netdev - Re: Much higher CPU usage when generating UDP vs. TCP traffic

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Date:   Mon, 16 Jan 2023 10:23:37 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Ignacy Gawędzki 
        <ignacy.gawedzki@...en-communications.fr>
Cc:     netdev@...r.kernel.org
Subject: Re: Much higher CPU usage when generating UDP vs. TCP traffic

On Mon, Jan 16, 2023 at 7:07 AM Ignacy Gawędzki
<ignacy.gawedzki@...en-communications.fr> wrote:
>
> Hi,
>
> While running some network performance tests, I discovered that the
> CPU usage when generating UDP traffic is much higher than when
> generating TCP traffic.  Note that no significant difference of CPU
> usage was observed when simply forwarding UDP vs. TCP traffic.
>
> This happens on Ethernet, on Wi-Fi, even on veth links (but not on the
> loopback interface), so it seems unrelated to any specific hardware
> driver.  BTW this difference in CPU usage decreases quite notably when
> generating UDP paquets of maximum size, resulting in 64 KiB IP paquest
> getting immediately fragmented to fit a standard MTU of 1500 bytes.

Probably a large factor here is the TCP support for TSO/GSO.

There are probably mn
This article has a nice discussion of some of the main issues in UDP
vs TCP performance:
  https://www.fastly.com/blog/measuring-quic-vs-tcp-computational-efficiency

neal