[Bloat] Trying to *really* understand Linux pacing

[Dave Taht]

On Wed, Feb 7, 2024 at 7:08 AM Michael Welzl <michawe at ifi.uio.no> wrote:
>
> Whoa… and now I’m even more afraid   :-)
>
> My sincere apologies to anyone whose code I may have mis-represented!  I just tried to get it…  sorry if there are silly mistakes in there!

I think his invention of packet pacing alone, of Eric Dumazet's many
inventions, deserves him a place in the Internet Hall of Fame. It make
short buffered switches saner, allows interactive traffic to fit in
between other traffic (essentially host based FQ), and as the Linux
default, has helped power the expansion of ever more bandwidth to ever
more people without overloading the edges far more, than his fq_codel
did. I also love, that by making it just work, he thoroughly disproved
a formerly influential paper on it, circa 2004 or so.

And I do not understand the implementation either, and have been
trying to come up with ways of improving slow start by varying it. I
hope more enlightenment spreads, and your attempt to document it
improves.

https://docs.google.com/document/d/1-uXnPDcVBKmg5krkG5wYBgaA2yLSFK_kZa7xGDWc7XU/edit#heading=h.7624xn94jzf6

>
>
> > On 7 Feb 2024, at 13:05, Dave Taht <dave.taht at gmail.com> wrote:
> >
> > Dear Michael:
> >
> > Thank you for digging deeply into packet pacing, TSQ, etc. I think
> > there are some interesting new possibilities in probing (especially
> > during slow start) that could make the core idea even more effective
> > than it is. I also tend to think that attempting it in various cloudy
> > environments and virtualization schemes, and with certain drivers, the
> > side effects are not as well understood as I would like. For example,
> > AWS's nitro lacks BQL as does virtio-net.
> >
> > I think the netdev community, now cc'd, would be interested in your
> > document and explorations so far, below. I hope for more
> > enlightenment.
> >
> > On Wed, Feb 7, 2024 at 6:57 AM Michael Welzl via Bloat
> > <bloat at lists.bufferbloat.net> wrote:
> >>
> >> Dear de-bloaters of buffers,
> >> Esteemed experts of low delay and pacing!
> >>
> >> I have no longer been satisfied with high-level descriptions of how pacing works in Linux, and how it interacts with TSQ (I’ve seen some, in various papers, over the years) - but I wanted to REALLY understand it. So, I have dug through the code.
> >>
> >> I documented this experience here:
> >> https://docs.google.com/document/d/1-uXnPDcVBKmg5krkG5wYBgaA2yLSFK_kZa7xGDWc7XU/edit?usp=sharing
> >> but it has some holes and may have mistakes.
> >>
> >> Actually, my main problem is that I don’t really know what goes on when I configure a larger IW… things seem to get quite “off” there. Why? Anyone up for solving that riddle?  ;-)
> >> (see the tests I documented towards the end of the document)
> >>
> >> Generally, if someone who has their hands on files such as tcp_output.c all the time could take a look, and perhaps “fill” my holes, or improve anything that might be wrong, that would be fantastic!   I think that anyone should be allowed to comment and make suggestions in this doc.
> >>
> >> MANY thanks to whoever finds the time to take a look !
> >>
> >> Cheers,
> >> Michael
> >>
> >> _______________________________________________
> >> Bloat mailing list
> >> Bloat at lists.bufferbloat.net
> >> https://lists.bufferbloat.net/listinfo/bloat
> >
> >
> >
> > --
> > 40 years of net history, a couple songs:
> > https://www.youtube.com/watch?v=D9RGX6QFm5E
> > Dave Täht CSO, LibreQos
>


-- 
40 years of net history, a couple songs:
https://www.youtube.com/watch?v=D9RGX6QFm5E
Dave Täht CSO, LibreQos