MIME-Version: 1.0
From: Daniel Halperin <dhalperi@cs.washington.edu>
Date: Fri, 11 Mar 2011 00:11:14 -0800
Message-ID: <AANLkTi=taszKd9-kyijN100oEegnFi=0u5L4WCp7R_sJ@mail.gmail.com>
Subject: bug: iwlwifi, aggregation, and mac80211's reorder buffer
To: ipw3945-devel@lists.sourceforge.net, linux-wireless@vger.kernel.org
Content-Type: text/plain; charset=ISO-8859-1
Sender: linux-wireless-owner@vger.kernel.org

I'm doing some performance debugging of iwlwifi.  My test setup has
one machine with iwlwifi-5300 (3 TX streams) and ar9280 (2 TX streams)
connected to a 3-stream iwl5300 AP.  There's no/not much external
interference in my setup.  I'm doing bandwidth tests using iperf.
Both cards gets something like 200 Mbps using UDP, and Atheros gets
130--150 Mbps using TCP while iwlwifi gets ~60 Mbps(!!).  This seems
bad, especially since iwl5300 has 3 TX streams.  And they're
connecting from the same computer to the same AP while getting similar
UDP performance, so I don't think it's a hardware difference.

In looking at various TCP effects, I noticed that with
ath9k+minstrel_ht, there are never any TCP retransmissions or
selective acknowledgements.  However, with iwlwifi these are rampant
and large bursts of these are highly correlated with network-layer
dead time even up to ~100ms(!!).

I first suspected rate selection, so I hacked iwlwifi to only use MCS
that work very well for my test link.  I confirmed that there is very
little loss in my experiments.  I then moved on to aggregation.

One thing that Intel does that ath9k does not is transmit packets out
of sequence number order inside a batch.  (This is legal in the 802.11
standard).  I figured that one explanation for the TCP SACKs would be
if, somehow, frames got released to the network stack out of order;
indeed, many of the "holes" covered by the SACKs are filled quickly
(within ~4ms, about the length of one aggregation batch).  Note that
iwlwifi defaults to an aggregation frame limit, hence buffer size, of
31 frames.  mac80211 honors this buffer size specification by
releasing frames to the network stack that are >= 31 sequence numbers
below the highest received frame.

It looks like Intel doesn't honor its own frame limit, as I often saw
it have more than 31 frames outstanding, causing mac80211 on the
receiver to release many frames early.  Changing iwlwifi's default agg
limit to 63 frames on both ends dramatically reduced the prevalence of
SACKs/TCP retransmissions and improved avg TCP performance to ~100
Mbps (ranging 83-110).

A few questions:

(1) Why does iwlwifi default to an aggregation frame limit of 31? I
didn't see any negative effects from 63 frame limit and performance
improved dramatically
(2) Is there a way to make iwlwifi honor the aggregation limit?  I
know that agg is controlled by a hardware scheduler, so this may be
difficult.
(3) mac80211's reorder buffer code could probably also be relaxed.  It
probably wouldn't hurt to have the buffer be twice the transmitter's
advertised buffer, and might compensate for devices that don't
properly honor frame limits.  Also, mac80211 only flushes the reorder
buffer every 100 ms.  That seems like a LONG time given that batches
are limited to 4ms -- 100ms is room for at least 10, maybe 20
retransmissions to attempt to fill in the holes(!).
(4) even after this fix, I see a few SACKs, and even when there aren't
SACKs I still see TCP "dead time" up to ~100ms.  What else would you
use to debug this setup?

Thanks
Dan