On Mon, 29 May 2023 23:02:45 +0800
Fengkai Sun <qcloud1014@gmail.com> wrote:

> Hi list,
>
> I'm curious how DPDK programs the NIC to receive/transmit packets when
> there are multiple queues on a single port.
>
> As for RX, the answer might be clear.
> The NIC can only receive a packet once at a time, since the cable only
> outputs one signal (0 or 1) at a time (correct me if I'm wrong).
> Therefore the NIC can receive a packet, check it's information, and finally
> put in into the right queue via some policies, e.g. RSS, all sequentially.
>
> However, it confuses me when it comes to TX.
> As there are multiple TX queues on the same port, the NIC must decide which
> queue to get packets from when it's idle.
> This is where scheduling lies. How does the NIC select the queue?
> Round-Robin? Does it have to enforce fairness among the queues?
>
> I'm wondering where I can find some documentation on this issue. Thank you!

Transmit scheduling is up to the hardware (not DPDK).
Generally I assume it is round-robin
but there maybe cases like priority queues (like DCB) or large packets
with segment offload.