[Cerowrt-devel] Equivocal results with using 3.10.28-14

Dave Taht dave.taht at gmail.com
Mon Feb 24 10:51:59 EST 2014

On Mon, Feb 24, 2014 at 9:36 AM, Rich Brown <richb.hanover at gmail.com> wrote:
> CeroWrt 3.10.28-14 is doing a good job of keeping latency low. But... it has two other effects:
> - I don't get the full "7 mbps down, 768 kbps up" as touted by my DSL provider (Fairpoint). In fact, CeroWrt struggles to get above 6.0/0.6 mbps.

0) try the tcp_upload or tcp_download or tcp_bidir tests to get
results closer to what your provider claims.

since your plots are pretty sane, you can get cleaner ones with using
the 'totals' plot type
and/or comparing multiple runs to get a cdf

-p totals or -p icmp (theres a few different ones, --list-plots

-i somerun.json.gz -i somerun2.json.gz

1) http://richb-hanover.com/wp-content/uploads/2014/02/6854-777-dflt-sqm-disabled1.png

is your baseline without SQM?

If so why do you compare the providers stated rate...

   with the measured rate with/without SQM?

These are two measures of the truth - one with and without a change.

Vs a providers claim for link rate that doesn't account for real
packet dynamics.

2) the netperf reporting interval is too high to get good measurements
at below a few
mbit, so you kind of have to give up on the upload chart at these
rates. (totals chart is

Note that the tcp acks are invisible - you are getting >6mbit down,
and sending back approximately
150kbit in acks which we can't easily measure. The overhead in the
measurement streams is
relative to the RTT as well.

I'd really like to get to a test that emulated tcp and got a fully
correct measurement.

3) Generally using a larger fq_codel target will give you better
upload throughput and
better utiliziation at these rates. try target 40ms as a start. We've
embedded a version
of the calculation in the latest cero build attempts (but other stuff is broke)

nfq_codel seems also do to give a better balance between up and
downloads at low rates,
also with a larger target.

it looks like overhead 44 is about right and your first set of charts
about right.

> - When I adjust the SQM parameters to get close to those numbers, I get increasing levels of packet loss (5-8%) during a concurrent ping test.

Shows the pings are now accruing delay.

> So my question to the group is whether this behavior makes sense: that we can have low latency while losing ~10% of the link capacity, or that getting close to the link capacity should induce large packet loss...

You never had the 10% in the first place.

> Experimental setup:
> I'm using a Comtrend 583-U DSL modem, that has a sync rate of 7616 kbps down, 864 kbps up. Theoretically, I should be able to tell SQM to use numbers a bit lower than those values, with an ATM plus header overhead with default settings.
> I have posted the results of my netperf-wrapper trials at http://richb-hanover.com - There are a number of RRUL charts, taken with different link rates configured, and with different link layers.
> I welcome people's thoughts for other tests/adjustments/etc.
> Rich Brown
> Hanover, NH USA
> PS I did try the 3.10.28-16, but ran into troubles with wifi and ethernet connectivity. I must have screwed up my local configuration - I was doing it quickly - so I rolled back to

manually adjust the target.

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Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html

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