Relation between DPDK queue and descriptors

[Stephen Hemminger]

On Thu, 3 Oct 2024 11:34:36 +0300
Dmitry Kozlyuk <dmitry.kozliuk at gmail.com> wrote:

> 2024-10-03 07:37 (UTC+0000), Mikael R Carlsson:
> > Hi!
> > 
> > Thanks. 
> > 
> > According to chatgpt the descriptors are shared over all TX queues. 
> > 
> > So, in a 4 TX queue and 1024 descriptors scenario I would be able to get maximum 256 descriptors per TX queue (If I want same amount on all queues). But if I only used 1 TX queue, I would get all 1024 descriptors on that single TX queue.  
> 
> This is not so, ChatGPT errs.
> The number of advertised and configured descriptors is per queue
> (the latter is per the specific queue being configured, actually).
> You won't get more descriptors per queue if you use fewer queues.
> Note, however, that queues consume NIC resources and larger queues stress
> CPU cache, so it is not always the best to have many queues or large queues.

Also, large numbers of descriptors means the mbuf pool much be larger which can exhaust
the available huge page memory.  If you had 4 Tx queue * 1024 descriptors per queue
the Tx side could consume as much as 4K * 2K  = 8 Meg of hugepage memory.
And on Rx side same applies.