[Bloat] powerboost and sqm

[Benjamin Cronce]

Strict token bucket without fair queuing can cause packetloss bursts for
all flows. In my personal experience when dealing with a low(single digit)
RTT, I would find that my ex-50Mb connection would accept a 1Gb burst and
ACK all of the data. Then the sender would think I had a 1Gb link and keep
sending at 1Gb. Around the 200ms mark, there would be a steep slope where
all of my traffic would suddenly see ~5% loss for the rest of that second.
Once steady state was reached, it was fine. The issue seemed to have a
baseline relative to the ratio between the provisioned rate and the burst
rate, with a dynamic multiplier  not-quite-linearly driven by the link's
current utilization. At ~0% average utilization, bursts that lasted longer
than the bucket could induce maximum, and not much of an issue past 80%.

I could reliably recreate the issue by loading a high bandwidth video on
youtube and jumping around the timeline to unbuffered segments. I had
anywhere from 6ms to 12ms latency to youtube CDNs depending on the route
and which datacenter. Not only could I measure the issue with icmp at 100
samples per second, but I could reliably see issues in-game with either UDP
or TCP based games. Simply shaping to 1-2Mb below my provisioned rate and
enabling Codel seemed to alleviate the issue into not-noticing.

On Sun, Jul 1, 2018 at 4:50 PM Jonas Mårtensson <martensson.jonas at gmail.com>
wrote:

>
>
> On Sat, Jun 30, 2018 at 9:46 AM Pete Heist <pete at heistp.net> wrote:
>
>>
>> On Jun 30, 2018, at 8:26 AM, Jonas Mårtensson <martensson.jonas at gmail.com>
>> wrote:
>>
>>>
>>> I played around with flent a bit, here are some example plots:
>>
>> https://dl.dropbox.com/s/facariwkp5x5dh1/flent.zip?dl=1
>>
>> The short spikes are not seen with flent so I'm led to believe these are
>> just a result of running the "Hi-Res" dslreports test in a browser. In the
>> flent rrul test, up to about 10 ms induced latency can be seen during the
>> "powerboost" phase but after that it is almost zero. I'm curious about how
>> this is implemented on the ISP side. If anything, sqm seems to induce a bit
>> more latency during the "steady-state" phase.
>>
>>
>> You may also want to try running flent with --socket-stats and making a
>> tcp_rtt plot. You should see a significant difference in TCP RTT between
>> sfq and anything that uses CoDel.
>>
>
> In case anyone is curious I tried this and the tcp rtt plot looks very
> similar to the ping rtt plot, i.e. the latencies are the same.
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