Re: [Make-wifi-fast] Where is the bloat in WiFi?

[Michael Welzl <michawe@ifi.uio.no>]

Hi, and thanks for a quick answer!

But, it's not quite what I was looking for.... see below:

> On 6 Oct 2020, at 13:47, Toke Høiland-Jørgensen <toke@toke.dk> wrote:
> 
> Michael Welzl <michawe@ifi.uio.no> writes:
> 
>> Hi all,
>> 
>> A simple question to y'all who spent so much time on Cake and things
>> ... in a household using WiFi, which buffer is usually bloated? Where
>> does the latency really come from?
>> 
>> Is it:
>> 1. the access point's downlink queue, feeding into the WiFi network,
> 
> This we mostly fixed, but only if you're on a recent OpenWrt with the
> right WiFi drivers.

Well okay... I was curious about where the bottleneck is. I can translate my question into: "if Cake is installed everywhere, where does it have the most work to do?".


> Otherwise, this is a major source of latency *if*
> the WiFi link is faster than the downlink from the internet.

Huh? Slower, you mean?


> This
> depends on both the internet connection and the current rate each WiFi
> station operates at, so it can vary wildly, and on very short time
> scales.

Sure... I was asking for the "if" in your statement above - since this is an operationally-oriented list: what do people see? What is the more common case?


>> 2. the modem's downlink queue, feeding into the access point,
> 
> If your internet (downlink) connection is slower than your WiFi link,
> this is where you'll get the queueing.

Yes sure  :-)     see above: I wanted to know what the more common case is, in households that people on this list have dealt with.


>> 3. the modem's uplink queue,
> 
> As above, but in the other direction - but as uplinks tend to be
> asymmetric, this direction is often more of a problem.
> 
>> 4. the access point's uplink queue towards the modem   (hm, that seems
>> silly, surely the AP-modem connection is fast... so perhaps, instead:
>> the queue in the host, as it wants to send data towards the access
>> point)
> 
> Yeah, that would be in the host; but host drivers can suffer from severe
> bufferbloat as well, especially as rates drop (since the buffers are
> often tuned for the maximum throughput the device can deliver in
> best-case signal conditions).
> 
>> or is it a combination of these?
> 
> Usually it's a combination; especially since the WiFi capacity varies
> wildly with signal conditions (as devices move around relative to the
> AP), general link usage (more devices active mean less available
> capacity for each device, exacerbated by airtime unfairness), and
> interference. Also there are things like excessive retries causing HoL
> blocking.

Man, what an academic answer!    Makes me think you have a PhD, or something!  What *theoretically* can happen is not what I was fishing for   :-D


>> I guess that, with openwrt, Cake is operating on the queue that's
>> feeding the wifi network, as the modem's queue is out of its
>> control... so: is this where the bottleneck usually is?
> 
> It certainly used to be; but as uplink connection speeds improve, the
> bottleneck moves to the WiFi link.

Yessss, that's why I was asking....


> The extent to which this happens
> depends on where you are in the world; personally I've been bottlenecked
> on the WiFi link ever since I got a fibre upstream (and with 802.11ax
> rates maxing at >1Gbps, maybe that'll change again?).

THIS is what I was after   :)   one data point, cool - so far, so good...

Cheers,
Michael