Relation between DPDK queue and descriptors

[Stephen Hemminger]

On Wed, 2 Oct 2024 15:21:45 +0000
Mikael R Carlsson <mikael.r.carlsson at tietoevry.com> wrote:

> Hi experts!
> 
> I have a hard time to finds a good documentation about the relation between DPDK TX queue and descriptors.
> 
> Queue as in rte_eth_tx_queue_setup
> Descriptor as in rte_eth_dev_adjust_nb_rx_rx_desc
> 
> We suspect we run out of descriptors in TX path, we are not sure here. We use more than one TX queue. Will we get more descriptors if we only use one single TX queue? Does anyone know if there is some good documentation regarding the TX queue and the descriptors?
> 
>   / Mikael
> 

A typical driver has a hardware ring buffer between the driver and the hardware.
One ring for transmit, and another for receive.
The entries in the ring are hardware specific data structure called descriptors.
Each descriptor usually has physical memory address, size, and flags.

The number of Rx descriptors determines the number of unread frames the
driver can hold. Too small, and you risk dropping packets; too large and
under stress load the driver can end up buffering excessively causing latency (bufferbloat).
Similar on Tx but less of a problem because typically the network is faster
than the application can send packets.