Re: UDP send costs in Linux

Subodh Iyengar <subodh@fb.com> Wed, 04 April 2018 02:50 UTC

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From: Subodh Iyengar <subodh@fb.com>
To: Ian Swett <ianswett=40google.com@dmarc.ietf.org>, IETF QUIC WG <quic@ietf.org>
Subject: Re: UDP send costs in Linux
Thread-Topic: UDP send costs in Linux
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Date: Wed, 4 Apr 2018 02:50:31 +0000
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Thanks for sharing this Ian.


This definitely matches some of the observations we've seen as well in the UDP write path. Some of the other paths that we saw that added overhead was the route table lookup in linux udp stack. Connected UDP sockets did amortize that.


I'm looking forward to a smarter sendmmsg with GSO and zero copy. Is there any indication of the timeline for these patches to make it to linux? Would be happy to try any of these out to help iron out the API.


Subodh

________________________________
From: QUIC <quic-bounces@ietf.org>; on behalf of Ian Swett <ianswett=40google.com@dmarc.ietf.org>;
Sent: Tuesday, April 3, 2018 5:20:08 PM
To: IETF QUIC WG
Subject: UDP send costs in Linux

One challenge with QUIC at the moment is the increased CPU cost of sending UDP packets vs TCP payloads.  I've seen this across every platform Google has deployed QUIC on, so it's a widespread issue.

Here's an excellent presentation on what's causing the increased CPU consumption on Linux from Willem de Bruijn(UDP starts on slide 9).
http://vger.kernel.org/netconf2017_files/rx_hardening_and_udp_gso.pdf

And while you're thinking of CPU usage, it's worth looking at the presentation on timing wheel based packet pacing(which is minimum release time based) and is ideal for QUIC(and TCP for that matter): https://conferences.sigcomm.org/sigcomm/2017/files/program/ts-9-4-carousel.pdf<https://urldefense.proofpoint.com/v2/url?u=https-3A__conferences.sigcomm.org_sigcomm_2017_files_program_ts-2D9-2D4-2Dcarousel.pdf&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=h3Ju9EBS7mHtwg-wAyN7fQ&m=tgghgvFkps7jYaFNdNyZBNFf0epVxFZbOGhhybFwPiE&s=87gOGfz3S0lLbw8jy-lz3M9vPGChkmgtiJVzUxbMfvY&e=>

-Ian