Re: [Make-wifi-fast] [PATCH v2 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Sebastian Moeller <moeller0@gmx.de> writes:

> On October 19, 2019 2:14:42 PM GMT+02:00, "Toke Høiland-Jørgensen" <toke@redhat.com> wrote:
>>Sebastian Moeller <moeller0@gmx.de> writes:
>>
>>>> On Oct 18, 2019, at 16:15, Johannes Berg <johannes@sipsolutions.net>
>>wrote:
>>>> 
>>>> On Thu, 2019-10-17 at 18:11 -0700, Kan Yan wrote:
>>>>> I don't think it is hard to take care of extra header size for
>>frames
>>>>> with VLAN tags
>>>> 
>>>> VLAN tags are payload as far as wifi is concerned, so no need to
>>take
>>>> that into account ...
>>>
>>> 	Ah, good to know; but just out of curiosity is any of the
>>> 	following 7 Byte Preamble + 1 Byte start of frame delimiter
>>> 	(SFD) + 12 Byte inter frame gap (IFG) actually packaged into
>>> 	ethernet frames inside 802.11 packets? I would have guessed that
>>> 	at least the IFG would be dropped as it does not really exist as
>>> 	"data"?
>>
>>No, those are accounted in the airtime calculation in airtime.c (Felix'
>>code). E.g.,:
>>
>>		duration = 20 + 16; /* premable + SIFS */
>
>
> Looks like apples and pears to me. These seem to be the wifi preamble
> and short interframe space in microseconds. Sure you need to add those
> to the airtime estimate as these will hog airtime.
>
> But the 12 byte interframe gap of the Ethernet packets that are
> transmitted over a 802.11 link surely will not be actually transmitted
> as a stretch of zeros? Same for the Ethernet preamble and the start of
> frame marker, as those can/will be trivially added by the Ethernet NIC
> that will handle the encapsulated Ethernet frame after the wifi link,
> no?
>
> As far as I can tell, the wifi SIFS is constant time independent of
> wifi link speed while the IFG size is constant but it's duration is
> not?
>
> Again, I am trying to understand this conceptually, which seems
> orthogonal to the question of whether 38 is the correct number....
>
> Best Regards
>         Sebastian
>
> P.S.: Is there a repository I could look into to try to figure this
> out myself?

This paper has a fairly comprehensive description of how frames look
when transmitted over the air: http://dx.doi.org/10.1155/2015/548109
(see Figure 1 and Table 2, and the text in Section 2).

Otherwise you'll have to go read the standards, I guess (though I can't
say I'd recommend it as light reading :P)

-Toke