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

[Bev SCHWARTZ]

I am writing a bridge program.  Originally, I based my implementation on skeleton/basicfwd.c.  I next wanted to support using multi-core, so I found l2fwd.c as a simple model for tying queues to cores.  However, l2fwd.c uses rte_eth_tx_buffer.  Not understanding enough about dpdk, I switched over to using rte_eth_tx_buffer because I wrongly thought that it had to be used with multi-core.

I have changed my code back to using rte_eth_tx_burst, and that has solved my problem.  However, on very unbalanced traffic, using rte_eth_tx_buffer caused an 80% performance degradation.  That seems rather extreme for such a small change, so I was inquiring to see if people understood why.  And given this degradation, I'm surprised that l2fwd uses rte_eth_tx_buffer instead of rte_eth_tx_burst.

-Bev

________________________________________
From: Manish Kumar <manish.jangid08 at gmail.com>
Sent: Monday, July 13, 2020 2:32 AM
To: Suraj R Gupta
Cc: Bev SCHWARTZ; users at dpdk.org
Subject: [External] Re: [dpdk-users] Significant performance degradation when using tx buffers rather than rte_eth_tx_burst

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<mailto: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<mailto: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