DPDK and DMA

[fwefew 4t4tg]

Thank you for taking time to provide a nice reply. The upshot here is that
DPDK
already uses DMA in a smart way to move packet data into TXQs. I presume the
reverse also happens: NIC uses DMA to move packets out of its HW RXQs into
the host machine's memory using the mempool associated with it.



On Wed, Jan 11, 2023 at 6:26 AM Dmitry Kozlyuk <dmitry.kozliuk at gmail.com>
wrote:

> 2023-01-08 16:05 (UTC-0500), fwefew 4t4tg:
> > Consider a valid DPDK TXQ with its mempool of rte_mbufs. Application code
> > will allocate a mbuf from the pool and prepare it with headers, data, and
> > so on.
> >
> > When the mbuf(s) are enqueued to the NIC with rte_eth_tx_burst() does
> DPDK
> > DMA the memory into the NIC? Is this an optimization worth considering?
>
> DPDK is SW running on CPU.
> DMA is a way for HW to access RAM bypassing CPU (thus it is "direct").
>
> What happens in rte_eth_tx_burst():
> DPDK fills the packet descriptor and requests the NIC to send the packet.
> The NIC subsequently and asynchronously uses DMA to read the packet data.
>
> Regarding optimizations:
> 1. Even if the NIC has some internal buffer where it stores packet data
> before sending it to the wire, those buffers are not usually exposed.
> 2. If the NIC has on-board memory to store packet data,
> this would be implemented by a mempool driver working with such memory.
>
> > DPDK provides a DMA example here:
> > http://doc.dpdk.org/api/examples_2dma_2dmafwd_8c-example.html
> >
> > Now, to be fair, ultimately whether or not DMA helps must be evidenced
> by a
> > benchmark. Still, is there any serious reason to make mempools and its
> > bufs DMA into and out of the NIC?
>
> DMA devices in DPDK allow the CPU to initiate an operation on RAM
> that will be performed asynchronously by some special HW.
> For example, instead of memset() DPDK can tell DMA device
> to zero a memory block and avoid spending CPU cycles
> (but CPU will need to ensure zeroing completion later).
>
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