Unstable local network throughput

[Ben RUBSON]

> On 04 Aug 2016, at 11:40, Ben RUBSON <ben.rubson at gmail.com> wrote:
> 
>> On 02 Aug 2016, at 22:11, Ben RUBSON <ben.rubson at gmail.com> wrote:
>> 
>>> On 02 Aug 2016, at 21:35, Hans Petter Selasky <hps at selasky.org> wrote:
>>> 
>>> The CX-3 driver doesn't bind the worker threads to specific CPU cores by default, so if your CPU has more than one so-called numa, you'll end up that the bottle-neck is the high-speed link between the CPU cores and not the card. A quick and dirty workaround is to "cpuset" iperf and the interrupt and taskqueue threads to specific CPU cores.
>> 
>> My CPUs : 2x E5-2620v3 with DDR4 at 1866.
> 
> OK, so I cpuset all Mellanox interrupts to one NUMA, as well as the iPerf processes, and I'm able to reach max bandwidth.
> Choosing the wrong NUMA (or both, or one for interrupts, the other one for iPerf, etc...) totally kills throughput.
> 
> However, full-duplex throughput is still limited, I can't manage to reach 2x40Gb/s, throttle is at about 45Gb/s.
> I tried many different cpuset layouts, but I never went above 45Gb/s.
> (Linux allowed me to reach 2x40Gb/s so hardware is not a bottleneck)
> 
>>> Are you using "options RSS" and "options PCBGROUP" in your kernel config?
> 
> I will then give RSS a try.

Without RSS :
A ---> B : 40Gbps (unidirectional)
A <--> B : 45Gbps (bidirectional)

With RSS :
A ---> B : 28Gbps (unidirectional)
A <--> B : 28Gbps (bidirectional)

Sounds like RSS does not help :/

Why, without RSS, do I have difficulties to reach 2x40Gbps (full-duplex) ?

Thank U !