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

Hi,

MPWQE apparently causes this in mlx5. MPWQE does not provide true 
zero-copy for packet sizes less than 256bytes. In order not to saturate 
the PCIe bus, MPWQE "copies" multiple small packets into one fixed-size 
memory block and then send them in one work queue entry which explains 
the degrading performance from packet sizes 64B to 256B.

https://github.com/torvalds/linux/blob/5aaef24b5c6d4246b2cac1be949869fa36577737/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h#L159

MPWQE can be turned off using: ethtool --set-priv-flags enp2s0np0 
xdp_tx_mpwqe off
Performance reaches 25M pps on 64B and 128B, and 18M on 256 saturating a 
40Gbps link.

Best,
Jalal

On 10/26/22 12:34, Mostafa, Jalal (IPE) wrote:
> 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
>