TCP does not start transmitting and reports bandwidth of 0 bps

[cedyd]

Context

I am using Iperf3 with Centos7, I have four Ethernet controllers, hence four ports, and we are doing a loopback from channel 1 - channel 2; channel 3 - channel 4. Our test is running at 71 C, so our CPU is throttling a bit, however, with the provided cooling. We are supposed to run all day.

It seems that after sometime, when starting iperf in the 71 C condition, Iperf, seem not able to establish a negotiation and reports the following:

--Std Out: Connecting to host 10.60.104.1, port 5201
[ 5] local 192.168.103.1 port 59686 connected to 10.60.104.1 port 5201
[ ID] Interval Transfer Bandwidth Retr Cwnd
[ 5] 0.00-5.00 sec 42.4 KBytes 69.5 Kbits/sec 3 1.41 KBytes (omitted)
[ 5] 0.00-5.00 sec 0.00 Bytes 0.00 bits/sec 0 1.41 KBytes
[ 5] 5.00-10.00 sec 0.00 Bytes 0.00 bits/sec 1 1.41 KBytes
[ 5] 10.00-15.00 sec 0.00 Bytes 0.00 bits/sec 0 1.41 KBytes

• Version of iperf3:
  Iperf3.1.3

• Hardware:

• Operating system (and distribution, if any):

Linux - Centos7

• Other relevant information (for example, non-default compilers,
  libraries, cross-compiling, etc.):

Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate.

Bug Report

• Expected Behavior

• Actual Behavior

• Steps to Reproduce

• Possible Solution

Please submit patches or code changes as a pull request.

Enhancement Request

• Current behavior

• Desired behavior

• Implementation notes

If submitting a proposed implementation of an enhancement request,
please use the pull request mechanism.

[bmah888]

You could try this with a newer iperf3 (3.1.3 is pretty old) although I don't believe this would make a difference. Note that iperf3 was designed to test performance between two hosts, not between multiple interfaces on the same host. It's very possible the Linux networking stack (particularly the part that handles routing) is not doing exactly what you want it to do.

[janmaris]

Have you checked the MTU situation? perhaps it's lower than 1500. Best to control with the -M parameter: TCP max. segment size

[bmah888]

Closing for lack of additional information.