Re: [ath9k-devel] [RFC] ath9k: improve aggregation throughput by using only first rate

[Felix Fietkau <nbd@openwrt.org>]

On 2010-07-26 9:23 PM, Björn Smedman wrote:
> 2010/7/26 Felix Fietkau <nbd@openwrt.org>:
>> On 2010-07-26 7:10 PM, Björn Smedman wrote:
>>> I think there are some (in theory) simple improvements that can be
>>> done to the tx aggregation / rate control logic. A proof of concept of
>>> one such improvement is provided below. Basically, it's a hack that
>> I think it makes sense to rely less on on-chip MRR for fallback, but I
>> think to make this workable, we really should use the MRR table for
>> something, otherwise the rate control algorithm will take much longer to
>> adapt.
>> It's probably better to fix this properly after I'm done with my A-MPDU
>> rewrite, because then I can more easily push parts of the software
>> retransmission behaviour into minstrel_ht directly.
> 
> Sounds very reasonable. I'm sure you've thought of it but now that
> it's fresh in my head it would be great if the new aggregation design
> allowed us to experiment with stuff like this:
> 
> * The rate control logic treats the average aggregate length as a
> measured independent variable, when in fact it depends heavily on the
> rates selected (via the 4 ms txop limit).
Yes, with the new design maybe we could use the initial rate lookup only
for setting the sampling flag, and then doing a separate per-AMPDU
lookup, which properly takes the AMPDU length into account.

> * When tx is aggregated most rate control probe frames end up inside
> aggregates and are never used for probing (effective probe frequency
> is divided by average aggregate length).
Nope, a probing frame never ends up inside an aggregate. It's always
sent out as a single frame, which is why I had to make the decision
about sending a probing frame more complex in minstrel_ht, compared to
minstrel - the previous 10% stuff was limiting aggregation size.

> * When setting up a hardware MRR for an aggregate the focus should be
> on throughput (as explained earlier in this thread). But there are
> situations when reliability is important: e.g. when a subframe in the
> aggregate is about to expire (because of time or block ack window). It
> may even be advantageous to tx the subframes that are about to expire
> in their own aggregate with lower / more reliable bitrate?
Yes, that's what I was thinking as well. We should probably make this
decision based on the number of sw-retransmitted frames, and maybe
consider the offset of seqno vs baw_tail as well.

> * In many busy radio environments the packet success rate depends very
> much on the protection method being used (none, cts-to-self or
> rts-cts), often more so than on the bitrate itself. It would be
> interesting to experiment with including the protection method in the
> rate selection, i.e. to probe for the optimal protection method and
> bitrate combination.
Sounds good.

> * In order to have the best possible rate control in very dynamic rf
> environments it's important to keep the hardware queue short and
> select rates as late as possible (to not introduce unnecessary delay
> when selecting new rates). I have no idea how to do this but it would
> be great if the tx queue could be kept long enough to never stall tx,
> but no longer.
This would work with what I suggested above - per-AMPDU rate lookup.
With software scheduling that's easy to do, since we already restrict
the queue to max. 2 AMPDUs

> * If I understand correctly the Atheros hardware does not adjust the
> rts / cts-to-self duration field when going through the MRR
> (correct?). In that case it may be even more advantageous to use
> software retry as much as possible when some form of protection is
> enabled.
Not sure, but I think it does adjust the duration field according to the
rate, while transmitting.

> Looking forward to the new aggregation code!
That will still take some time, I recently came up with some better
design ideas, which require some larger changes to the code that I
already wrote.

- Felix