[Cake] [Cerowrt-devel] ingress rate limiting falling short

[Dave Taht]

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.

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
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