[Cerowrt-devel] Ideas on how to simplify and popularize bufferbloat control for consideration.

[David Lang]

On Sun, 27 Jul 2014, Sebastian Moeller wrote:

> Hi David,
>
> On Jul 26, 2014, at 23:45 , David Lang <david at lang.hm> wrote:
>
>> On Sat, 26 Jul 2014, Sebastian Moeller wrote:
>>
>>> On Jul 26, 2014, at 22:39 , David Lang <david at lang.hm> wrote:
>>>
>>>> by how much tuning is required, I wasn't meaning how frequently to tune, 
>>>> but how close default settings can come to the performance of a expertly 
>>>> tuned setup.
>>>
>>> 	Good question.
>>>
>>>>
>>>> Ideally the tuning takes into account the characteristics of the hardware 
>>>> of the link layer. If it's IP encapsulated in something else (ATM, PPPoE, 
>>>> VPN, VLAN tagging, ethernet with jumbo packet support for example), then 
>>>> you have overhead from the encapsulation that you would ideally take into 
>>>> account when tuning things.
>>>>
>>>> the question I'm talking about below is how much do you loose compared to 
>>>> the idea if you ignore this sort of thing and just assume that the wire is 
>>>> dumb and puts the bits on them as you send them? By dumb I mean don't even 
>>>> allow for inter-packet gaps, don't measure the bandwidth, don't try to pace 
>>>> inbound connections by the timing of your acks, etc. Just run BQL and 
>>>> fq_codel and start the BQL sizes based on the wire speed of your link 
>>>> (Gig-E on the 3800) and shrink them based on long-term passive observation 
>>>> of the sender.
>>>
>>> 	As data talks I just did a quick experiment with my ADSL2+ koine at 
>>> home. The solid lines in the attached plot show the results for proper 
>>> shaping with SQM (shaping to 95% of del link rates of downstream and 
>>> upstream while taking the link layer properties, that is ATM encapsulation 
>>> and per packet overhead into account) the broken lines show the same system 
>>> with just the link layer adjustments and per packet overhead adjustments 
>>> disabled, but still shaping to 95% of link rate (this is roughly equivalent 
>>> to 15% underestimation of the packet size). The actual theist is 
>>> netperf-wrappers RRUL (4 tcp streams up, 4 tcp steams down while measuring 
>>> latency with ping and UDP probes). As you can see from the plot just getting 
>>> the link layer encapsulation wrong destroys latency under load badly. The 
>>> host is ~52ms RTT away, and with fq_codel the ping time per leg is just 
>>> increased one codel target of 5ms each resulting in an modest latency 
>>> increase of ~10ms with proper shaping for a total of ~65ms, with improper 
>>> shaping RTTs increase to ~95ms (they almost double), so RTT increases by 
>>> ~43ms. Also note how the extremes for the broken lines are much worse than 
>>> for the solid lines. In short I would estimate that a slight misjudgment 
>>> (15%) results in almost 80% increase of latency under load. In other words 
>>> getting the rates right matters a lot. (I should also note that in my setup 
>>> there is a secondary router that limits RTT to max 300ms, otherwise the 
>>> broken lines might look even worse...)
>>
>> what is the latency like without BQL and codel? the pre-bufferbloat version? 
>> (without any traffic shaping)
>
> 	So I just disabled SQM and the plot looks almost exactly like the broken 
> line plot I sent before (~95ms RTT up from 55ms unloaded, with single pings 
> delayed for > 1000ms, just as with the broken line, with proper shaping even 
> extreme pings stay < 100ms). But as I said before I need to run through my ISP 
> supplied primary router (not just a dumb modem) that also tries to bound the 
> latencies under load to some degree. Actually I just repeated the test 
> connected directly to the primary router and get the same ~95ms average ping 
> time with frequent extremes > 1000ms, so it looks like just getting the 
> shaping wrong by 15% eradicates the buffer de-bloating efforts completely...

just so I understand this completely

you have

debloated box <-> ISP router <-> ADSL <-> Internet <-> debloated server?

and are you measuring the latency impact when uploading or downloading?

I think a lot of people would be happy with 95ms average pings on a loaded 
connection, even with occasional outliers. It's far better than sustained 
multi-second ping times which is what I've seen with stock setups.

but if no estimate is this bad, how bad is it if you use as your estimate the 
'rated' speed of your DSL (i.e. what the ISP claims they are providing you) 
instead of the fully accurate speed that includes accounting for ATM 
encapsulation?

It's also worth figuring out if this problem would remain in place if you didn't 
have to go through the ISP router and were runing fq_codel on that router. As 
long as fixing bufferbloat involves esoteric measurements and tuning, it's not 
going to be solved, but if it could be solved by people flahing openwrt onto 
their DSL router and then using the defaults, it could gain traction fairly 
quickly.

>> I agree that going from 65ms to 95ms seems significant, but if the stock 
>> version goes into up above 1000ms, then I think we are talking about things 
>> that are ‘close'
>
> 	Well if we include outliers (and we should as enough outliers will 
> degrade the FPS and voip suitability of an otherwise responsive system 
> quickly) stock and improper shaping are in the >1000ms worst case range, while 
> proper SQM bounds this to 100ms.
>
>>
>> assuming that latency under load without the improvents got >1000ms
>>
>> fast-slow (in ms)
> ideal=10
> untuned=43
> bloated > 1000
>
> 	The sign seems off as fast < slow? I like this best ;)

yep, I reversed fast/slow in all of these

>>
>> fast/slow
>> ideal = 1.25
>> untuned = 1.83
>> bloated > 19
>
> 	But Fast < Slow and hence this ration should be <0?

1 not 0, but yes, this is really slow/fast

>> slow/fast
>> ideal = 0.8
>> untuned = 0.55
>> bloated = 0.05
>>
>
> 	and this >0?

and this is really fast/slow

David Lang