale for large llc domains
and will only get worse with more cores in future.

This patch solves the scalability problem by:
- Setting an upper and lower limit of idle cpu search in select_idle_cpu
to keep search time low and constant
- Adding a new sched feature SIS_CORE to disable select_idle_core

Additionally it also introduces a new per-cpu variable next_cpu to track
the limit of search so that every time search starts from where it ended.
This rotating search window over cpus in LLC domain ensures that idle
cpus are eventually found in case of high load.

Following are the performance numbers with various benchmarks with SIS_CORE
true (idle core search enabled).

Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine
(lower is better):
groups baseline %stdev patch %stdev
1 0.5816 8.94 0.5903 (-1.5%) 11.28
2 0.6428 10.64 0.5843 (9.1%) 4.93
4 1.0152 1.99 0.9965 (1.84%) 1.83
8 1.8128 1.4 1.7921 (1.14%) 1.76
16 3.1666 0.8 3.1345 (1.01%) 0.81
32 5.6084 0.83 5.5677 (0.73%) 0.8

Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
threads baseline %stdev patch %stdev
8 2095.45 1.82 2102.6 (0.34%) 2.11
16 4218.45 0.06 4221.35 (0.07%) 0.38
32 7531.36 0.49 7607.18 (1.01%) 0.25
48 10206.42 0.21 10324.26 (1.15%) 0.13
64 12053.73 0.1 12158.3 (0.87%) 0.24
128 14810.33 0.04 14840.4 (0.2%) 0.38

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine
(normalized, higher is better):
users baseline %stdev patch %stdev
20 1 0.9 1.0068 (0.68%) 0.27
40 1 0.8 1.0103 (1.03%) 1.24
60 1 0.34 1.0178 (1.78%) 0.49
80 1 0.53 1.0092 (0.92%) 1.5
100 1 0.79 1.0090 (0.9%) 0.88
120 1 0.06 1.0048 (0.48%) 0.72
140 1 0.22 1.0116 (1.16%) 0.05
160 1 0.57 1.0264 (2.64%) 0.67
180 1 0.81 1.0194 (1.94%) 0.91
200 1 0.44 1.028 (2.8%) 3.09
220 1 1.74 1.0229 (2.29%) 0.21

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with
message size = 8k (higher is better):
threads baseline %stdev patch %stdev
8 45.36 0.43 46.28 (2.01%) 0.29
16 87.81 0.82 89.67 (2.12%) 0.38
32 151.19 0.02 153.5 (1.53%) 0.41
48 190.2 0.21 194.79 (2.41%) 0.07
64 190.42 0.35 202.9 (6.55%) 1.66
128 323.86 0.28 343.56 (6.08%) 1.34

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline patch
1 629.8 603.83 (-4.12%)
2 1159.65 1155.75 (-0.34%)
4 2121.61 2093.99 (-1.3%)
8 2620.52 2641.51 (0.8%)
16 2879.31 2897.6 (0.64%)
32 2791.24 2936.47 (5.2%)
64 1853.07 1894.74 (2.25%)
128 1484.95 1494.29 (0.63%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline patch
1 256.41 255.8 (-0.24%)
2 509.89 504.52 (-1.05%)
4 999.44 1003.74 (0.43%)
8 1982.7 1976.42 (-0.32%)
16 3891.51 3916.04 (0.63%)
32 6819.24 6845.06 (0.38%)
64 8542.95 8568.28 (0.3%)
128 15277.6 15754.6 (3.12%)

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile baseline %stdev patch %stdev
50 94 2.82 92 (2.13%) 2.17
75 124 2.13 122 (1.61%) 1.42
90 152 1.74 151 (0.66%) 0.66
95 171 2.11 170 (0.58%) 0
99 512.67 104.96 208.33 (59.36%) 1.2
99.5 2296 82.55 3674.66 (-60.05%) 22.19
99.9 12517.33 2.38 12784 (-2.13%) 0.66

Hackbench process on 2 socket, 16 core and 128 threads SPARC machine
(lower is better):
groups baseline %stdev patch %stdev
1 1.3085 6.65 1.2213 (6.66%) 10.32
2 1.4559 8.55 1.5048 (-3.36%) 4.72
4 2.6271 1.74 2.5532 (2.81%) 2.02
8 4.7089 3.01 4.5118 (4.19%) 2.74
16 8.7406 2.25 8.6801 (0.69%) 4.78
32 17.7835 1.01 16.759 (5.76%) 1.38
64 36.1901 0.65 34.6652 (4.21%) 1.24
128 72.6585 0.51 70.9762 (2.32%) 0.9

Following are the performance numbers with various benchmarks with SIS_CORE
false (idle core search disabled). This improves throughput of certain
workloads but increases latency of other workloads.

Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine
(lower is better):
groups baseline %stdev patch %stdev
1 0.5816 8.94 0.5835 (-0.33%) 8.21
2 0.6428 10.64 0.5752 (10.52%) 4.05
4 1.0152 1.99 0.9946 (2.03%) 2.56
8 1.8128 1.4 1.7619 (2.81%) 1.88
16 3.1666 0.8 3.1275 (1.23%) 0.42
32 5.6084 0.83 5.5856 (0.41%) 0.89

Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
threads baseline %stdev patch %stdev
8 2095.45 1.82 2084.72 (-0.51%) 1.65
16 4218.45 0.06 4179.69 (-0.92%) 0.18
32 7531.36 0.49 7623.18 (1.22%) 0.39
48 10206.42 0.21 10159.16 (-0.46%) 0.21
64 12053.73 0.1 12087.21 (0.28%) 0.19
128 14810.33 0.04 14894.08 (0.57%) 0.08

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine
(normalized, higher is better):
users baseline %stdev patch %stdev
20 1 0.9 1.0056 (0.56%) 0.34
40 1 0.8 1.0173 (1.73%) 0.13
60 1 0.34 0.9995 (-0.05%) 0.85
80 1 0.53 1.0175 (1.75%) 1.56
100 1 0.79 1.0151 (1.51%) 1.31
120 1 0.06 1.0244 (2.44%) 0.5
140 1 0.22 1.034 (3.4%) 0.66
160 1 0.57 1.0362 (3.62%) 0.07
180 1 0.81 1.041 (4.1%) 0.8
200 1 0.44 1.0233 (2.33%) 1.4
220 1 1.74 1.0125 (1.25%) 1.41

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with
message size = 8k (higher is better):
threads baseline %stdev patch %stdev
8 45.36 0.43 46.94 (3.48%) 0.2
16 87.81 0.82 91.75 (4.49%) 0.43
32 151.19 0.02 167.74 (10.95%) 1.29
48 190.2 0.21 200.57 (5.45%) 0.89
64 190.42 0.35 226.74 (19.07%) 1.79
128 323.86 0.28 348.12 (7.49%) 0.77

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline patch
1 629.8 600.19 (-4.7%)
2 1159.65 1162.07 (0.21%)
4 2121.61 2112.27 (-0.44%)
8 2620.52 2645.55 (0.96%)
16 2879.31 2828.87 (-1.75%)
32 2791.24 2760.97 (-1.08%)
64 1853.07 1747.66 (-5.69%)
128 1484.95 1459.81 (-1.69%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline patch
1 256.41 258.11 (0.67%)
2 509.89 509.13 (-0.15%)
4 999.44 1016.58 (1.72%)
8 1982.7 2006.53 (1.2%)
16 3891.51 3964.43 (1.87%)
32 6819.24 7376.92 (8.18%)
64 8542.95 9660.45 (13.08%)
128 15277.6 15438.4 (1.05%)

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile baseline %stdev patch %stdev
50 94 2.82 94.67 (-0.71%) 2.2
75 124 2.13 124.67 (-0.54%) 1.67
90 152 1.74 154.33 (-1.54%) 0.75
95 171 2.11 176.67 (-3.31%) 0.86
99 512.67 104.96 4130.33 (-705.65%) 79.41
99.5 2296 82.55 10066.67 (-338.44%) 26.15
99.9 12517.33 2.38 12869.33 (-2.81%) 0.8

Hackbench process on 2 socket, 16 core and 128 threads SPARC machine
(lower is better):
groups baseline %stdev patch %stdev
1 1.3085 6.65 1.2514 (4.36%) 11.1
2 1.4559 8.55 1.5433 (-6%) 3.05
4 2.6271 1.74 2.5626 (2.5%) 2.69
8 4.7089 3.01 4.5316 (3.77%) 2.95
16 8.7406 2.25 8.6585 (0.94%) 2.91
32 17.7835 1.01 17.175 (3.42%) 1.38
64 36.1901 0.65 35.5294 (1.83%) 1.02
128 72.6585 0.51 71.8821 (1.07%) 1.05

Following are the schbench performance numbers with SIS_CORE false and
SIS_PROP false. This recovers the latency increase by having SIS_CORE
false.

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile baseline %stdev patch %stdev
50 94 2.82 93.33 (0.71%) 1.24
75 124 2.13 122.67 (1.08%) 1.7
90 152 1.74 149.33 (1.75%) 2.35
95 171 2.11 167 (2.34%) 2.74
99 512.67 104.96 206 (59.82%) 8.86
99.5 2296 82.55 3121.67 (-35.96%) 97.37
99.9 12517.33 2.38 12592 (-0.6%) 1.67

Changes since v1
- Compute the upper and lower limit based on number of cpus in a core
- Split up the search limit and search window rotation into separate
patches
- Add new sched feature to have option of disabling idle core search

subhra mazumdar (5):
sched: limit cpu search in select_idle_cpu
sched: introduce per-cpu var next_cpu to track search limit
sched: rotate the cpu search window for better spread
sched: add sched feature to disable idle core search
sched: SIS_CORE to disable idle core search

kernel/sched/core.c | 2 ++
kernel/sched/fair.c | 31 +++++++++++++++++++++++--------
kernel/sched/features.h | 1 +
kernel/sched/sched.h | 1 +
4 files changed, 27 insertions(+), 8 deletions(-)

--
2.9.3

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