This looks like a CPU issue, not an iperf issue.

I would try running separate iperf3 processes and manually binding them to a 
CPU/core on each host.

Example for #2:
Host1: iperf3 -s -p 5000
Host1: iperf3 -s -p 5001
Host2: iperf3 -c host1 -p5000  -A0,0
Host2: iperf3 -c host1 -p5001 -A1,1 -R

Since you are able to get full line-rate in one direction, I suspect you don’t 
need to tweak TCP window sizes or frame/packet sizes.


Ryan Harden
Research and Advanced Networking Architect
University of Chicago - ASN160
P: 773.834.5441

> On Aug 9, 2017, at 4:51 AM, CiscoNSP List <> wrote:
> Hi James - Yes I tested this still will not go above 
> first I though it may be the carriers link, but I also tested the 
> bidirectional/simultaneous test, and it produced similar results (Single 
> direction test were fine.)  Single direction tests are fine.
> I think its a problem with IPerf, potentiallly the server nics...Or maybe 
> IPerf is "limited" to the bandwith of the link (1G, but can only send/receive 
> a "total" (50% in both directions)
> There are a few reports of issues with the bidirectional tests, but I cant 
> locate any "fixes" Id prefer to spend time looking at a more robust 
> (proven) testing tool.
> Cheers
> ________________________________
> From: cisco-nsp <> on behalf of James 
> Bensley <>
> Sent: Wednesday, 9 August 2017 5:47 PM
> To:
> Subject: Re: [c-nsp] IPerf alternative
> On 9 August 2017 at 08:39, CiscoNSP List <> wrote:
>> Thanks Ryan - They did not include bidirectional simultaneous test in V3 I 
>> dont think? (this is a test we definitely require, and one that is causing 
>> us the most issues in v2 (It appears to be somewhat broken...perhaps that is 
>> what they dint include it in v3 ?
> Run two copies simultaneously? That way you can also tweak the two
> unidirectional streams independently.
> Cheers,
> James.
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