Re: Enabling Byte Queue Limits in the ath5k wireless driver

[Anirudh Sivaraman <sk.anirudh@gmail.com>]

On Sun, Oct 6, 2013 at 5:03 PM, Simon Barber <simon@superduper.net> wrote:
> Indeed - but this is based on underruns while there is more to send - once
> it has established a length that does a good job of keeping the hardware fed
> it should not change much. This control loop will slowly catch up with the
> different speeds wireless packets are sent at (if a single link is in use,
> not multiple links) but it would be better controlled and also handle
> multiple links on one interface (e.g. Access point mode, or 11z) properly if
> packet duration was considered.
>

Ok, that makes a lot of sense. I guess I 'll try implementing BQL
first, followed by TQL, and see what I find.

Anirudh

> Simon
>
>
> On 10/6/2013 9:36 AM, Anirudh Sivaraman wrote:
>>
>> On Sun, Oct 6, 2013 at 11:01 AM, Simon Barber <simon@superduper.net>
>> wrote:
>>>
>>> BQL or TQL are open-loop - there is no feedback. The amount of bytes or
>>> total packet transmission time in the queue is counted when packets enter
>>> and leave the queue, and a hard limit of total bytes or total time in
>>> queue
>>> is set. When I say time - I mean that on a packet entering the queue the
>>> total time the packet will take to transmit is calculated, and that time
>>> is
>>> added to a count to the total time currently in queue. When a packet has
>>> been transmitted the time is subtracted from the total time in queue.
>>> There
>>> would be no interference with CODEL running in front of this.
>>>
>> I thought, based on http://lwn.net/Articles/454390/, that the queue
>> length is dynamically adjusted by DQL/BQL/TQL. This is the device
>> driver control loop I was referring to. This queue length in turn
>> dictates when the dequeue function is called at the qdisc layer, where
>> CoDel is running. Please correct me if I am wrong here.
>>
>> Anirudh
>>
>>> Simon
>>>
>>>
>>> On 10/6/2013 6:31 AM, Anirudh Sivaraman wrote:
>>>>
>>>> On Sat, Oct 5, 2013 at 11:39 PM, Simon Barber <simon@superduper.net>
>>>> wrote:
>>>>>
>>>>> BQL does make sense, but TQL would be even better - 'Time Queue
>>>>> Limits'.
>>>>> Which would work based on an estimate of how much time each packet will
>>>>> take
>>>>> to send, and limiting the hardware queue to contain a total mount of
>>>>> 'time'.
>>>>> The purpose of the hardware queue is to mask interrupt and other
>>>>> latencies
>>>>> involved in refilling an empty queue - a time based phenomenon. Hence
>>>>> time
>>>>> is the best metric to control it - with fixed speed interfaced like
>>>>> ethernet
>>>>> bytes=time, but not so on wireless.
>>>>>
>>>> Thank you for your reply. TQL does make more sense for wireless at
>>>> least. However, assuming that someone implement a qdisc like CoDel and
>>>> attaches it to the wireless interface, doesn't TQL (or for that matter
>>>> BQL) lead to two possibly competing control loops (one at the qdisc,
>>>> and one at the device driver)?
>>>>
>>>> Anirudh
>>>>>
>>>>> Simon
>>>>>
>>>>>
>>>>> On 10/3/2013 11:49 AM, Anirudh Sivaraman wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I am looking into enabling BQL for the ath5k driver, and was wondering
>>>>>> if anyone here knows of any prior efforts in this direction. In
>>>>>> particular, is BQL even a sensible strategy for wireless drivers?
>>>>>> Thank you in advance for any advice you may have in this regard.
>>>>>>
>>>>>> Anirudh
>>>>>> _______________________________________________
>>>>>> Bloat-devel mailing list
>>>>>> Bloat-devel@lists.bufferbloat.net
>>>>>> https://lists.bufferbloat.net/listinfo/bloat-devel
>>>>>
>>>>>
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>>>
>