[dpdk-users] Significant performance degradation when using tx buffers rather than rte_eth_tx_burst

[Manish Kumar]

I agree with Suraj on the same. @Bev : Were you trying to use
rte_eth_tx_buffer function as part of just an experiment ? As per your
email you already got performance with the rte_eth_tx_burst function.

Regards
Manish

On Wed, Jul 8, 2020 at 1:42 PM Suraj R Gupta <surajrgupta at iith.ac.in> wrote:

> Hi bev,
> If my understanding is right,   rte_eth_tx_burst transmits output packets
> immediately with a specified number of packets.
> While,  'rte_eth_tx_buffer' buffers the packet in the queue of the port,
> the packets would be transmitted only when buffer is or
> rte_eth_tx_buffer_flush is called.
> Since you are buffering packets one by one and then you are calling flush,
> this may have contributed to the delay.
> Thanks and Regards
> Suraj R Gupta
>
>
> On Wed, Jul 8, 2020 at 10:53 PM Bev SCHWARTZ <bev.schwartz at raytheon.com>
> wrote:
>
> > I am writing a bridge using DPDK, where I have traffic read from one port
> > transmitted to the other.  Here is the core of the program, based on
> > basicfwd.c.
> >
> > while (!force_quit) {
> >     nb_rx = rte_eth_rx_burst(rx_port, rx_queue, bufs, BURST_SIZE);
> >     for (i = 0; i < nb_rx; i++) {
> >         /* inspect packet */
> >     }
> >     nb_tx = rte_eth_tx_burst(tx_port, tx_queue, bufs, nb_rx);
> >     for (i = nb_tx; i < nb_rx; i++) {
> >         rte_pktmbuf_free(bufs[i]);
> >     }
> > }
> >
> > (A bunch of error checking and such left out for brevity.)
> >
> > This worked great, I got bandwidth equivalent to using a Linux Bridge.
> >
> > I then tried using tx buffers instead.  (Initialization code left out for
> > brevity.)  Here is the new loop.
> >
> > while (!force_quit) {
> >     nb_rx = rte_eth_rx_burst(rx_port, rx_queue, bufs, BURST_SIZE);
> >     for (i = 0; i < nb_rx; i++) {
> >         /* inspect packet */
> >         rte_eth_tx_buffer(tx_port, tx_queue, tx_buffer, bufs[i]);
> >     }
> >     rte_eth_tx_buffer_flush(tx_port, tx_queue, tx_buffer);
> > }
> >
> > (Once again, error checking left out for brevity.)
> >
> > I am running this on 8 cores, each core has its own loop.  (tx_buffer is
> > created for each core.)
> >
> > If I have well balanced traffic across the cores, then my performance
> goes
> > down, about 5% or so.  If I have unbalanced traffic such as all traffic
> > coming from a single flow, my performance goes down 80% from about 10 gbs
> > to 2gbs.
> >
> > I want to stress that the ONLY thing that changed in this code is
> changing
> > how I transmit packets.  Everything else is the same.
> >
> > Any idea why this would cause such a degradation in bit rate?
> >
> > -Bev
>
>
>
> --
> Thanks and Regards
> Suraj R Gupta
>


-- 
Thanks
Manish Kumar