Re: [PATCH v11 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

[Felix Fietkau <nbd@nbd.name>]

On 2020-02-27 12:07, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd@nbd.name> writes:
> 
>>> Not sure I fully understand your concerns here. The AQL budget is per
>>> STA, per TID, so frames queued in the driver's special queue for one
>>> station won't impact another station's budget. Those frames are
>>> counted towards the per interface pending airtime, which could trigger
>>> AQL to switch to use the lower queue limit. In this case, that could
>>> be the desirable behavior when there is heavy traffic.
>> Yes, the per interface limit is what I'm concerned about. I'm not sure
>> if it will be an issue in practice, it's just something that I noticed
>> while reviewing the code.
> 
> Ah, right. Note that it's not a hard limit, though; it's just a
> threshold for when the per-station limit switches to a smaller value.
> The idea is that when there are many stations with outstanding data,
> each station's latency will be higher since they have to also wait for
> their turn to transmit. And so we compensate for this by lowering each
> station's queue limit, since the hardware is unlikely to become idle
> when there are many stations to send to.
> 
> The lower limit is still 5ms of data, though, so there should still be
> enough for a full aggregate queued for each station. Arguably the limit
> could actually be made *smaller*: On a very busy network with lots of
> stations transmitting at once, IMO we *don't* want to send full-sized
> aggregates as that will add up to way too much latency. Better to
> sacrifice a bit of network efficiency to raise responsiveness :)
Makes sense, thanks.

- Felix