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LPC2018 - Optimizing UDP for Content Delivery with GSO, Pacing and Zerocopy

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Published on Dec 3, 2018

url: https://linuxplumbersconf.org/event/2...
speaker: Willem de Bruijn (Google)


UDP is a popular foundation for new protocols. It is available across
operating systems without superuser privileges and widely supported
by middleboxes. Shipping protocols in userspace on top of
a robust UDP stack allows for rapid deployment, experimentation
and innovation of network protocols.

But implementing protocols in userspace has limitations. The
environment lacks access to features like high resolution timers
and hardware offload. Transport cost can be high. Cycle count of
transferring large payloads with UDP can be up to 3x that of TCP.

In this talk we present recent and ongoing work, both by the authors
and others, at improving UDP for content delivery.

UDP Segmentation offload amortizes transmit stack traversal by
sending as many as 64 segments as one large fused large packet.
The kernel passes this through the stack as one datagram, then
splits it into multiple packets and replicates their network and
transport headers just before handing to the network device.

Some devices can offload segmentation for exact multiples of
segment size. We discuss how partial GSO support combines the
best of software and hardware offload and evaluate the benefits of
segmentation offload over standard UDP.

With these large buffers, MSG_ZEROCOPY becomes effective at
removing the cost of copying in sendmsg, often the largest
single line item in these workloads. We extend this to UDP and
evaluate it on top of GSO.

Bursting too many segments at once can cause drops and retransmits.
SO_TXTIME adds a release time interface which allows offloading of
pacing to the kernel, where it is both more accurate and cheaper.
We will look at this interface and how it is supported by queuing
disciplines and hardware devices.

Finally, we look at how these transmit savings can be extended to
the forwarding and receive paths through the complement of GSO,
GRO, and local delivery of fused packets.

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