[Cerowrt-devel] SQM Question #5: Link Layer Adaptation Overheads

Dave Taht dave.taht at gmail.com
Mon Jan 6 10:03:34 EST 2014

On Jan 6, 2014 5:56 AM, "Sebastian Moeller" <moeller0 at gmx.de> wrote:
> Hi Dave, hi List,
> On Jan 6, 2014, at 04:29 , Dave Taht <dave.taht at gmail.com> wrote:
> > On Sat, Jan 4, 2014 at 10:40 AM, Fred Stratton <fredstratton at imap.cc>
> >> Link Names:
> >>
> >> For consistency, if ADSL is used as a portmanteau term, them VDSL
should be
> >> used as the equivalent for VDSL and VDSL2.
> >>
> >> CeroWRT has to decide whether it is an experimental build, or
something that
> >> will eventually be used in production, so these decisions can be made
> >> consistently.
> >
> > Well, what I was aiming for was for us to get the sqm scripts and gui
> > up to where they were better than the standard openwrt qos scripts and
> > then push them up to openwrt to where they could be more widely
> > deployed.
> >
> > Aside from being able to dynamically assign priorities in the gui, we
> > are there.  Except that nfq_codel is currently getting better results
> > than fq_codel at low bandwidths, and I'm tempted to pour all of
> > simple.qos into C.
>         Since you wore nfq_codel, what is the secret sauce here?

1) It uses a 'tighter' version of Codel than what is currently in Linux. It
doesn't work as well on longer rtts but holds down queue lengths at shorter
rtts better and responds quicker than normal codel.

This is a slightly more expensive version of codel too in that it uses two
invsqrt via newtons method to get more accurate results.

2) it rotates the flow list more like how sfq does yielding better mixing
which leads to higher survival rates for sparse flows and more balance
across all flows.

(This is a one line change to fq-codel)

At higher bandwidths (say, 50mbit) being more drr like (fqcodel) actually
tends to do better than sfqlike as bunching up some packet deliveries makes
hosts respond quicker.

3) common to all the codels in this was elimination of the maxpacket check
which mildly increases drop probability.

Compared to the orders of magnitude we already get from fq codel the sum
benefit of these fixes is in the very small percentage points. Without an
extensive testing and simulation campaign I've been reluctant to attempt
pushing them upstream. What I have mostly thought about instead was
bundling up simple.QoS into c (call it cake or broadbandeq),
Using these mods, adding in fixes for things that are hard now, like full
diffserv support and something lighter than htb.

But enotime, funding etc. Until 3 hit seeing benefit from nfqcodel was even
harder to see, and I'd like to drop out 3 and revisit the data to see if
the improvement is a chimera or not.

> >
> > As for cero's future - certainly since all the snowden revelations
> > I've been going around saying that "friends don't let friends run
> > factory firmware". I would like a stable build of sqm and cerowrt to
> > emerge, and to then go off and work on improving wifi. Regrettably
> > what seems to be happening is more backwards than forwards on the
> > former, and ramping up on the ath9k and ath10k is taking more time
> > than I'd like, and it seems likely I'll be working on those primarily
> > on another platform and only eventually pushing the results out to
> > cero, mainline kernel
> >
> > So it's still at the "keep plugging away" point for sqm, ipv6, cero in
> > general, with the stable release always just out of sight.
> >
> > Tackling the ipv6 problem is next on my agenda on cero, and getting a
> > test suite going is next on my day job.
>         Any further hints on the nature of your day job possible :)
> >
> >> I concur with your ADSL setup suggestion as default. I have been
running the
> >> Sebastian Moeller ping script overnight to calculate ADSL overhead for
> >> last several days. After several hours of curve fitting using Octave,
> >> overhead result is displayed. This novel approach works well.
> >
> > It would be nice to get to where we could autoconfigure a router using
> > tools like these with no human intervention. This includes bandwidth
> > estimation.
>         I fully agree that it would be nice. Also it ail e hard unless we
take control over the actual bottleneck link… With DSL connections, the DSL
modem knows a lot about the link properties, if the modem would be onboard
we could programmatically as about bandwidth and encapsulation; for the
more typical case with an independent modem or even modem router we have no
clear path accessing that information. With cable I have even less hope, as
will never get the modem into the router (then again DOCSIS 3.1's typically
faster speeds and mandatory pie in the modem might make the situation less
dire than on the DSL side).
>         So for ATM based links, I think we can estimate a number of
relevant parameters about encapsulation and an aggregate up- and
down-bandwidth measurement (which alas is not too helpful unless we know
the degree of asymmetry or one of the bandwidth, in both cases we are
likely to know everything a priori anyway). But the current prototype code
I have is really slow (3 hours measurement time with otherwise quiet home
network; ~20 processing) and memory demanding (the ascii ping traces take
up ~260MB) so will not be able to run on the router (also the current
implementation in matlab/octave does not lend itself well for an embedded
> Best Regards
>         Sebastian
> >
> >> The overhead for the particular setup I use was 40 for PPPoE, and 10
> >> PPPoA.
> >>
> >> The default you suggest is a suitable starting point, I suggest.
> >>
> >>
> >> On 04/01/14 18:16, Rich Brown wrote:
> >>>
> >>> QUESTION #5: I still don’t have any great answers for the Link Layer
> >>> Adaptation overhead descriptions and recommendations. In an earlier
> >>> (see
> >>>
> >>> and following messages), Fred Stratton described the overheads
carried by
> >>> various options, and Sebastian Moeller also gave some useful advice.
> >>>
> >>> After looking at the options, I despair of giving people a clear
> >>> recommendation that would be optimal for their equipment.
Consequently, I
> >>> believe the best we can do is come up with “good enough”
> >>> that are not wrong, and still give decent performance.
> >>>
> >>> In this spirit, I have changed Draft #3 of the “Setting up SQM” page
> >>> reflect this understanding. See
> >>>
> >>>
> >>>        ADSL/ATM link: Choose “ADSL/ATM", and set Per Packet Overhead
> >>> 40
> >>>        VDSL2 link: Choose “VDSL”, and set Per Packet Overhead to 8
> >>>        Other kind of link (e.g., Cable, Fiber, Ethernet, other not
> >>> listed): Choose “None (default)”, and set Per Packet Overhead to 0
> >>>
> >>> NB: I have changed the first menu choice to “ADSL/ATM” and the second
> >>> “VDSL” in the description. I would ask that we change to GUI to
> >>> those names as well. This makes it far easier/less confusing to talk
> >>> the options.
> >>>
> >>> As always, I welcome help in setting out clear recommendations that
> >>> well for the vast majority of people who try CeroWrt. Thanks.
> >>>
> >>> Rich
> >>> _______________________________________________
> >>> Cerowrt-devel mailing list
> >>> Cerowrt-devel at lists.bufferbloat.net
> >>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >>
> >>
> >> _______________________________________________
> >> Cerowrt-devel mailing list
> >> Cerowrt-devel at lists.bufferbloat.net
> >> https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >
> >
> >
> > --
> > Dave Täht
> >
> > Fixing bufferbloat with cerowrt:
> > _______________________________________________
> > Cerowrt-devel mailing list
> > Cerowrt-devel at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/cerowrt-devel
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