Question: DPDK thread model

[Stephen Hemminger]

On Sun, 16 Jan 2022 16:27:11 -0500
fwefew 4t4tg <7532yahoo at gmail.com> wrote:

> I completed code to initialize an AWS ENA adapter with RX, TX queues. With
> this work in hand, DPDK creates one thread pinned to the right core as per
> the --lcores argument. So far so good.
> The DPDK documentation and example code is fairly clear here.

Look at examples/l3fwd

> 
> What's not as clear is how RX packets are handled. As far as I can tell the
> canonical way to deal with RX packets is running 'rte_eth_add_rx_callback'
> for each RXQ. This allows one to process each received packet (for a given
> RXQ) via a provided callback in the same lcore/hardware-thread that DPDK
> created for me. As such, there is no need to create additional threads.
> Correct?

rte_eth_add_rx_callback is not the droid you are looking for!
Applications use rte_eth_rx_burst to get packets. Most applications
use RSS and/or multiple queues so that each thread reads one or more
receive queues.


> Furthermore, I hope the mbufs the callback gets somehow correspond to mbufs
> associated with the RX descriptors provided to the RXQs so there's no need
> for copying packets after the NIC receives them before the callback acts on
> it. As far as I can this hope is ill-founded.. A lot of DPDK code I've seen
> allocates more mbufs per RXQ than the number of RX descriptors. To me this
> seems to imply DPDK's RXQ threads put copies of the
> received-off-the-wire-packets into a copy for delivery to app code.

You are digging in the wrong place.


> TX is less clear to me.
> 
> For TX there seems to be no way to transmit packets (burst or otherwise)
> without creating another thread --- that is, another thread beyond what
> DPDK makes for me. This other thread must at the appropriate time
> prepare mbufs and call 'rte_eth_tx_burst' on the correct TXQ. DPDK seems to
> want to keep its thread for it's own work. Yes, DPDK provides
> 'rte_eth_add_tx_callback' but that only works after the mbufs have been
> created and told to transmit, which is after the fact of creation. Putting
> this together, DPDK requires me to create new threads unlike RX. Correct?
> 
> While creating additional threads for TX is not the end of the world, I do
> not want the DPDK TX thread to copy mbufs; I want zero-copy. Here, then, I
> gather DPDK's TXQ thread takes the mbufs the helper TX thread provides in
> the 'rte_eth_tx_burst' call and provides them to the TXQS descriptors so
> they go out on the wire without copying. Is this correct?
> 
> Now, it's worth pointing out here that 'rte_eth_tx_queue_setup' unlike the
> RX equivalent does not accept a mempool.  So in addition to the above
> points, those additional TX helper threads (those which call
> rte_eth_tx_burst) will need to arrange for its own mempool. That's not hard
> to do, but I just want confirmation.

The common way to handle Transmit is to have one transmit queue per thread.
You don't want to be creating your own threads; those threads would not end up
being part of DPDK and it that creates lots of issues.