From: Andrew Bainbridge <andbain@microsoft.com>
To: "users@dpdk.org" <users@dpdk.org>
Subject: [dpdk-users] Low level understanding of mbufs needed.
Date: Mon, 4 Sep 2017 10:44:02 +0000 [thread overview]
Message-ID: <HE1PR83MB0220FF34AE789E0245EA01ADAE910@HE1PR83MB0220.EURPRD83.prod.outlook.com> (raw)
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
next reply other threads:[~2017-09-04 10:44 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-04 10:44 Andrew Bainbridge [this message]
2017-09-04 11:03 ` Andriy Berestovskyy
2017-09-07 1:47 ` Stephen Hemminger
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