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SubjectRe: [RFC PATCH 00/11] Reconcile NUMA balancing decisions with the load balancer
Hi Mel,

On Wed, 12 Feb 2020 at 10:36, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> The NUMA balancer makes placement decisions on tasks that partially
> take the load balancer into account and vice versa but there are
> inconsistencies. This can result in placement decisions that override
> each other leading to unnecessary migrations -- both task placement and
> page placement. This is a prototype series that attempts to reconcile the
> decisions. It's a bit premature but it would also need to be reconciled
> with Vincent's series "[PATCH 0/4] remove runnable_load_avg and improve
> group_classify"
>
> The first three patches are unrelated and are either pending in tip or
> should be but they were part of the testing of this series so I have to
> mention them.
>
> The fourth and fifth patches are tracing only and was needed to get
> sensible data out of ftrace with respect to task placement for NUMA
> balancing. Patches 6-8 reduce overhead and reduce the changes of NUMA
> balancing overriding itself. Patches 9-11 try and bring the CPU placement
> decisions of NUMA balancing in line with the load balancer.

Don't know if it's only me but I can't find patches 9-11 on mailing list

>
> In terms of Vincent's patches, I have not checked but I expect conflicts
> to be with patches 10 and 11.
>
> Note that this is not necessarily a universal performance win although
> performance results are generally ok (small gains/losses depending on
> the machine and workload). However, task migrations, page migrations,
> variability and overall overhead are generally reduced.
>
> Tests are still running and take quite a long time so I do not have a
> full picture. The main reference workload I used was specjbb running one
> JVM per node which typically would be expected to split evenly. It's
> an interesting workload because the number of "warehouses" does not
> linearly related to the number of running tasks due to the creation of
> GC threads and other interfering activity. The mmtests configuration used
> is jvm-specjbb2005-multi with two runs -- one with ftrace enabling
> relevant scheduler tracepoints.
>
> The baseline is taken from late in the 5.6 merge window plus patches 1-4
> to take into account patches that are already in flight and the tracing
> patch I relied on for analysis.
>
> The headline performance of the series looks like
>
> baseline-v1 lboverload-v1
> Hmean tput-1 37842.47 ( 0.00%) 42391.63 * 12.02%*
> Hmean tput-2 94225.00 ( 0.00%) 91937.32 ( -2.43%)
> Hmean tput-3 141855.04 ( 0.00%) 142100.59 ( 0.17%)
> Hmean tput-4 186799.96 ( 0.00%) 184338.10 ( -1.32%)
> Hmean tput-5 229918.54 ( 0.00%) 230894.68 ( 0.42%)
> Hmean tput-6 271006.38 ( 0.00%) 271367.35 ( 0.13%)
> Hmean tput-7 312279.37 ( 0.00%) 314141.97 ( 0.60%)
> Hmean tput-8 354916.09 ( 0.00%) 357029.57 ( 0.60%)
> Hmean tput-9 397299.92 ( 0.00%) 399832.32 ( 0.64%)
> Hmean tput-10 438169.79 ( 0.00%) 442954.02 ( 1.09%)
> Hmean tput-11 476864.31 ( 0.00%) 484322.15 ( 1.56%)
> Hmean tput-12 512327.04 ( 0.00%) 519117.29 ( 1.33%)
> Hmean tput-13 528983.50 ( 0.00%) 530772.34 ( 0.34%)
> Hmean tput-14 537757.24 ( 0.00%) 538390.58 ( 0.12%)
> Hmean tput-15 535328.60 ( 0.00%) 539402.88 ( 0.76%)
> Hmean tput-16 539356.59 ( 0.00%) 545617.63 ( 1.16%)
> Hmean tput-17 535370.94 ( 0.00%) 547217.95 ( 2.21%)
> Hmean tput-18 540510.94 ( 0.00%) 548145.71 ( 1.41%)
> Hmean tput-19 536737.76 ( 0.00%) 545281.39 ( 1.59%)
> Hmean tput-20 537509.85 ( 0.00%) 543759.71 ( 1.16%)
> Hmean tput-21 534632.44 ( 0.00%) 544848.03 ( 1.91%)
> Hmean tput-22 531538.29 ( 0.00%) 540987.41 ( 1.78%)
> Hmean tput-23 523364.37 ( 0.00%) 536640.28 ( 2.54%)
> Hmean tput-24 530613.55 ( 0.00%) 531431.12 ( 0.15%)
> Stddev tput-1 1569.78 ( 0.00%) 674.58 ( 57.03%)
> Stddev tput-2 8.49 ( 0.00%) 1368.25 (-16025.00%)
> Stddev tput-3 4125.26 ( 0.00%) 1120.06 ( 72.85%)
> Stddev tput-4 4677.51 ( 0.00%) 717.71 ( 84.66%)
> Stddev tput-5 3387.75 ( 0.00%) 1774.13 ( 47.63%)
> Stddev tput-6 1400.07 ( 0.00%) 1079.75 ( 22.88%)
> Stddev tput-7 4374.16 ( 0.00%) 2571.75 ( 41.21%)
> Stddev tput-8 2370.22 ( 0.00%) 2918.23 ( -23.12%)
> Stddev tput-9 3893.33 ( 0.00%) 2708.93 ( 30.42%)
> Stddev tput-10 6260.02 ( 0.00%) 3935.05 ( 37.14%)
> Stddev tput-11 3989.50 ( 0.00%) 6443.16 ( -61.50%)
> Stddev tput-12 685.19 ( 0.00%) 12999.45 (-1797.21%)
> Stddev tput-13 3251.98 ( 0.00%) 9311.18 (-186.32%)
> Stddev tput-14 2793.78 ( 0.00%) 6175.87 (-121.06%)
> Stddev tput-15 6777.62 ( 0.00%) 25942.33 (-282.76%)
> Stddev tput-16 25057.04 ( 0.00%) 4227.08 ( 83.13%)
> Stddev tput-17 22336.80 ( 0.00%) 16890.66 ( 24.38%)
> Stddev tput-18 6662.36 ( 0.00%) 3015.10 ( 54.74%)
> Stddev tput-19 20395.79 ( 0.00%) 1098.14 ( 94.62%)
> Stddev tput-20 17140.27 ( 0.00%) 9019.15 ( 47.38%)
> Stddev tput-21 5176.73 ( 0.00%) 4300.62 ( 16.92%)
> Stddev tput-22 28279.32 ( 0.00%) 6544.98 ( 76.86%)
> Stddev tput-23 25368.87 ( 0.00%) 3621.09 ( 85.73%)
> Stddev tput-24 3082.28 ( 0.00%) 2500.33 ( 18.88%)
>
> Generally, this is showing a small gain in performance but it's
> borderline noise. However, in most cases, variability between
> the JVM performance is much reduced except at the point where
> a node is almost fully utilised.
>
> The high-level NUMA stats from /proc/vmstat look like this
>
> NUMA base-page range updates 1710927.00 2199691.00
> NUMA PTE updates 871759.00 1060491.00
> NUMA PMD updates 1639.00 2225.00
> NUMA hint faults 772179.00 967165.00
> NUMA hint local faults % 647558.00 845357.00
> NUMA hint local percent 83.86 87.41
> NUMA pages migrated 64920.00 45254.00
> AutoNUMA cost 3874.10 4852.08
>
> The percentage of local hits is higher (87.41% vs 84.86%). The
> number of pages migrated is reduced by 30%. The downside is
> that there are spikes when scanning is higher because in some
> cases NUMA balancing will not move a task to a local node if
> the CPU load balancer would immediately override it but it's
> not straight-forward to fix this in a universal way and should
> be a separate series.
>
> A separate run gathered information from ftrace and analysed it
> offline.
>
> 5.5.0 5.5.0
> baseline lboverload-v1
> Migrate failed no CPU 1934.00 4999.00
> Migrate failed move to idle 0.00 0.00
> Migrate failed swap task fail 981.00 2810.00
> Task Migrated swapped 6765.00 12609.00
> Task Migrated swapped local NID 0.00 0.00
> Task Migrated swapped within group 644.00 1105.00
> Task Migrated idle CPU 14776.00 750.00
> Task Migrated idle CPU local NID 0.00 0.00
> Task Migrate retry 2521.00 7564.00
> Task Migrate retry success 0.00 0.00
> Task Migrate retry failed 2521.00 7564.00
> Load Balance cross NUMA 1222195.00 1223454.00
>
> "Migrate failed no CPU" is the times when NUMA balancing did not
> find a suitable page on a preferred node. This is increased because
> the series avoids making decisions that the LB would override.
>
> "Migrate failed swap task fail" is when migrate_swap fails and it
> can fail for a lot of reasons.
>
> "Task Migrated swapped" is also higher but this is somewhat positive.
> It is when two tasks are swapped to keep load neutral or improved
> from the perspective of the load balancer. The series attempts to
> swap tasks that both move to their preferred node for example.
>
> "Task Migrated idle CPU" is also reduced. Again, this is a reflection
> that the series is trying to avoid NUMA Balancer and LB fighting
> each other.
>
> "Task Migrate retry failed" happens when NUMA balancing makes multiple
> attempts to place a task on a preferred node.
>
> So broadly speaking, similar or better performance with fewer page
> migrations and less conflict between the two balancers for at least one
> workload and one machine. There is room for improvement and I need data
> on more workloads and machines but an early review would be nice.
>
> include/trace/events/sched.h | 51 +++--
> kernel/sched/core.c | 11 --
> kernel/sched/fair.c | 430 ++++++++++++++++++++++++++++++++-----------
> kernel/sched/sched.h | 13 ++
> 4 files changed, 379 insertions(+), 126 deletions(-)
>
> --
> 2.16.4
>

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