[Bug 280386] if_bridge throws output errors under load

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280386

Cheng Cui <cc@FreeBSD.org> changed:

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--- Comment #25 from Cheng Cui <cc@FreeBSD.org> ---
(In reply to Kevin Bowling from comment #22)
(In reply to pascal.guitierrez from comment #23)

I had a similar experience last year when I was debugging a ENOBUFS error
returned to TCP on using bce NICs. But I am not sure if you can find a similar
solution.

<snap from 2023>
Turns out the root cause is the default NIC send queue length is too small. The
enobufs error came from the _IF_QFULL check in ifq.h.

However, tuning "sysctl net.link.ifqmaxlen" directly does not work. There is a
per NIC interface setup in the driver to setup device tx/rx queues. I have to
increase the tx queue "ifq_maxlen" from the device sysctl "hw.bce.tx_pages".
After tuning that, I can achieve a stable 1Gbps x 100ms delay BDP.
</snap from 2023>

Talking about review D4295, it reminds me of `Linux has some work like TCP
small queue at the sender side.`

Talking about workaround, you may also test the following two patches I
prepared in stable/14 branch as a workaround of improving TCP performance in
congestion control.

https://reviews.freebsd.org/D47218     << apply this patch firstly
https://reviews.freebsd.org/D47213     << apply this patch secondly

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