Hi there !

I'm using a couple of Pi 3B+, most of then running PiOS but one of them
running Fedora 33. On all the PiOS ones, a "wget" download is going up
to roughly 30 MB/s, which fits to my Internet downlink and is near the
maximum interface speed of this box. But on the Fedora machine, the
exact same command gets stuck at a fraction of this, even though it's
connected to the same switch as one of the PiOS machines.

The Fedora box doesn't have a high load or something else which might be
an obvious reason for this effect. It's acting as a DHCP and DNS server.
Is Fedora's network stack that bad? Maybe I'll try a newer kernel
version (currently 5.11.15 whereas 5.14.18 is available).

Merry X-mas!

fw
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Frank Winkler <usenet@f.winkler-ka.de> wrote:

Hi there !

I'm using a couple of Pi 3B+, most of then running PiOS but one of them running Fedora 33. On all the PiOS ones, a "wget" download is going up
to roughly 30 MB/s, which fits to my Internet downlink and is near the maximum interface speed of this box. But on the Fedora machine, the
exact same command gets stuck at a fraction of this, even though it's connected to the same switch as one of the PiOS machines.

The Fedora box doesn't have a high load or something else which might be
an obvious reason for this effect. It's acting as a DHCP and DNS server.
Is Fedora's network stack that bad? Maybe I'll try a newer kernel
version (currently 5.11.15 whereas 5.14.18 is available).

I'm not familiar with the problem (or Fedora, or the Pi 3B+!) but
it sounds like yours is connected via the onboard Ethernet
interface, which talks to the CPU as a USB 2.0 device. This makes
the USB driver a likely point of difference between RPi OS and
Fedora, because RPi OS by default uses the dwc-otg USB driver which
is optimised for the Pi, including using the FIQ "Fast" interrupt
which bypasses some of the kernel's usual interrupt handling
process. That driver hadn't been adopted into the official Linux
kernel, so most other Linux distros for the Pi use the dwc2 driver
instead, which is officially part of Linux.

RPi OS can be told to load the dwc2 driver instead of dwc-otg by
adding this line to the config.txt file in the boot/FAT partition: dtoverlay=dwc2

Maybe try that and see if network performance then becomes the same
as with Fedora. In which case I'm not sure if there's an easy way
to get dwc-otg into Fedora - it might require manually swapping
over the kernel and modules from a RPi OS release with a similar
kernel version, which would likely bring up a whole lot of
problems.


Another description of the driver difference which I found when
confirming that I wasn't mis-remembering: https://revolutionpi.com/forum/viewtopic.php?t=1563#p6083

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Frank Winkler <usenet@f.winkler-ka.de> wrote:

Hi there !

I'm using a couple of Pi 3B+, most of then running PiOS but one of them running Fedora 33. On all the PiOS ones, a "wget" download is going up
to roughly 30 MB/s, which fits to my Internet downlink and is near the maximum interface speed of this box. But on the Fedora machine, the
exact same command gets stuck at a fraction of this, even though it's connected to the same switch as one of the PiOS machines.

Bad network cable causing one to be stuck at 100Mbps?

Try swapping the cables over, and separately swapping the SD cards.
See if you can nail down if it's the hardware or the software.

The Fedora box doesn't have a high load or something else which might be
an obvious reason for this effect. It's acting as a DHCP and DNS server.
Is Fedora's network stack that bad? Maybe I'll try a newer kernel
version (currently 5.11.15 whereas 5.14.18 is available).

There shouldn't be a material difference.

That assumes both are 3B+ (which has true gigabit ethernet) not 3B (which
has ethernet hung off USB 2 as mentioned in another post). Swapping parts around would tell you this.

Theo
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Theo <theom+news@chiark.greenend.org.uk> wrote:

That assumes both are 3B+ (which has true gigabit ethernet) not 3B (which
has ethernet hung off USB 2 as mentioned in another post).

I'll admit I'm continually confused by all the different Pi models,
but I did try to check this and according to this the Pi 3B+'s
Ethernet is still connected via USB, it just uses its own "USB
channel" (which I assume means no onboard USB hub in-between).

https://magpi.raspberrypi.com/articles/raspberry-pi-3bplus-specs-benchmarks

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Computer Nerd Kev <not@telling.you.invalid> wrote:

Theo <theom+news@chiark.greenend.org.uk> wrote:

That assumes both are 3B+ (which has true gigabit ethernet) not 3B (which has ethernet hung off USB 2 as mentioned in another post).

I'll admit I'm continually confused by all the different Pi models,
but I did try to check this and according to this the Pi 3B+'s
Ethernet is still connected via USB, it just uses its own "USB
channel" (which I assume means no onboard USB hub in-between).

https://magpi.raspberrypi.com/articles/raspberry-pi-3bplus-specs-benchmarks

You're right, I misremembered.

The 3B+ has 'gigabit ethernet' but it's only so far as running the ethernet link at gigabit - it's still behind a single 480Mbps USB2 link from the SoC. That link is still shared with the USB ports, whose hub is on the same chip
as the ethernet (as it is on all ethernet Pis before 4). In practice you
got about 300Mbps.

On prior Pis like 3B the ethernet link was only 100Mbps.

The 4 has a proper RGMII link to a Broadcom ethernet PHY chip, which can run true gigabit without using any USB channels.

Your link doesn't say it's not shared USB on the 3B+ - if you used it for a
NAS with a USB disc you'd have to transfer everything twice. It's just that previous Pis were limited to 100M ethernet so the remaining USB bandwidth
would have been wasted in that instance.

Theo
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