[Make-wifi-fast] Diagram of the ath9k TX path

David Lang david at lang.hm
Fri May 13 16:49:30 EDT 2016

On Fri, 13 May 2016, Bob McMahon wrote:

> On driver delays, from a driver development perspective the problem isn't
> to add delay or not (it shouldn't) it's that the TCP stack isn't presenting
> sufficient data to fully utilize aggregation.  Below is a histogram
> comparing aggregations of 3 systems (units are mpdu per ampdu.)  The lowest
> latency stack is in purple and it's also the worst performance with respect
> to average throughput.   From a driver perspective, one would like TCP to
> present sufficient bytes into the pipe that the histogram leans toward the
> blue.

The problem isn't providing sufficent bytes into the pipe, it's that to do 
aggregations sanely you need lots of bytes to a particular station in a seprate 
queue than the lots of bytes for each of the other stations.

current qdisc settings don't provide this to the driver, they provide a packet 
to host A, a packet to host B, a packet to host C, etc.

What's needed is for the higher level (fq_codel, etc) to provide a queue for 
host A, a queue for host B, a queue for host C and throttle what goes into these 
queues, and then the driver grabs an aggregates worth from the first queue and 
sends it to host A, an aggregates worth for host B (which is a different number 
of bytes because B has a different data rate than A), etc.

the current model of the tcp stack having one set of queues and the driver 
having a separate set of queues just doesn't work for this. One of the sets of 
queues needs to go away and the queues that remain need to be able to be filled 
with fairness between flows in mind (fq_codel or similar) and drained with 
fairness of airtime in mind (and efficient aggregation)

David Lang

> [image: Inline image 1]
> I'm not an expert on TCP near congestion avoidance but maybe the algorithm
> could benefit from RTT as weighted by CWND (or bytes in flight) and hunt
> that maximum?
> Bob
> On Mon, May 9, 2016 at 8:41 PM, David Lang <david at lang.hm> wrote:
>> On Mon, 9 May 2016, Dave Taht wrote:
>> On Mon, May 9, 2016 at 7:25 PM, Jonathan Morton <chromatix99 at gmail.com>
>>> wrote:
>>>> On 9 May, 2016, at 18:35, Dave Taht <dave.taht at gmail.com> wrote:
>>>>> should we always wait a little bit to see if we can form an aggregate?
>>>> I thought the consensus on this front was “no”, as long as we’re making
>>>> the decision when we have an immediate transmit opportunity.
>>> I think it is more nuanced than how david lang has presented it.
>> I have four reasons for arguing for no speculative delays.
>> 1. airtime that isn't used can't be saved.
>> 2. lower best-case latency
>> 3. simpler code
>> 4. clean, and gradual service degredation under load.
>> the arguments against are:
>> 5. throughput per ms of transmit time is better if aggregation happens
>> than if it doesn't.
>> 6. if you don't transmit, some other station may choose to before you
>> would have finished.
>> #2 is obvious, but with the caviot that anytime you transmit you may be
>> delaying someone else.
>> #1 and #6 are flip sides of each other. we want _someone_ to use the
>> airtime, the question is who.
>> #3 and #4 are closely related.
>> If you follow my approach (transmit immediately if you can, aggregate when
>> you have a queue), the code really has one mode (plus queuing). "If you
>> have a Transmit Oppertunity, transmit up to X packets from the queue", and
>> it doesn't matter if it's only one packet.
>> If you delay the first packet to give you a chance to aggregate it with
>> others, you add in the complexity and overhead of timers (including
>> cancelling timers, slippage in timers, etc) and you add "first packet,
>> start timers" mode to deal with.
>> I grant you that the first approach will "saturate" the airtime at lower
>> traffic levels, but at that point all the stations will start aggregating
>> the minimum amount needed to keep the air saturated, while still minimizing
>> latency.
>> I then expect that application related optimizations would then further
>> complicate the second approach. there are just too many cases where small
>> amounts of data have to be sent and other things serialize behind them.
>> DNS lookup to find a domain to then to a 3-way handshake to then do a
>> request to see if the <web something> library has been updated since last
>> cached (repeat for several libraries) to then fetch the actual page
>> content. All of these thing up to the actual page content could be single
>> packets that have to be sent (and responded to with a single packet),
>> waiting for the prior one to complete. If you add a few ms to each of
>> these, you can easily hit 100ms in added latency. Once you start to try and
>> special cases these sorts of things, the code complexity multiplies.
>> So I believe that the KISS approach ends up with a 'worse is better'
>> situation.
>> David Lang
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>> Make-wifi-fast at lists.bufferbloat.net
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