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SubjectBug report: UDP ~20% degradation
Hi all,

Our performance verification team spotted a degradation of up to ~20% in
UDP performance, for a specific combination of parameters.

Our matrix covers several parameters values, like:
IP version: 4/6
MTU: 1500/9000
Msg size: 64/1452/8952 (only when applicable while avoiding ip
fragmentation).
Num of streams: 1/8/16/24.
Num of directions: unidir/bidir.

Surprisingly, the issue exists only with this specific combination:
8 streams,
MTU 9000,
Msg size 8952,
both ipv4/6,
bidir.
(in unidir it repros only with ipv4)

The reproduction is consistent on all the different setups we tested with.

Bisect [2] was done between these two points, v5.19 (Good), and v6.0-rc1
(Bad), with ConnectX-6DX NIC.

c82a69629c53eda5233f13fc11c3c01585ef48a2 is the first bad commit [1].

We couldn't come up with a good explanation how this patch causes this
issue. We also looked for related changes in the networking/UDP stack,
but nothing looked suspicious.

Maybe someone here can help with this.
We can provide more details or do further tests/experiments to progress
with the debug.

Thanks,
Tariq

[1]
commit c82a69629c53eda5233f13fc11c3c01585ef48a2
Author: Vincent Guittot <vincent.guittot@linaro.org>
Date: Fri Jul 8 17:44:01 2022 +0200

sched/fair: fix case with reduced capacity CPU

The capacity of the CPU available for CFS tasks can be reduced
because of
other activities running on the latter. In such case, it's worth
trying to
move CFS tasks on a CPU with more available capacity.




The rework of the load balance has filtered the case when the CPU
is


classified to be fully busy but its capacity is reduced.







Check if CPU's capacity is reduced while gathering load balance
statistic


and classify it group_misfit_task instead of group_fully_busy so we
can


try to move the load on another CPU.







Reported-by: David Chen <david.chen@nutanix.com>



Reported-by: Zhang Qiao <zhangqiao22@huawei.com>



Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>



Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>



Tested-by: David Chen <david.chen@nutanix.com>



Tested-by: Zhang Qiao <zhangqiao22@huawei.com>



Link:
https://lkml.kernel.org/r/20220708154401.21411-1-vincent.guittot@linaro.org




[2]

Detailed bisec steps:

+--------------+--------+-----------+-----------+
| Commit | Status | BW (Gbps) | BW (Gbps) |
| | | run1 | run2 |
+--------------+--------+-----------+-----------+
| 526942b8134c | Bad | --- | --- |
+--------------+--------+-----------+-----------+
| 2e7a95156d64 | Bad | --- | --- |
+--------------+--------+-----------+-----------+
| 26c350fe7ae0 | Good | 279.8 | 281.9 |
+--------------+--------+-----------+-----------+
| 9de1f9c8ca51 | Bad | 257.243 | --- |
+--------------+--------+-----------+-----------+
| 892f7237b3ff | Good | 285 | 300.7 |
+--------------+--------+-----------+-----------+
| 0dd1cabe8a4a | Good | 305.599 | 290.3 |
+--------------+--------+-----------+-----------+
| dfea84827f7e | Bad | 250.2 | 258.899 |
+--------------+--------+-----------+-----------+
| 22a39c3d8693 | Bad | 236.8 | 245.399 |
+--------------+--------+-----------+-----------+
| e2f3e35f1f5a | Good | 277.599 | 287 |
+--------------+--------+-----------+-----------+
| 401e4963bf45 | Bad | 250.149 | 248.899 |
+--------------+--------+-----------+-----------+
| 3e8c6c9aac42 | Good | 299.09 | 294.9 |
+--------------+--------+-----------+-----------+
| 1fcf54deb767 | Good | 292.719 | 301.299 |
+--------------+--------+-----------+-----------+
| c82a69629c53 | Bad | 254.7 | 246.1 |
+--------------+--------+-----------+-----------+
| c02d5546ea34 | Good | 276.4 | 294 |
+--------------+--------+-----------+-----------+

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