Relation between DPDK queue and descriptors

[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.

  / Mikael




-----Original Message-----
From: Pathak, Pravin <pravin.pathak at intel.com> 
Sent: Wednesday, October 2, 2024 6:07 PM
To: Mikael R Carlsson <mikael.r.carlsson at tietoevry.com>; Stephen Hemminger <stephen at networkplumber.org>
Cc: users at dpdk.org
Subject: RE: Relation between DPDK queue and descriptors

[Du f?r inte e-post ofta fr?n pravin.pathak at intel.com. L?s om varf?r det h?r ?r viktigt p? https://aka.ms/LearnAboutSenderIdentification ]

Hi Mikael -
ChatGpt provides a good description of the relation between these two. If you ask ChatGPT to do a deep dive, it will provide a good bit of programming and optimization details.
Regards
Pravin


> -----Original Message-----
> From: Mikael R Carlsson <mikael.r.carlsson at tietoevry.com>
> Sent: Wednesday, October 2, 2024 12:05 PM
> To: Stephen Hemminger <stephen at networkplumber.org>
> Cc: users at dpdk.org
> Subject: RE: Relation between DPDK queue and descriptors
>
> Hi!
>
> Thanks for the response.
>
> I think I get the descriptor part, but what is the relation to queues? 
> If the hardware supports 1024 descriptors and I need 6 queues, do I 
> have 1024 descriptors on each TX queue?
>
>   / Mikael
>
>
> -----Original Message-----
> From: Stephen Hemminger <stephen at networkplumber.org>
> Sent: Wednesday, October 2, 2024 5:29 PM
> To: Mikael R Carlsson <mikael.r.carlsson at tietoevry.com>
> Cc: users at dpdk.org
> Subject: Re: Relation between DPDK queue and descriptors
>
> 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.