Re: No KERNCONFs w/ RSS?

W dniu 20.10.2025 o 19:48, Ravi Pokala pisze:
> Hi folks,
>
> I remember watching support for Receive-Side Scaling going into the source tree years ago. AFAIK, it is a fairly mature feature at this point.
>
> I was surprised to see that it is not included in any KERNCONF, or even in NOTES so it is part of LINT. Is there a particular reason why it's not at least in NOTES?
>
> Thanks,
>
> Ravi (rpokala@)
>
When I first tried to learn about RSS implementation a few years ago, it 
seemed like a mystery - almost a "holy grail". But as I investigated 
further, step by step, I realized that hardly anyone is actually using 
it. The code still exists here and there, but it’s rarely built. People 
seem either cautious or unconvinced about any real advantages of these 
code samples hidden under #ifdefs. At times, RSS feels like a kind of 
"snake oil" among kernel options.

That said, the code is still buildable. One genuinely beneficial feature 
for me is the change in ECMP routing - the way hashing, tuples, etc, are 
calculated differs depending on whether "options RSS" is enabled. When 
RSS is used, the paths are just more predictable for UDP. I have not 
really read or studied the code, but rather looked how packets are 
forwarded. ECMP routing with FreeBSD is quite a new feature, introduced 
by melifaro@ with netlink(4). People rarely use FreeBSD as a router 
nowadays, so the feedback is limited and very narrow.

Another, perhaps more subjective, observation is that when a BGP session 
is established and the router is pulling the full-view routing table - 
or when it’s stressed by a high packet-per-second load in a congested 
network - performance seems better with both "options RSS" and 
net.isr.dispatch="deferred" enabled. In these cases, routing feels 
faster, packet latency is lower, and the improvement is noticeable, for 
example, in the audio quality of VoIP calls, I had not taken any 
measures though.

Good luck with adding "options RSS" to one of the standard kernel configs!

Cheers!

Marek