Re: [dpdk-users] Interesting considerate but annoying behavior of MLX5 driver

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 26 Apr 2018 22:26:36 +0000
"Wu, Xiaoban" <Xiaoban_Wu@student.uml.edu> wrote:

> Dear All,
> 
> 
> I have two Dell PowerEdge R740 servers A and B running with ubuntu 16.04, each one has a Mellanox MCX556A-ECAT NIC installed on the PCIe x16 slot. And, the two NICs are directly connected back to back with a copper cable.
> 
> 
> On server A, it runs a RX program which runs a function to process and analyze the received packets. On server B, it runs a packet-gen program which generates and sends packets out. They are both compiled with dpdk-17.11
> 
> 
> Now the interesting thing is that on server B, it is kind of smart and considerate enough to automatically adjust its TX throughput according to how fast server A processes the packets. If server A processes the packets faster, then server B sends packets with a higher throughput. Similarly, if server A processes the packets slower, then server B sends packets with a lower throughput. Please note that once the program on server B starts, it is never interrupted by any way.
> 
> 
> However, I think the server B should send out the packets with a constant throughput no matter how fast server A processes the packets.
> 
> 
> Does anybody else notice this interesting behavior of MLX5 driver? Can anybody help me disable this feature permanently? Thanks very much for your help.
> 
> 
> Best wishes,
> 
> Xiaoban

Turn off Ethernet flow control.  It was a design mistake that NIC vendors always seem to like because it gives 0 loss benchmarks.