[Cerowrt-devel] Managed to break 802.11n (on a 3800)

Thu Jan 16 17:50:54 EST 2014

On Thu, Jan 16, 2014 at 3:10 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:

> Hi Aaron,
>
>
> On Jan 16, 2014, at 20:08 , Aaron Wood <woody77 at gmail.com> wrote:
>
>
> Sebastian, after sorting out the router, it's still biased, but far
> less
> so, about a 2:1 ratio between upload and download.
>
>
> So I See offen 10:1 and worse @165Mbit/s raw wireless rate
>
>
> I get mixed results, but they aren't good.
>
>
It's hard to comment on each graph in email, but I'll try.

I generally run rrul with the --disable-log option. Log scales helped back
when we were still
comparing against pfifo fast.

The really bad download graph. "Crazy results"

Download bandwith is bad because the upload starts and fills the queue
first, the download
has to wait to fill the queue and generally gets dropped earlier than the
upload. This is
one of the many reasons I don't care for IW10....

The upload gets better slowly due to how slow tcp is ramping up over the
half-duplex
wifi channel.

>
> I just checked again and I get crazy results for both RRUL and
> RRUL_NOCLASSIFICATION:
>
> in both cases I get ~ 10:1 out-in imbalance.
>

I think that with a larger quantum on the AP they will be in less
imbalance, and you should try nfq_codel also.

The larger quantum will also hurt, too.... right answer has always been per
station queues.

> And even crazier just had one rrul where both in and out came up almost
> perfectly at 1:1
>

Hmm. Wifi is weird, isn't it? It's not like ethernet at all. Too bad the
universe insists on trying to
defy the laws of physics by trying to make it act like ethernet....

> . Interestingly the classification really works in giving different
> bandwidth for the different classes. (And in rrul_noclassification, where
> the still classified UDP probes make it through the EF flow gets shorter
> latencies…).
>

having 4 full queues and a txop each is far worse than 1 queue with better
aggregation, IMHO.

>
> Note that measuring through cerowrt to a wired host (with too restrictive
> firewall settings) get:
>

You are seeing the upload ramp up along tcp's lines and the download ramp
down as it gets progressively more starved.

> with the MacBooks uplink still dominant (actually continually getting more
> bandwidth…).
>

Well, you only have X bandwidth, in the air, total. A better way of saying
it might be the macbook is taking better advantage
of it's txops to ship more data in an aggregate.

Since I my only wireless connected machines are macs and nobody else
> complained about this issue I assume it is an osx issue
>

I honestly think that aside from benchmarks, bandwidth is irrelevant on
wifi. Lower latency is something that you
actually feel, and when accessing the web or doing a videoconference,
that's the part that matters.

it IS possible to get the best of both worlds, but that's going to take
some driver rework.

>
> For comparison an RRUL test from the wired linux host to cerowrt, where
> things look much better...
>
>
>
> IIRC, apple really changed something about the media access in 10.8, I'll
> look into that.  And see if my wife will let me install netperf on her
> laptop (I think it's still running 10.7)
>
>
> Yeah, good question whether this is the same in all macosx versions?
> (Sonner or later I will switch to 10.9 and repeat the measurements…) The
> saving grace is that I usually either upload or download at home between my
> 2 computers so I rarely feel the full force of this unfortunate macosx
> behavior. Just checked using SMB to copy a file to the wired machine and
> from the wired machine at the same time, nicely splits the bandwidth evenly
> between up and download, so this might be netsurf related...
>
>
Single threaded tests will generally work ok, which is why nobody before
has complained... which is why rrul exists to beat up things like
torrent-like ,web like videoconferencing-like and voip-like behaviors.

>
>
> Also, my understanding was that with rts/cts, the router was in control
> of
> that aspect of things?
>
>
>     That is what I thought AS well, but it is not what I See with osx 10.8.
>
>
> It may be a case of the station aggressively asking to send, and the AP
> granting instead of sending data to the station that's waiting.
>
>
> I think we agree that the AP should show more self-confidence and reject
> such requests more firmly :)
>
>
>
> It should be clear in a monitor-mode tcpdump (or a statistical summary of
> packets).
>
>
> I am not really equipped to do this, with just one wireless notebook at my
> disposal :)
>
> best
> Sebastian
>
>
Now that you have your laptops running this stuff (AWESOME thank you!)

If I can encourage you all to go outside, to like your nearest cybercafe,
or conference center, and run your rrul tests there...

... you'll find out just how bad the rest of the world is... (and get some
good data).

I routinely see 4-6 seconds of latency, and a bare megabit or two of actual
bandwidth. The ONLY place Iv'e ever had decent wifi performance in a busy
area has been at ietf conferences....

>
> --Aaron
>
>
>
> _______________________________________________
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> Cerowrt-devel at lists.bufferbloat.net
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>
>

-- 
Dave Täht

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