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

On 02/04/2016 12:56 PM, Grumbach, Emmanuel wrote:

On 02/04/2016 10:46 PM, Ben Greear wrote:

quoted

On 02/04/2016 12:16 PM, Emmanuel Grumbach wrote:

quoted

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.

Per-peer queues will probably be nice, especially if we can keep the
buffer bloat manageable.

quoted

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)

I think all the details are in the firmware, at least for now.

The firmware details are probably not something I should go into, but suffice it to say
its complex and varies between firmware versions in non-trivial ways.

Thanks,
Ben

-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com

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