netdev - Re: [PATCH] r8169: add support for 16K jumbo frames on RTL8125B

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Message-ID: <CALjTZva9+ufCR5+QhJXL+7CHDRJVLQqb4uPwumEO5BqssGKPMw@mail.gmail.com>
Date: Sat, 1 Mar 2025 11:45:37 +0000
From: Rui Salvaterra <rsalvaterra@...il.com>
To: Heiner Kallweit <hkallweit1@...il.com>
Cc: nic_swsd@...ltek.com, netdev@...r.kernel.org
Subject: Re: [PATCH] r8169: add support for 16K jumbo frames on RTL8125B

Hi, Heiner,

On Fri, 28 Feb 2025 at 20:22, Heiner Kallweit <hkallweit1@...il.com> wrote:
>
> This has been proposed and discussed before. Decision was to not increase
> the max jumbo packet size, as vendor drivers r8125/r8126 also support max 9k.

I did a cursory search around the mailing list, but didn't find
anything specific. Maybe I didn't look hard enough. However…

> And in general it's not clear whether you would gain anything from jumbo packets,
> because hw TSO and c'summing aren't supported for jumbo packets.

… I actually have numbers to justify it. For my use case, jumbo frames
make a *huge* difference. I have an Atom 330-based file server, this
CPU is too slow to saturate the link with a MTU of 1500 bytes. The
situation, however, changes dramatically when I use jumbo frames. Case
in point…


MTU = 1500 bytes:

Accepted connection from 192.168.17.20, port 55514
[  5] local 192.168.17.16 port 5201 connected to 192.168.17.20 port 55524
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   241 MBytes  2.02 Gbits/sec
[  5]   1.00-2.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   2.00-3.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   3.00-4.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   4.00-5.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   5.00-6.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   6.00-7.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   7.00-8.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   8.00-9.00   sec   242 MBytes  2.03 Gbits/sec
[  5]   9.00-10.00  sec   242 MBytes  2.03 Gbits/sec
[  5]  10.00-10.00  sec   128 KBytes  1.27 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec  2.36 GBytes  2.03 Gbits/sec                  receiver


MTU = 9000 bytes:

Accepted connection from 192.168.17.20, port 53474
[  5] local 192.168.17.16 port 5201 connected to 192.168.17.20 port 53490
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   1.00-2.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   2.00-3.00   sec   294 MBytes  2.47 Gbits/sec
[  5]   3.00-4.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   4.00-5.00   sec   294 MBytes  2.47 Gbits/sec
[  5]   5.00-6.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   6.00-7.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   7.00-8.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   8.00-9.00   sec   295 MBytes  2.47 Gbits/sec
[  5]   9.00-10.00  sec   295 MBytes  2.47 Gbits/sec
[  5]  10.00-10.00  sec   384 KBytes  2.38 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec  2.88 GBytes  2.47 Gbits/sec                  receiver


MTU = 12000 bytes (with my patch):

Accepted connection from 192.168.17.20, port 59378
[  5] local 192.168.17.16 port 5201 connected to 192.168.17.20 port 59388
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   1.00-2.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   2.00-3.00   sec   295 MBytes  2.48 Gbits/sec
[  5]   3.00-4.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   4.00-5.00   sec   295 MBytes  2.48 Gbits/sec
[  5]   5.00-6.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   6.00-7.00   sec   295 MBytes  2.48 Gbits/sec
[  5]   7.00-8.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   8.00-9.00   sec   296 MBytes  2.48 Gbits/sec
[  5]   9.00-10.00  sec   294 MBytes  2.47 Gbits/sec
[  5]  10.00-10.00  sec   512 KBytes  2.49 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec  2.89 GBytes  2.48 Gbits/sec                  receiver


This demonstrates that the bottleneck is in the frame processing. With
a larger frame size, the number of checksum calculations is also
lower, for the same amount of payload data, and the CPU is able to
handle them.


Kind regards,

Rui Salvaterra