Unstable local network throughput

[Hans Petter Selasky]

On 08/04/16 17:24, Ben RUBSON wrote:
>
>> 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) ?
>

Hi,

Possibly because the packets are arriving at the wrong CPU compared to 
what RSS expects. Then RSS will invoke a taskqueue to process the 
packets on the correct CPU, if I'm not mistaken.

The mlx4 driver does not fully support RSS. Then mlx5 does.

--HPS