AF_XDP-example/txonly: strange performance with different packet sizes

Hi,

I am running the txonly microbenchmark from the AF_XDP-example sample 
program
in bpf-examples. Howerver, I noticed strange performance with different 
tx packet sizes.

With the following command I generate the below results:

./xdpsock -i enp2s0np0 -q 4 -t -b 2048 -m -s 128 -m

Driver: mlx5 on 40Gbps ConnectX-6
Kernel version: 6.0.0-rc6+

| Pkt Size | Native-ZC PPS | Native-C PPS | Generic PPS  |
| ----------- | -------------------- | ------------------ | 
----------------- |
| 64         | 16.5M              | 1.73M            | 1.71M          |
| 128       | 9.42M              | 1.72M            | 1.66M    |
| 256       | 7.78M              | 1.64M            | 1.66M          |
| 512       | 9.39M              | 1.62M            | 1.59M          |
| 1024     | 4.78M              | 1.42M            | 1.38M    |

At size 128B, I expect 16.5M packets (the limiting performance of 
AF_XDP) since
the link is not saturated. The problem is more obvious at size 256B, 
only 7.78B
even though it jumps again to 9.39M at 512. So I think the problem is 
related to
the packet size and not to a limited performance in the xsk engine in 
the kernel.
Or what do you think?

I have already opened an issue on github here:

https://github.com/xdp-project/bpf-examples/issues/61

Best,
Jalal