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Subject: Re: [PATCH v11 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on
 packet dequeue
To:     =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        Kan Yan <kyan@google.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Make-Wifi-fast <make-wifi-fast@lists.bufferbloat.net>,
        Yibo Zhao <yiboz@codeaurora.org>,
        John Crispin <john@phrozen.org>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kevin Hayes <kevinhayes@google.com>
References: <20191119060610.76681-1-kyan@google.com>
 <20191119060610.76681-5-kyan@google.com>
 <789d592c-5b1b-b785-6d9c-86b7cc7d57f4@nbd.name> <87k149xbb4.fsf@toke.dk>
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From:   Felix Fietkau <nbd@nbd.name>
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Message-ID: <c1a8b4a9-3cc7-fe23-b9f9-2a28d5ec2158@nbd.name>
Date:   Thu, 27 Feb 2020 13:00:27 +0100
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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