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

[Toke Høiland-Jørgensen]

Michael Welzl <michawe at 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. Otherwise, this is a major source of latency *if*
the WiFi link is faster than the downlink from the internet. 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.

> 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.

> 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.

> 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. 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?).

-Toke