On Thu, Aug 01, 2019 at 04:40:16PM +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.
>
> Patch 1 has already been sent separatly and only consolidate asym policy
> in one place and help the review of the changes in load_balance.
>
> Patch 2 renames the sum of h_nr_running in stats.
>
> Patch 3 removes meaningless imbalance computation to make review of
> patch 4 easier.
>
> Patch 4 reworks load_balance algorithm and fixes some wrong task placement
> but try to stay conservative.
>
> Patch 5 add the sum of nr_running to monitor non cfs tasks and take that
> into account when pulling tasks.
>
> Patch 6 replaces runnable_load by load now that the metrics is only used
> when overloaded.
>
> Patch 7 improves the spread of tasks at the 1st scheduling level.
>
> Patch 8 uses utilization instead of load in all steps of misfit task
> path.
>
> Some benchmarks results based on 8 iterations of each tests:
> - small arm64 dual quad cores system
>
> tip/sched/core w/ this patchset improvement
> schedpipe 54981 +/-0.36% 55459 +/-0.31% (+0.97%)
>
> hackbench
> 1 groups 0.906 +/-2.34% 0.906 +/-2.88% (+0.06%)
>
> - large arm64 2 nodes / 224 cores system
>
> tip/sched/core w/ this patchset improvement
> schedpipe 125665 +/-0.61% 125455 +/-0.62% (-0.17%)
>
> hackbench -l (256000/#grp) -g #grp
> 1 groups 15.263 +/-3.53% 13.776 +/-3.30% (+9.74%)
> 4 groups 5.852 +/-0.57% 5.340 +/-8.03% (+8.75%)
> 16 groups 3.097 +/-1.08% 3.246 +/-0.97% (-4.81%)
> 32 groups 2.882 +/-1.04% 2.845 +/-1.02% (+1.29%)
> 64 groups 2.809 +/-1.30% 2.712 +/-1.17% (+3.45%)
> 128 groups 3.129 +/-9.74% 2.813 +/-6.22% (+9.11%)
> 256 groups 3.559 +/-11.07% 3.020 +/-1.75% (+15.15%)
>
> dbench
> 1 groups 330.897 +/-0.27% 330.612 +/-0.77% (-0.09%)
> 4 groups 932.922 +/-0.54% 941.817 +/*1.10% (+0.95%)
> 16 groups 1932.346 +/-1.37% 1962.944 +/-0.62% (+1.58%)
> 32 groups 2251.079 +/-7.93% 2418.531 +/-0.69% (+7.44%)
> 64 groups 2104.114 +/-9.67% 2348.698 +/-11.24% (+11.62%)
> 128 groups 2093.756 +/-7.26% 2278.156 +/-9.74% (+8.81%)
> 256 groups 1216.736 +/-2.46% 1665.774 +/-4.68% (+36.91%)
>
> tip/sched/core sha1:
> a1dc0446d649 ('sched/core: Silence a warning in sched_init()')
>
> Changes since v1:
> - fixed some bugs
> - Used switch case
> - Renamed env->src_grp_type to env->balance_type
> - split patches in smaller ones
> - added comments
>
> Vincent Guittot (8):
> sched/fair: clean up asym packing
> sched/fair: rename sum_nr_running to sum_h_nr_running
> sched/fair: remove meaningless imbalance calculation
> sched/fair: rework load_balance
> sched/fair: use rq->nr_running when balancing load
> sched/fair: use load instead of runnable load
> sched/fair: evenly spread tasks when not overloaded
> sched/fair: use utilization to select misfit task
>
> kernel/sched/fair.c | 769
> ++++++++++++++++++++++++++++-----------------------
> kernel/sched/sched.h | 2 +-
> 2 files changed, 419 insertions(+), 352 deletions(-)
>
> --
> 2.7.4
>
I keep expecting a v3 so I have not dug into all the patches in detail.
However, I've
been working with them from Vincent's tree while they were under development so
I thought
I'd add some results.
The workload is a test our perf team came up with to illustrate the issues we
were seeing
with imbalance in the presence of group scheduling.
On a 4-numa X 20 cpu system (smt on) we run a 76 thread lu.C benchmark from the
NAS Parallel
suite. And at the same time run 2 stress cpu burn processes. The GROUP test
puts the
benchmark and the stress processes each in its own cgroup. The NORMAL case
puts them all
in the first cgroup. The results show first the average number of threads of
each type
running on each of the numa nodes based on sampling taken during the run. This
is followed
by the lu.C benchmark results. There are 3 runs of GROUP and 2 runs of NORMAL
shown.
Before (linux-5.3-rc1+ @ a1dc0446d649)
lu.C.x_76_GROUP_1.stress.ps.numa.hist Average 0.00 1.00 1.00
lu.C.x_76_GROUP_2.stress.ps.numa.hist Average 0.00 1.00 1.00
lu.C.x_76_GROUP_3.stress.ps.numa.hist Average 0.00 1.00 1.00
lu.C.x_76_NORMAL_1.stress.ps.numa.hist Average 1.15 0.23 0.00 0.62
lu.C.x_76_NORMAL_2.stress.ps.numa.hist Average 1.67 0.00 0.00 0.33
lu.C.x_76_GROUP_1.ps.numa.hist Average 30.45 6.95 4.52 34.08
lu.C.x_76_GROUP_2.ps.numa.hist Average 32.33 8.94 9.21 25.52
lu.C.x_76_GROUP_3.ps.numa.hist Average 30.45 8.91 12.09 24.55
lu.C.x_76_NORMAL_1.ps.numa.hist Average 18.54 19.23 19.69 18.54
lu.C.x_76_NORMAL_2.ps.numa.hist Average 17.25 19.83 20.00 18.92
============76_GROUP========Mop/s===================================
min q1 median q3 max
2119.92 2418.1 2716.28 3147.82 3579.36
============76_GROUP========time====================================
min q1 median q3 max
569.65 660.155 750.66 856.245 961.83
============76_NORMAL========Mop/s===================================
min q1 median q3 max
30424.5 31486.4 31486.4 31486.4 32548.4
============76_NORMAL========time====================================
min q1 median q3 max
62.65 64.835 64.835 64.835 67.02
After (linux-5.3-rc1+ @ a1dc0446d649 + this v2 series pulled from
Vincent's git on ~8/15)
lu.C.x_76_GROUP_1.stress.ps.numa.hist Average 0.36 1.00 0.64
lu.C.x_76_GROUP_2.stress.ps.numa.hist Average 1.00 1.00
lu.C.x_76_GROUP_3.stress.ps.numa.hist Average 1.00 1.00
lu.C.x_76_NORMAL_1.stress.ps.numa.hist Average 0.23 0.15 0.31 1.31
lu.C.x_76_NORMAL_2.stress.ps.numa.hist Average 1.00 0.00 0.00 1.00
lu.C.x_76_GROUP_1.ps.numa.hist Average 18.91 18.36 18.91 19.82
lu.C.x_76_GROUP_2.ps.numa.hist Average 18.36 18.00 19.91 19.73
lu.C.x_76_GROUP_3.ps.numa.hist Average 18.17 18.42 19.25 20.17
lu.C.x_76_NORMAL_1.ps.numa.hist Average 19.08 20.00 18.62 18.31
lu.C.x_76_NORMAL_2.ps.numa.hist Average 18.09 19.91 19.18 18.82
============76_GROUP========Mop/s===================================
min q1 median q3 max
32304.1 33176 34047.9 34166.8 34285.7
============76_GROUP========time====================================
min q1 median q3 max
59.47 59.68 59.89 61.505 63.12
============76_NORMAL========Mop/s===================================
min q1 median q3 max
29825.5 32454 32454 32454 35082.5
============76_NORMAL========time====================================
min q1 median q3 max
58.12 63.24 63.24 63.24 68.36
I had initially tracked this down to two issues. The first was picking the wrong
group in find_busiest_group due to using the average load. The second was in
fix_small_imbalance(). The "load" of the lu.C tasks was so low it often failed
to move anything even when it did find a group that was overloaded (nr_running
> width). I have two small patches which fix this but since Vincent was
> embarking
on a re-work which also addressed this I dropped them.
We've also run a series of performance tests we use to check for regressions
and
did not find any bad results on our workloads and systems.
So...
Tested-by: Phil Auld <pa...@redhat.com>
Cheers,
Phil
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