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

[Sebastian Moeller]

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...

> 
> 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 ;)

> 
> fast/slow
> ideal = 1.25
> untuned = 1.83
> bloated > 19

	But Fast < Slow and hence this ration should be <0?

> slow/fast
> ideal = 0.8
> untuned = 0.55
> bloated = 0.05
> 

	and this >0?

> rather than looking at how much worse it is than the ideal, look at how much closer it is to the ideal than to the bloated version.
> 
> David Lang
>