UDP iperf2 throughput cannot higher than 600Mbps

### Context
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I'm troubleshooting the company product, but there's some issue with the UDP iperf2 throughput

* **Version of iperf**
iperf 2.0.9
iperf 3.1.3

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### Bug Report
I set my pc as the server and the product's kernal as the client, the bandwidth is 1100M:

**For Iperf3**
[ 5] local 192.168.1.2 port 5001 connected to 192.168.1.1 port 50031
[ ID] Interval Transfer Bandwidth Jitter Lost/Total Datagrams
[ 5] 0.00-1.00 sec 95.0 MBytes 797 Mbits/sec 0.053 ms 115/12278 (0.94%)
[ 5] 1.00-2.00 sec 111 MBytes 931 Mbits/sec 0.066 ms 207/14407 (1.4%)
[ 5] 2.00-3.00 sec 108 MBytes 908 Mbits/sec 0.073 ms 273/14130 (1.9%)
[ 5] 3.00-4.00 sec 110 MBytes 923 Mbits/sec 0.067 ms 303/14384 (2.1%)
[ 5] 4.00-5.00 sec 111 MBytes 934 Mbits/sec 0.056 ms 179/14425 (1.2%)
[ 5] 5.00-6.00 sec 109 MBytes 913 Mbits/sec 0.054 ms 217/14140 (1.5%)
[ 5] 6.00-7.00 sec 112 MBytes 943 Mbits/sec 0.053 ms 0/14394 (0%)
[ 5] 7.00-8.00 sec 105 MBytes 883 Mbits/sec 0.057 ms 0/13466 (0%)
[ 5] 8.00-9.00 sec 107 MBytes 900 Mbits/sec 0.056 ms 0/13734 (0%)
iperf3: OUT OF ORDER - incoming packet = 68767 and received packet = 134302 AND SP = 5
[ 5] 9.00-10.00 sec 105 MBytes 879 Mbits/sec 0.075 ms 1/13416 (0.0075%)
[ 5] 10.00-10.03 sec 3.45 MBytes 876 Mbits/sec 0.074 ms 0/441 (0%)
[ ID] Interval Transfer Bandwidth Jitter Lost/Total Datagrams
[ 5] 0.00-10.03 sec 0.00 Bytes 0.00 bits/sec 0.074 ms 1295/139215 (0.93%)
[SUM] 0.0-10.0 sec 1 datagrams received out-of-order

-> It looks normal

**For iperf2**

[ 3] local 192.168.1.2 port 5001 connected with 192.168.1.1 port 44077
[ ID] Interval Transfer Bandwidth Jitter Lost/Total Datagrams
[ 3] 0.0- 1.0 sec 66.4 MBytes 557 Mbits/sec 0.032 ms 22/47394 (0.046%)
[ 3] 1.0- 2.0 sec 66.4 MBytes 557 Mbits/sec 0.027 ms 33/47425 (0.07%)
[ 3] 2.0- 3.0 sec 66.5 MBytes 558 Mbits/sec 0.024 ms 24/47462 (0.051%)
[ 3] 3.0- 4.0 sec 66.5 MBytes 558 Mbits/sec 0.029 ms 0/47460 (0%)
[ 3] 4.0- 5.0 sec 66.5 MBytes 558 Mbits/sec 0.026 ms 0/47461 (0%)
[ 3] 5.0- 6.0 sec 66.5 MBytes 558 Mbits/sec 0.030 ms 26/47463 (0.055%)
[ 3] 6.0- 7.0 sec 66.5 MBytes 558 Mbits/sec 0.027 ms 35/47450 (0.074%)
[ 3] 7.0- 8.0 sec 66.3 MBytes 556 Mbits/sec 0.028 ms 186/47452 (0.39%)
[ 3] 8.0- 9.0 sec 66.5 MBytes 558 Mbits/sec 0.025 ms 0/47452 (0%)
[ 3] 0.0-10.0 sec 665 MBytes 557 Mbits/sec 0.028 ms 359/474363 (0.076%)

-> with no packet loss, no additional commands; why the the throughput drop to 500Mbps?

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**Expected Behavior**
UDP can get 800~900Mbps throughput for iperf2 and iperf3, since I use cat5 cable for connection (1GB)

**Actual Behavior**
Iperf2 get no more than 600Mbps, I have tried -l , -w , -P, but all of them not work.

**Possible Solutions**
Unknown. Since I mainly use iperf2, I wish I can figure out this iperf2 issue.