netdev - Re: [RFC v2] iwlwifi: pcie: transmit queue auto-sizing

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Date:	Thu, 4 Feb 2016 20:56:01 +0000
From:	"Grumbach, Emmanuel" <emmanuel.grumbach@...el.com>
To:	Ben Greear <greearb@...delatech.com>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Dave Taht <dave.taht@...il.com>,
	"Jonathan Corbet" <corbet@....net>
Subject: Re: [RFC v2] iwlwifi: pcie: transmit queue auto-sizing



On 02/04/2016 10:46 PM, Ben Greear wrote:
> On 02/04/2016 12:16 PM, Emmanuel Grumbach wrote:
>> As many (all?) WiFi devices, Intel WiFi devices have
>> transmit queues which have 256 transmit descriptors
>> each and each descriptor corresponds to an MPDU.
>> This means that when it is full, the queue contains
>> 256 * ~1500 bytes to be transmitted (if we don't have
>> A-MSDUs). The purpose of those queues is to have enough
>> packets to be ready for transmission so that when the device
>> gets an opportunity to transmit (TxOP), it can take as many
>> packets as the spec allows and aggregate them into one
>> A-MPDU or even several A-MPDUs if we are using bursts.
> I guess this is only really usable if you have exactly one
> peer connected (ie, in station mode)?
>
> Otherwise, you could have one slow peer and one fast one,
> and then I suspect this would not work so well?

Yes. I guess this one (big) limitation. I guess that what would happen
in this case is that the the latency would constantly jitter. But I also
noticed that I could reduce the transmit queue to 130 descriptors
(instead of 256) and still reach maximal throughput because we can
refill the queues quickly enough.
In iwlwifi, we have plans to have one queue for each peer.
This is under development. Not sure when it'll be ready. It also requires
firmware change obviously.

> For reference, ath10k has around 1400 tx descriptors, though
> in practice not all are usable, and in stock firmware, I'm guessing
> the NIC will never be able to actually fill up it's tx descriptors
> and stop traffic.  Instead, it just allows the stack to try to
> TX, then drops the frame...

1400 descriptors, ok... but they are not organised in queues?
(forgive my ignorance of athX drivers)

>
> Thanks,
> Ben
>