Re: [Cake] making split_gso_threshold configurable

[Dave Taht <dave.taht@gmail.com>]

On Wed, Jul 25, 2018 at 3:07 AM Sebastian Moeller <moeller0@gmx.de> wrote:
>
> Hi Toke,
>
>
> > On Jul 25, 2018, at 12:02, Toke Høiland-Jørgensen <toke@toke.dk> wrote:
> >
> > Dave Taht <dave.taht@gmail.com> writes:
> >
> >> I really wanted cake to always optimize for low latency. I wanted it
> >> to "just work" at line rate on dsl, on 100mbit, even 10mbit ethernet,
> >> to work against pause frames, etc, without configuration. I wanted to
> >> defeat drivers like the mvneta that can do 64k of software GRO
> >> automagically.
> >
> > So I think that Eric's point was that the GSO logic itself should
> > throttle back and not build so big packets when the link rate is lower.
> > Might be worth it to check if this is actually the case, and if not to
> > get GSO fixed rather than disabling it.

It doesn't and I think I should just put forth the patch and face the
howling from the 40gbit guys.

> >
> > Also, are there actually any modems that will negotiate ethernet line
> > rates less than a gigabit?
>
>         Yes, there are XDSL Modem(routers) that only have fast-ethernet ports. Not sure how rare these are though.

And there are also many devices that ecert hardware flow control.

> Best Regards
>         Sebastian
>
>
> >
> >> Along the way to mainlining it, cake's behavior got changed, to scale
> >> well to 50Gbit by allowing GSO above a 1gbit shaped rate, and *always
> >> allowing GSO at line rate*.
> >>
> >>        if (q->rate_bps && q->rate_bps <= CAKE_SPLIT_GSO_THRESHOLD)
> >>                q->rate_flags |= CAKE_FLAG_SPLIT_GSO;
> >>        else
> >>                q->rate_flags &= ~CAKE_FLAG_SPLIT_GSO;
> >>
> >> I don't care about 50gbit. I'm delighted, even astonished, it does
> >> 50Gbit. I want it to do 1mbit with 13ms interpacket latency when hit
> >> by an IW10 burst from somewhere else, not 130, with hardware pause
> >> frames in use on the dsl modem or elsewhere.
> >
> > Getting pause frames to work properly in this setup kinda requires the
> > buffering in the modem to be reasonable in the first place...
> >
> >> so, I'd like to A) default to always split shaped or unshaped. And B)
> >> expose the split threshold to be configured via userspace. This latter
> >> option would give a tunable for those struggling a bit for cpu to play
> >> with, also.
> >
> > I very much doubt that we can get upstream to accept split as the
> > default. It may be possible to get it to be user tunable. I don't think
> > we should deviate from upstream unless we absolutely have to (that was
> > kinda the whole point of the upstreaming exercise), and I'm not
> > convinced that this is a good enough reason...
> >
> > -Toke
> > _______________________________________________
> > Cake mailing list
> > Cake@lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/cake
>


-- 

Dave Täht
CEO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-669-226-2619