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

[Suraj R Gupta]

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