[Bloat] [Cerowrt-devel] capturing packets and applying qdiscs

Dave Taht dave.taht at gmail.com
Fri Mar 27 23:31:35 EDT 2015

wonderful dataset isaac! A lot to learn there and quite a bit I can
explain, which might take me days to do with graphs and the like.

But it's late, and unless you are planning on doing another test run I will

It is mildly easier to look at this stuff in bulk, so I did a wget -l 1- m
http://candelatech.com/downloads/wifi-reports/trial1/ on the data.

Quick top level notes rather than write a massive blog with graph entry....

-1) These are totally artificial tests, stressing out queue management.
There are no
winners, or losers per se', only data. Someday we can get to winners and
but we have a zillion interrelated variables to isolate and fix first. So
consider this data a *baseline* for what wifi - at the highest rate
possible - looks like today - and I'd dearly like some results that are
below mcs4 on average also as a baseline....

Typical wifi traffic looks nothing like rrul, for example. rrul vs rrul_be
is useful for showing how badly 802.11e queues actually work today, however.

0) Pretty hard to get close to the underlying capability of the mac, isn't
it? Plenty of problems besides queue management could exist, including
running out of cpu....

1) SFQ has a default packet limit of 128 packets which does not appear to
be enough at these speeds. Bump it to 1000 for a more direct comparison to
the other qdiscs.

You will note a rather big difference in cwnd on your packet captures, and
bandwidth usage more similar to pfifo_fast. I would expect, anyway.

2) I have generally felt that txops needed more of a "packing" approach to
wedging packets into a txop rather than a pure sfq or drr approach, as
losses tend to be bursty, and maximizing the number of flows in a txop a
goodness.  SFQ packs better than DRR.

That said there are so many compensation stuff (like retries) getting in
the way right now...

3) The SFQ results being better than the fq_codel results in several cases
are also due in part to an interaction of the drr quantum and a not high
enough target to compensate for wifi jitter.

But in looking at SFQ you can't point to a lower latency and say that's
"better" when you also have a much lower achieved bandwidth.

So I would appreciate a run where the stations had a fq_codel quantum 300
and target 30ms. APs, on the other hand, would be better a larger
(incalculable, but say 4500) quantum, a similar target, and a per dst
filter rather than the full 5 tuple.

On Fri, Mar 27, 2015 at 12:00 PM, Isaac Konikoff <konikofi at candelatech.com>

>  Thanks for pointing out horst.
> I've been trying wireshark io graphs such as:
> retry comparison:  wlan.fc.retry==0 (line) to wlan.fc.retry==1 (impulse)
> beacon delays:  wlan.fc.type_subtype==0x08 AVG frame.time_delta_displayed
> I've uploaded my pcap files, netperf-wrapper results and lanforge script
> reports which have some aggregate graphs below all of the pie charts. The
> pcap files with 64sta in the name correspond to the script reports.
> candelatech.com/downloads/wifi-reports/trial1
> I'll upload more once I try the qdisc suggestions and I'll generate
> comparison plots.
> Isaac
> On 03/27/2015 10:21 AM, Aaron Wood wrote:
> On Fri, Mar 27, 2015 at 8:08 AM, Richard Smith <smithbone at gmail.com>
> wrote:
>> Using horst I've discovered that the major reason our WiFi network sucks
>> is because 90% of the packets are sent at the 6mbit rate.  Most of the rest
>> show up in the 12 and 24mbit zone with a tiny fraction of them using the
>> higher MCS rates.
>> Trying to couple the radiotap info with the packet decryption to discover
>> the sources of those low-bit rate packets is where I've been running into
>> difficulty.  I can see the what but I haven't had much luck on the why.
>> I totally agree with you that tools other than wireshark for analyzing
>> this seem to be non-existent.
>  Using the following filter in Wireshark should get you all that 6Mbps
> traffic:
>  radiotap.datarate == 6
>  Then it's pretty easy to dig into what those are (by wifi frame-type, at
> least).  At my network, that's mostly broadcast traffic (AP beacons and
> whatnot), as the corporate wifi has been set to use that rate as the
> broadcast rate.
>  without capturing the WPA exchange, the contents of the data frames
> can't be seen, of course.
>  -Aaron

Dave Täht
Let's make wifi fast, less jittery and reliable again!

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