Why using the -P parameter in a container will get different results

# Context

* Version of iperf3: 3.0.7

* Operating system (and distribution, if any): centos7, kernel: 3.10, containerd

# See The Phenomenon
in different containers on different nodes(vxlan connect), use -p or not will get different results, the test results are as follows:
* Not use -P parameter

server:

```shell
iperf3 -s -p 5001 -i1
-----------------------------------------------------------
Server listening on 5001
-----------------------------------------------------------
Accepted connection from 10.244.4.173, port 34416
[ 5] local 10.244.3.53 port 5001 connected to 10.244.4.173 port 34418
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-1.00 sec 605 MBytes 5.07 Gbits/sec
[ 5] 1.00-2.00 sec 624 MBytes 5.24 Gbits/sec
[ 5] 2.00-3.00 sec 713 MBytes 5.98 Gbits/sec
[ 5] 3.00-4.00 sec 798 MBytes 6.70 Gbits/sec
[ 5] 4.00-5.00 sec 756 MBytes 6.34 Gbits/sec
[ 5] 5.00-5.04 sec 27.7 MBytes 6.60 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 5] 0.00-5.04 sec 3.44 GBytes 5.87 Gbits/sec 3155 sender
[ 5] 0.00-5.04 sec 3.44 GBytes 5.87 Gbits/sec receiver
```

agent:

```shell
iperf3 -c 10.244.3.53 -p5001 -t5
Connecting to host 10.244.3.53, port 5001
[ 4] local 10.244.4.173 port 34418 connected to 10.244.3.53 port 5001
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 4] 0.00-1.00 sec 626 MBytes 5.25 Gbits/sec 634 343 KBytes
[ 4] 1.00-2.00 sec 627 MBytes 5.26 Gbits/sec 725 442 KBytes
[ 4] 2.00-3.00 sec 727 MBytes 6.10 Gbits/sec 371 419 KBytes
[ 4] 3.00-4.00 sec 798 MBytes 6.69 Gbits/sec 741 388 KBytes
[ 4] 4.00-5.00 sec 749 MBytes 6.28 Gbits/sec 684 456 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-5.00 sec 3.44 GBytes 5.92 Gbits/sec 3155 sender
[ 4] 0.00-5.00 sec 3.44 GBytes 5.91 Gbits/sec receiver
```

* Use -P parameter:

server:

```shell
iperf3 -s -p 5001 -i1
...
...
[ ID] Interval Transfer Bandwidth Retr
[ 5] 0.00-5.02 sec 1013 MBytes 1.69 Gbits/sec 303 sender
[ 5] 0.00-5.02 sec 1008 MBytes 1.68 Gbits/sec receiver
[ 7] 0.00-5.02 sec 1020 MBytes 1.70 Gbits/sec 471 sender
[ 7] 0.00-5.02 sec 1015 MBytes 1.70 Gbits/sec receiver
[ 9] 0.00-5.02 sec 996 MBytes 1.66 Gbits/sec 183 sender
[ 9] 0.00-5.02 sec 993 MBytes 1.66 Gbits/sec receiver
[ 11] 0.00-5.02 sec 987 MBytes 1.65 Gbits/sec 317 sender
[ 11] 0.00-5.02 sec 984 MBytes 1.64 Gbits/sec receiver
[ 13] 0.00-5.02 sec 1004 MBytes 1.68 Gbits/sec 303 sender
[ 13] 0.00-5.02 sec 1001 MBytes 1.67 Gbits/sec receiver
[SUM] 0.00-5.02 sec 4.90 GBytes 8.39 Gbits/sec 1577 sender
[SUM] 0.00-5.02 sec 4.88 GBytes 8.36 Gbits/sec receiver
```

agent:

```shell
iperf3 -c 10.244.3.53 -p5001 -t5 -P5
...
...
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-5.00 sec 1013 MBytes 1.70 Gbits/sec 303 sender
[ 4] 0.00-5.00 sec 1008 MBytes 1.69 Gbits/sec receiver
[ 6] 0.00-5.00 sec 1020 MBytes 1.71 Gbits/sec 471 sender
[ 6] 0.00-5.00 sec 1015 MBytes 1.70 Gbits/sec receiver
[ 8] 0.00-5.00 sec 996 MBytes 1.67 Gbits/sec 183 sender
[ 8] 0.00-5.00 sec 993 MBytes 1.67 Gbits/sec receiver
[ 10] 0.00-5.00 sec 987 MBytes 1.66 Gbits/sec 317 sender
[ 10] 0.00-5.00 sec 984 MBytes 1.65 Gbits/sec receiver
[ 12] 0.00-5.00 sec 1004 MBytes 1.68 Gbits/sec 303 sender
[ 12] 0.00-5.00 sec 1001 MBytes 1.68 Gbits/sec receiver
[SUM] 0.00-5.00 sec 4.90 GBytes 8.42 Gbits/sec 1577 sender
[SUM] 0.00-5.00 sec 4.88 GBytes 8.39 Gbits/sec receiver
```

# My Doubt
* why use or not -P parameter get different results?
* Should I use the-P parameter to test the bandwidth? Looks closer to actual bandwidth (10G) with-P parameter.
* Why is it possible to achieve a bandwidth close to 10G without using the-P parameter on bare metal?