linux-kernel - Re: [ath5k-devel] Unusually low speeds with ath5k and iwl3945

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Date:	Mon, 24 Nov 2008 10:43:15 +0100
From:	Felix Fietkau <nbd@...nwrt.org>
To:	Benoit PAPILLAULT <benoit.papillault@...e.fr>
CC:	Maxim Levitsky <maximlevitsky@...il.com>,
	Derek Smithies <derek@...ranet.co.nz>,
	ath5k-devel@...ts.ath5k.org, linux-wireless@...r.kernel.org,
	linux-kernel@...r.kernel.org, ipw3945-devel@...ts.sourceforge.net,
	wally@...blackmoor.net, Johannes Berg <johannes@...solutions.net>
Subject: Re: [ath5k-devel] Unusually low speeds with ath5k and iwl3945

Benoit PAPILLAULT wrote:
> I did a similar test here and results is very strange. AP was my good
> old linksys WRT54G running an iperf server. Client was a laptop running
> either ath5k or madwifi/trunk and an iperf client. Channel is 5. Both
> drivers show the same behaviour.
> 
> At the beginning, throughput was very low : 500 - 600 kbit/s. Suddenly
> (after few minutes), it jumps to 15 - 17 Mbit/s and then few minutes
> later (let's say 10 - 20 minutes maybe), it jumps back to 500 - 600
> kbit/s. Using a fixed rate has no effect.
> 
> I used my latest wireless monitoring tools and I did not saw lost of
> duplicates or lost packets. The only difference was the number of
> packets sent by seconds....
> 
> Looking a my syslog, I just saw few messages, unrelated in time with the
> throughput going up or down. They were:
> - ath5k : unsupported jumbo
> - switching to short barker preamble
> - switching to long barker preamble
> 
> I can repeat the same test with iwl3945 as well, if needed.
While reading the code for calculating the frame duration, i noticed
something odd: It doesn't seem to be taking into account the short
vs long preamble distinction for ERP rates. IMHO this might be causing
issues like this. I've seen similar behaviour a long time ago when testing
iwl3945 against a Broadcom AP with exactly the same throughput drop (500-
600 kbits/s).
When I analyzed the problem with an extra monitor mode card, I found out
that the throughput drop is caused by a huge number of retransmissions,
and if I remember correctly (I didn't look for this specifically back then),
the retransmissions went down the rate scaling table until they hit the
first non-ERP rate and that one worked on the first try.

Johannes, does that sound like a probable cause? If so, it should be easy
to fix.

- Felix
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