Re: [Make-wifi-fast] [PATCH v8 0/2] Implement Airtime-based Queue Limit (AQL)

[Dave Taht <dave@taht.net>]

Johannes Berg <johannes@sipsolutions.net> writes:

> On Wed, 2019-12-04 at 04:47 +0000, Kalle Valo wrote:
>> 
>> > Overall, I think AQL and fq_codel works well, at least with ath10k.
>> > The current target value of 20 ms is a reasonable default.

>> > It is
>> > relatively conservative that helps stations with weak signal to
>> > maintain stable throughput.

This statement is overbroad and largely incorrect.

>>> Although, a debugfs entry that allows
>> > runtime adjustment of target value could be useful.
>> 
>> Why not make it configurable via nl80211? We should use debugfs only for
>> testing and debugging, not in production builds, and to me the use case
>> for this value sounds like more than just testing.

I certainly lean towards making it configurable AND autotuning it
better.

> On the other hand, what application/tool or even user would be able to
> set this correctly?

The guideline from the theory ("Power") is the target should 5-10% of
the interval, and the interval fairly close to the most commonly
observed max RTT. I should try to stress (based on some statements made
here) - that you have to *consistently* exceed the target for the
interval, in order for codel to have any effect at all. Please try to
internalize that - the smoothing comes from the interval... 100ms is
quite a large interval....

Judging from kan's (rather noisy) data set 10ms is a good default on
5ghz. There is zero difference in throughput as near as I can tell.

It would be interesting to try 3ms (as there's up to 8ms of
buffering in the driver) to add to this dataset, helpful also
to be measuring the actual tcp rtt rather in addition to the fq behavior.

I see what looks like channel scan behavior in the data. (on the
client?) Running tests for 5 minutes will show the impact and frequency
of channel scans better.

The 20ms figure we used initially was due to a variety of factors:

* This was the first ever attempt at applying an AQM technology to wifi!!!
** FIXED: http://blog.cerowrt.org/post/real_results/
* We were debugging the FQ component, primarily.
** FIXED: http://blog.cerowrt.org/post/crypto_fq_bug/
* We were working on backports and on integrating a zillion other pieces
  all in motion.
** sorta FIXED. I know dang full well how many darn variables there
   are, as well as how much the network stack has changed since the initial work.
*  We were working on 2.4ghz which has a baseline rate of 1Mbit (13ms target)
   Our rule of thumb is that min target needs to MTU*1.5. There was also a
   a fudge factor to account for half duplex operation and the minimum
   size of a txop. 
** FIXED: 5ghz has a baseline rate of 6mbits.
* We didn't have tools to look at tcp rtts at the time
** FIXED: flent --socket-stats tcp_nup
* We had issues with power save
** Everybody has issues with powersave...
** These are still extant on many platforms, notably ones that wake up
   and dump all their accumulated mcast data into the link. Not our problem.
* channel scans: http://blog.cerowrt.org/post/disabling_channel_scans/
**  Non background channel scans are very damaging. I am unsure from this
    data if that's what we are seeing from the client? Or the ath10k?
    the ability to do these in the background or notmight be a factor in
    autotuning things better.
* We had MAJOR issues with TSQ
** FIXED: https://lwn.net/Articles/757643/

Honestly the TSQ interaction was the biggest barrier to figuring out
what was going wrong at the time we upstreamed this, and a tcp_nup test,
now, with TSQ closer to "right", AQL in place and the reduced target
should be interesting. I think the data we have now on TSQ vs wifi on
this chip, is now totally obsolete.

* We had issues with mcast
** I think we still have many issues with multicast but improving that
   is a separate problem entirely.
* We ran out of time and money, and had hit it so far out of the park
  ( https://lwn.net/Articles/705884/ ) 
  that it seemed like sleeping more and tweaking things less was a win.

Judging from the results we now get on 5ghz and on ac, it seems good to
reduce the target to 10ms (or less!) on 5ghz ghz, especially on ac,
which will result in a less path inflation and no loss in throughput.

I have been running with a 6ms target for several years now on my
802.11n 5ghz devices. (I advertise a 3ms rather than the default txop
size also) These are, admittedly, mostly used as backhaul
links (so I didn't have tsq, aql, rate changes, etc) , but seing a path
inflation of no more than 30ms under full bidirectional load is
nice. (and still 22ms worse than it could be in a more perfect world)

Another thing I keep trying to stress: TCP's ability to grab more
bandwidth is quadratic relative the delay.

>
> johannes