netdev - Re: [PATCH RFC net-next 00/11] udp gso

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Date:   Fri, 31 Aug 2018 09:08:59 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH RFC net-next 00/11] udp gso

On Fri, Aug 31, 2018 at 5:09 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> On Tue, 2018-04-17 at 17:07 -0400, Willem de Bruijn wrote:
> > That said, for negotiated flows an inverse GRO feature could
> > conceivably be implemented to reduce rx stack traversal, too.
> > Though due to interleaving of packets on the wire, it aggregation
> > would be best effort, similar to TCP TSO and GRO using the
> > PSH bit as packetization signal.
>
> Reviving this old thread, before I forgot again. I have some local
> patches implementing UDP GRO in a dual way to current GSO_UDP_L4
> implementation: several datagram with the same length are aggregated
> into a single one, and the user space receive a single larger packet
> instead of multiple ones. I hope quic can leverage such scenario, but I
> really know nothing about the protocol.
>
> I measure roughly a 50% performance improvement with udpgso_bench in
> respect to UDP GSO, and ~100% using a pktgen sender, and a reduced CPU
> usage on the receiver[1]. Some additional hacking to the general GRO
> bits is required to avoid useless socket lookups for ingress UDP
> packets when UDP_GSO is not enabled.
>
> If there is interest on this topic, I can share some RFC patches
> (hopefully somewhat next week).

As Eric pointed out, QUIC reception on mobile clients over the WAN
may not see much gain. But apparently there is a non-trivial amount
of traffic the other way, to servers. Again, WAN might limit whatever
gain we get, but I do want to look into that. And there are other UDP high
throughput workloads (with or without QUIC) between servers.

If you have patches, please do share them. I actually also have a rough
patch that I did not consider ready to share yet. Based on Tom's existing
socket lookup in udp_gro_receive to detect whether a local destination
exists and whether it has set an option to support receiving coalesced
payloads (along with a cmsg to share the segment size).

Converting udp_recvmsg to split apart gso packets to make this
transparent seems to me to be too complex and not worth the effort.

If a local socket is not found in udp_gro_receive, this could also be
tentative interpreted as a non-local path (with false positives), enabling
transparent use of GRO + GSO batching on the forwarding path.