Receiver reports double data rate at times

[manticode]

Context

• Version of iperf3: iperf 3.17.1 (cJSON 1.7.15)

• Hardware: Intel(R) Xeon(R) Gold 6354 CPU @ 3.00GHz (20 vcores)

• 56 GB memory

• Operating system (and distribution, if any): Oracle Linux Server release 8.10 64 bit

Please note: iperf3 is supported on Linux, FreeBSD, and macOS.
Support may be provided on a best-effort basis to other UNIX-like
platforms. We cannot provide support for building and/or running
iperf3 on Windows, iOS, or Android.

• Other relevant information (for example, non-default compilers,
  libraries, cross-compiling, etc.):
  CPU affinity setting, IPv6 flow label, SCTP, TCP congestion algorithm setting, sendfile / zerocopy, socket pacing, authentication, bind to device, support IPv4 don't fragment, POSIX threads

Sender is on a VM in OCI using "Optimized" shape with SR-IOV interfaces.

Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate. Note that submissions of bug fixes, new
features, etc. should be done as a pull request at
https://github.com/esnet/iperf/pulls

Bug Report

• Expected Behavior
  Sender and receiver report received speed correctly.

• Actual Behavior
  Intermittently, receiver reports back double data rate on some streams (when using parallel streams).

• Steps to Reproduce
  Run forward test with 50 Mbps with -P 10 and UDP
  Also occurs without -P option

[  5] local 10.218.11.55 port 56570 connected to 10.211.95.1 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  5.96 MBytes  50.0 Mbits/sec  39028
[  5]   1.00-2.00   sec  5.96 MBytes  50.0 Mbits/sec  39063
[  5]   2.00-3.00   sec  5.96 MBytes  50.0 Mbits/sec  39062
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-3.00   sec  17.9 MBytes  50.0 Mbits/sec  0.000 ms  0/117153 (0%)  sender
[  5]   0.00-3.01   sec  35.8 MBytes  99.7 Mbits/sec  0.008 ms  0/117153 (0%)  receiver

• Possible Solution
  Repeat a few times and problem may go away. Sometimes this only affects a few UDP parallel streams, but other times it affects all. Sometimes this issue does not happen.

Interesting to note that the total packets received correlate correctly.

[davidBar-On]

@manticode, I tried to run the test several time on my machine and there was no problem. Can you give more information:

1. What are the exact server and client commands? I.e. iperf3 -c ...., iperf3 -s .....
2. What is the server's output in these cases? It is important, so it will be clear if the root of the problem is in the server's side of the client's side.