[Cerowrt-devel] ingress rate limiting falling short

Wed Jun 3 18:34:24 EDT 2015

On Wed, Jun 3, 2015 at 3:27 PM, Dave Taht <dave.taht at gmail.com> wrote:

>
>
> On Wed, Jun 3, 2015 at 3:16 PM, Aaron Wood <woody77 at gmail.com> wrote:
>
>>
>> > On the 3800, it never meets the rate, but it's only off by maybe 5%.
>>>
>>>         As Jonathan pointed out already this is in the range of the
>>> difference between raw rates and tcp good put, so nothing to write home
>>> about ;)
>>>
>>
>> Yeah, I'm not too worried about that 5%, based on that explanation.
>>
>>
>>>
>>> > But on my new WRT1900AC, it's wildly off, even over the same
>>> performance range (I tested it from 80-220Mbps rates in 20Mbps jumps, and
>>> saw from 40-150Mbps.
>>>
>>>         So you started with the WRT1900AC where the wndr3800 dropped
>>> off? I wonder maybe the Belkin is also almost linear for the lower range?
>>
>>
>> Yeah, good point on a methodology fail.  I'll run another series of tests
>> walking up the same series of rate limits and see what I get.
>>
>>
>>> I also note we adjust the quantum based on the rates:
>>> from functions .sh:
>>> get_mtu() {
>>>
>> ... snip
>>
>>> }
>>>
>>> which we use in the htb invocations via this indirection:
>>> LQ="quantum `get_mtu $IFACE $CEIL`”
>>>
>>>
>> That is odd, and that's quite the aggressive curve on quantum, doubling
>> every 10-20Mbps.
>>
>> I did some math, and plotted out the quantum vs. bandwidth based on that
>> snippet of code (and assuming a 1500-byte MTU):
>>
>>
>>
>> And then plotted out the corresponding time in ms that each quantum
>> bytes (it is bytes, right?) is on the wire:
>>
>>
>> Which I think is a really interesting plot (and here are the points that
>> line up with the steps in the script):
>>
>> kbps = quantum = time
>> 20000 = 3000 = 1.2ms
>> 30000 = 6000 = 1.6ms
>> 40000 = 12000 = 2.4ms
>> 50000 = 24000 = 3.84ms
>> 60000 = 48000 = 6.4ms
>> 80000 = 96000 = 9.6ms
>>
>
>
>> So it appears that the goal of these values was to keep increases the
>> quantum as rates went up to provide more bytes per operation, but that's
>> going to risk adding latency as the time-per-quantum crosses the delay
>> target in fq_codel (if I'm understanding this correctly).
>>
>> So one thing that I can do is play around with this, and see if I can
>> keep that quantum time at a linear level (ie, 10ms, which seems _awfully_
>> long), or continue increasing it (which seems like a bad idea).  I'd love
>> to hear from whoever put this in as to what it's goal was (or was it just
>> empirically tuned?)
>>
>
> Empirical and tested only to about 60Mbits. I got back about 15% cpu to do
> it this way at the time I did it on the wndr3800.
>
> and WOW, thx for the analysis! I did not think much about this crossover
> point at the time - because we'd maxed on cpu long beforehand.
>

And most of my testing on x86 has been with this change to the htb quantum
entirely disabled and set to 1514.

the "production" sqm-scripts and my own hacked up version(s) have differed
in this respect for quite some time. (at least 6 months).

Great spot on this discrepancy.

:egg, otherwise, on face:

> I can certainly see this batching interacting with the codel target.
>
> On the other hand, you gotta not be running out of cpu in the first place.
> I am liking where cake is going.
>
> One of my daydreams is that once we have writable custom ethernet hardware
> that we can easily do hardware outbound rate limiting/shaping merely by
> programming a register to return a completion interrupt at the set rate
> rather than the actual rate.
>

>
>
>> >
>>> > I have no idea where to start looking for the cause.  But for now, I'm
>>> just setting my ingress rate MUCH higher than I should, because it's
>>> working out to the right value as a result.
>>>
>>>         It would be great to understand why we need to massively
>>> under-shape in that situation to get decent shaping and decent latency
>>> under load.
>>>
>>
>> Agreed.
>>
>> -Aaron
>>
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>>
>>
>
>
> --
> Dave Täht
> What will it take to vastly improve wifi for everyone?
> https://plus.google.com/u/0/explore/makewififast
>

-- 
Dave Täht
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