[Make-wifi-fast] bloated ath10k

Bob McMahon bob.mcmahon at broadcom.com
Sun Feb 10 22:03:26 EST 2019

Also, way off topic but the data center switch architects have proposed
something called HULL.


One model of the forwarding plane is that the chip firmware is indeed part
of the forwarding and the spectrum/AP provides the mesh or "switching
fabric."  It's somewhat similar to how data center switches are designed.
 In that context a STA, besides minimizing it's processing delay, may be
able to do the following interesting things:

   1. If RX queues are "full" (or hit the phantom queue depth) set the CE
   bit to signal TCP
   2. If TX queues are "full" and a standing queue (per bloat) is behind
   it, then drop.  If not, then advantage the air access via adaptive EDCA,
   e.g. reduce the AIFS (within the limits of the standards)  An AP might want
   to advertise a somewhat larger AIFS so a STA reducing its for a brief
   period will achieve the arbitration goals.

As a side note, we've done a lot of timing work in iperf 2.0.13 to help
fault isolate latency issues.  It works best if the device realtime clocks
are synchronized, e.g. to a GPS disciplined OCXO using PTP.   We also added
TCP connect times to monitor the 3WHS as well as trip times, timing the
data xfer phase.   Iperf 2.0.13 has been picked up by many distributions.


Thoughts welcomed and appreciated


On Mon, Feb 11, 2019 at 7:48 AM Bob McMahon <bob.mcmahon at broadcom.com>

> Just a reminder with 802.11ax uplink OFDMA that STA A and B transmits can
> be "concurrent."    Also, there are companies working on full duplex per
> self cancellation.  Kumu networks is one
> http://kumunetworks.com/
> Bob
> On Mon, Feb 11, 2019 at 12:30 AM David P. Reed <dpreed at deepplum.com>
> wrote:
>> Side note: between two stations talking through an AP, it's not half
>> duplex. It's kind-of quarter-duplex. Each packet between STA A and STA B
>> cannot  be concurrent with the subsequent packet from A to B, and both
>> transmissions of a packet from B to A.
>> That actually has a significant effect on "queue depth".  Please don't
>> call it "interference", because no packet corruption happens.
>> This is why merely fixing the queueing discipline in the AP alone doesn't
>> necessarily ameliorate bufferbloat. The queues in the STA's need to be
>> managed, too.
>> You guys know that implicitly, I know I'm not telling you anything you
>> don't know. But this queueing needs to be managed in such a way that the
>> backoff in TCP is signaled. That is, packets need to be dropped or marked.
>> You can't fix this in the forwarding paths alone.
>> -----Original Message-----
>> From: "Jonathan Morton" <chromatix99 at gmail.com>
>> Sent: Sunday, February 10, 2019 7:38am
>> To: "Adrian Popescu" <adriannnpopescu at gmail.com>
>> Cc: make-wifi-fast at lists.bufferbloat.net
>> Subject: Re: [Make-wifi-fast] bloated ath10k
>> > On 10 Feb, 2019, at 2:24 pm, Adrian Popescu <adriannnpopescu at gmail.com>
>> wrote:
>> >
>> > My attempts to use SQM and codel to reduce wifi bloat didn't seem to
>> get me very far. 802.11ac seems more reliable and it seems to be more
>> bloated. ath9k can go as low as 3-5 milliseconds. ath10k is usually in the
>> 20-50 milliseconds range (or more, based on the number of stations). I
>> usually test with a single client as I don't expect latency to improve with
>> more clients.
>> Some things are unavoidable when you move to a shared, half-duplex, noisy
>> medium like wifi versus a switched, full-duplex, error-free medium like
>> Ethernet.
>> If you're getting 20-50ms under load, then I think things are working
>> quite well with wifi. We can wish for better, but not long ago you could be
>> looking at multiple seconds in bad cases. At the levels you're now seeing,
>> ordinary interactive protocols like DNS and HTTP will work reliably and
>> quickly, and even VoIP should be able to cope; you'll likely only really
>> notice a problem with online games.
>> Ath9k has some advantages over ath10k in this area. Its MAC is managed at
>> a lower level by the driver, so we have much more control over it when
>> trying to debloat. I think we still have more control over ath10k than most
>> of the alternatives.
>> If low latency is mission critical, though - just run a wire.
>> - Jonathan Morton
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