[Ecn-sane] A brick wall threshold for SCE_threshold

[Dave Taht]

while I appreciated the experiments with making the assertion of
SCE_threshold spread out,
and not a brick wall, to me it seems best to coalesce on a brick wall
for it at a fixed period
of time the operator considers "queuing".

Does the SCE team agree with this approach?

I note, in trying to wrap my head around this as to how to make it
work at all, for wifi,
that I was kind of stuck on it happening after 1-2 txops, as that was
the minimum queue depth
that can be achieved today with today's hw. However, that is a minimum
!250us, and a maximum of
over 10ms, and that doesn't count at all the arbitratraton delay
potential from many stations
which can stretch out to hundreds of ms.

Nowadays I think about it as
merely being a fixed amount of time, not txops, in the 2.5ms range. I
do not like jitter and interference inducing long txops in the first
place, and present day 802.11ac can pack so much data into a single
aggregate, that finding ways to get rid of longer txops in general has
long been on my mind so as wifi could
multiplex better over many more stations.

as for LTE, gawd knows.

Still, that multiplexing over stations takes a LONG time, and perhaps
it would be better to
apply a delta from the last station service time to the sce_threshold
time, before considering it
too much queuing.

On Tue, Mar 9, 2021 at 7:28 AM Dave Taht <dave.taht at gmail.com> wrote:
>
> I have basically hoped to find some sysadmin out there with battle
> experience with a real dctcp deployment (such as the one at facebook)
> that could share his or her
> insights as to this debate.
>
> Most of the public testing with BBRv2 has instead been with a brick
> wall setting to CE_threshold, the published data on it was a 260us,
> which was about as low as raw iron linux can get. I tend to favor the
> brick wall approach over anything more complex for the AQM component
> of a SCE architecture, and to modify the transports to suit this
> assumption.
>
> On Tue, Mar 9, 2021 at 7:19 AM Rodney W. Grimes <4bone at gndrsh.dnsmgr.net> wrote:
> >
> > > I am of course much happier interacting here than over on tsvwg.
> > >
> > > I would like very much
> > >
> > > A large scale bit twiddling test from the larger providers of fq_codel
> > > and cake based hw and middleboxes.
> > >
> > > extensive testing on lte and wifi transports. even emulated.
> > > the sce patches polished up and submitted for review to netdev as what
> > > we call there, an RFC
> > > the ability to test both SCE and L4S on openwrt (backport thereof)
> > >
> > > I know there's been a lot of work on other qdiscs than cake, but
> > > haven't paid much attention. netdev review would be nice.
> > >
> > > A simple internet-wide test of say bittorrent, or using an adserver
> > > method like what apnic uses.
> > >
> > > Most of all I'd really like someone to take a stab at RFC3168 support
> > > for BBR. And by that, I don't mean a reduction by half, but to
> > > institute an RTT probe phase. A fixed rate reduction per RTT is simply
> > > not
> > > going to work IMHO, period, on lte and wifi. I'm sure dave miller
> > > would accept such a patch, and this would also lead towards better ecn
> > > support across the internet in general... at least from the
> > > perspective of the one provider I have an in with, dropbox.
> > >
> > > SCE support for BBRv2 would be nice also.
> > >
> > > And I know, a pony, all sparkly and stuff. I find it very difficult to
> > > summon the cojones to do a drop of work in this area, and I keep
> > > cheering you on - especially on the bug fixing and wonderful scripts
> > > and data you've provided so far, and I do wish I could find some way
> > > to help that didn't cause so much ptsd in me.
> > >
> > > I wish I merely knew more about how dctp was configured in the data
> > > center. So far as *I* know its on dedicated switches mostly. I would
> > > vastly prefer a dscp codepoint for this, also.
> >
> > If you want I can take a discussion on how DCTCP is configured in
> > data center switches in a side channel.  Its basically using a
> > drop function that has a starting queue depth, and slope that
> > as you go higher on the slope your propability of marking
> > increases.  The 40 gbit experiments we did with HPE modified
> > that part of the switch asic code to do SCE marks starting
> > at 1% mark probability for 1% Queue depth, up to 100% marking
> > at 100% Queue depth.  Sadly that was a slight mistake on
> > my part, and I thought we would get a chance to iterate
> > and retune these, I just pulled those values out as a WOG
> > and gave them to HPE to let them get on with setting it up.
> >
> > For most practical purposes an SCE mark and a DCTCP CE
> > mark convey very similiar, if not the same information,
> > as does a CE mark in L4S.
> >
> > Usually tunning of these queue depth and slope values involes
> > data collection and several iterations of experiments.
> >
> > TOR vs SPINE switches are usually configured with different values.
> >
> > I believe there is even some work by someone that does data
> > collection across the whole datacenter and tries to adjust
> > these automagically and dynamically.
> >
> > >
> > > On Mon, Mar 8, 2021 at 4:36 PM Pete Heist <pete at heistp.net> wrote:
> > > >
> > > > Sorry for reviving an old thread as I haven't been on this list in a
> > > > while:
> > > >
> > > > > > SCE proposes to use ect(1) as an indicator of some congestion and
> > > > does
> > > > > > not explictly
> > > > > > require a dscp codepoint in a FQ'd implementation.
> > > > >       Pretty much. I do think that a demonstration using an
> > > > > additional DSCP to create a similar HOV lane for SCE would have gone
> > > > > miles in convincing people in the WG that L4S might really not be as
> > > > > swell as its proponents argue, IMHO it won the day more with its
> > > > > attractive promise of low latency for all instead of what it
> > > > delivers.
> > > >
> > > > On that, I don't think any of us knows how things will end up or how
> > > > long it will take to get there...
> > > >
> > > > I do agree that the interim meeting leading up to the codepoint
> > > > decision could have gone better. Everything went great until it came to
> > > > how to deploy SCE in a small number of queues. We had dismissed the
> > > > idea of using DSCP, because we thought it would be panned for its poor
> > > > traversal over the Internet. That may still have been the case, but it
> > > > also may have worked if sold right. We thought that AF alone might be
> > > > enough to get past that part, but it wasn't.
> > > >
> > > > We already implemented a two-queue design that uses DSCP, but either
> > > > there wasn't much interest, or we didn't sell it enough. Plus, for
> > > > those who demand a two queue solution that requires no flow awareness
> > > > at all, DSCP alone may not get you there, because you still need some
> > > > reasonably fair way to schedule the two queues. So that might have been
> > > > the next line of criticism. Scheduling in proportion to the number of
> > > > flows each queue contains is one effective way to do that, but that
> > > > requires at least some concept of a flow. Perhaps there's another way
> > > > that doesn't introduce too much RTT unfairness, but I'm not sure.
> > > >
> > > > In our defense, there was already a lot of support built up for L4S,
> > > > and stepping in front of that was like stepping in front of a freight
> > > > train no matter what we did. I think we've made a decent argument in
> > > > the most recent version of the SCE draft that ECN is a "network
> > > > feature" which carries higher risks than drop-based signaling, and
> > > > warrants the requirement for unresponsive flow mitigation, for
> > > > starters. That of course requires some level of flow awareness, which
> > > > then makes various queueing designs possible. And, there may still be
> > > > deployment possibilities with DSCP, as Rodney mentioned.
> > > >
> > > > Anyway, there's progress being made on SCE, with some new ideas and
> > > > improvements to testing tools coming along.
> > > >
> > > > Pete
> > > >
> > > >
> > > > Ecn-sane at lists.bufferbloat.net
> > > dave at taht.net <Dave T?ht> CTO, TekLibre, LLC Tel: 1-831-435-0729
> > --
> > Rod Grimes                                                 rgrimes at freebsd.org
>
>
>
> --
> "For a successful technology, reality must take precedence over public
> relations, for Mother Nature cannot be fooled" - Richard Feynman
>
> dave at taht.net <Dave Täht> CTO, TekLibre, LLC Tel: 1-831-435-0729



-- 
"For a successful technology, reality must take precedence over public
relations, for Mother Nature cannot be fooled" - Richard Feynman

dave at taht.net <Dave Täht> CTO, TekLibre, LLC Tel: 1-831-435-0729