[dpdk-users] Low level understanding of mbufs needed.

Andriy Berestovskyy aber at semihalf.com
Mon Sep 4 13:03:53 CEST 2017

Hey Andrew,
Please note that each buffer has a headroom and a tailroom, so
basically you might just encapsulate and decapsulate mbufs in-place,
just modifying the headers and the adjusting the mbuf headroom.

For general description please have a look here:

The function you might want to use is rte_pktmbuf_adj():

and rte_pktmbuf_prepend():


On Mon, Sep 4, 2017 at 12:44 PM, Andrew Bainbridge
<andbain at microsoft.com> wrote:
> I'm a newbie. I want to learn more about to use mbufs to achieve the best throughput. My application is something like a VPN server. In pseudo code:
> while 1:
>     pkt = recv()
>     if pkt.ip.daddr == CLIENT:
>         new_pkt = encap(pkt)
>     else:
>         new_pkt = decap(pkt)
>     send(new_pkt)
> Where encap() prepends an IP and UDP header, and decap() does the opposite.
> Most of each packet I send is the same as one I just received. Is it possible to do the send without having to allocate a new mbuf and memcpy into it?
> I want to learn more about how the system works at the low level.
> My guess of how it works is that the NIC reads in a packet from the Ethernet cable and writes it into its on-chip SRAM. Once it has enough data buffered, or enough time has elapsed it does a PCIe write request to copy the data into system RAM. The simplest scheme would be to have a single large circular buffer in system RAM and for the packets to be written nose-to-tail into that buffer.
> Does DPDK do that? I guess not. I guess the supported cards all support scatter/gather, which AFAICT means the NICs are smart enough to understand an array of pointers to buffers.
> So what then? I have many 1500 byte buffers allocated, and I give the NIC an array of pointers to those buffers. The NIC then "scatters" the input stream into these buffers, one packet per buffer.
> I guess the best scheme for my application would be if I could tell the NIC to always leave 30 bytes or so of headroom on each packet, so that I can prepend the extra headers in the encap case. Can I request that when I configure the mbufs?
> If you can point me to some kind of tutorial or blog post that covers this area, that would also be helpful.
> Thanks,
> Andrew

Andriy Berestovskyy

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