Tim Shepard <shep@xxxxxxxxxxxx> writes:
>>
>> You're right that it doesn't check the max. However, this is less of a
>> problem now that there is no intermediate queueing in the driver; and
>> indeed the utility of haven the qlen_* tunables is somewhat questionable
>> with the patch applied: The only thing this is going to control is the
>> size of the retry queue, and possible limit the size of the retry queue.
>> [....]
>
> The driver queues things up for the hardware to DMA and transmit.
> Something has to limit the amount of packets handed over to the
> hardware. (We lack access to hardware documentation (grrrr!) but it
> appears to me that the hardware has a hard limit on how many packets
> can be handed to it.)
There's a ring buffer eight entries long that the aggregates (or
packets) are put on when actually being handed to the hardware.
This is in ath_txq->txq_fifo.
>> Because there's a second limit in play (which has always been there): in
>> ath_tx_sched_aggr() there is this check:
>>
>> if ((aggr && txq->axq_ampdu_depth >= ATH_AGGR_MIN_QDEPTH) ||
>> (!aggr && txq->axq_depth >= ATH_NON_AGGR_MIN_QDEPTH)) {
>> __skb_queue_tail(&tid->retry_q, bf->bf_mpdu);
>> *stop = true;
>> return false;
>> }
>>
>> The two constants are 2 and 8 respectively. This means that, with
>> aggregation enabled, no more than two full aggregates will be queued up.
>> The size of the aggregates is dynamically computed from the current
>> rate: they are limited a maximum of four milliseconds of (estimated)
>> airtime (for the BE queue; the others have different limits).
>>
>> So in a sense there's already a dynamic limit on the hardware queues.
>> Now, whether four milliseconds is the right maximum aggregate size might
>> be worth discussing. It is the maximum allowed by the standard. Dave and
>> I have been
>
> Ah that may be the clue that I lacked. There's got to be a dependency
> on processor speed (how quickly the system and driver can get another
> packet hooked up for transmission after completions) but perhaps with
> aggregates being so large in time, with full aggregates even the
> slowest processors are fast enough to avoid starvation.
>
> If there's no aggregation, a limit of some sort is needed (probably to
> prevent malfunction of the hardware/driver, but in any case to limit
> excess latency). And this limit will depend on processor speed (and
> will need to be autotuned at runtime).
ATH_NON_AGGR_MIN_QDEPTH is 8 -- so yeah, the limit is higher if there is
no aggregation.
These are hard-coded values, so presumably they are large enough to keep
the hardware busy on most platforms (or someone would have noticed and
changed them?). So I doubt there is much to be gained to add a mechanism
to dynamically tune them (between 0 and 2?).
The exception being in case pulling from the mac80211 queue is too slow
to keep the hardware busy at the current settings. I see no problems
with this on my hardware, but that's an x86 box. I would probably hold
off on the dynamic tuning until having proven that there's actually a
bottleneck, though... ;)
-Toke
--
To unsubscribe from this list: send the line "unsubscribe linux-wireless" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
