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

Dave Taht dave.taht at gmail.com
Fri May 13 13:49:51 EDT 2016

I try to stress that single tcp flows should never use all the bandwidth
for the sawtooth to function properly.

What happens when you hit it with 4 flows? or 12?

nice graph, but I don't understand the single blue spikes?

On Fri, May 13, 2016 at 10:46 AM, Bob McMahon <bob.mcmahon at broadcom.com>

> 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.
> [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
>> _______________________________________________
>> Make-wifi-fast mailing list
>> Make-wifi-fast at lists.bufferbloat.net
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Dave Täht
Let's go make home routers and wifi faster! With better software!
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