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
