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

[Kan Yan <kyan@google.com>]

Dave Taht (taht.net) writes:

> 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.

One large aggregation in 11ac can last 4-5 ms, with bursting,
firmware/hardware can complete as much 8 -10 ms worth of frames in one
shot and then try to dequeue more frames, so the jitter for the
sojourn time can be as high as 8-10 ms. Setting the default target to
something less than 10ms can cause unnecessary packet drop in some
occasions.


On Fri, Dec 6, 2019 at 11:53 AM Dave Taht <dave@taht.net> wrote:
>
> 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