[Bloat] powerboost and sqm

[Jonas Mårtensson]

Hi,

I have a 100/100 Mbit/s (advertised speed) connection over fiber (p2p, not
PON). The actual link rate is 1 Gbit/s. My ISP seems to be using
burst-tolerant shaping (similar to powerboost) as can be seen in this
speedtest where the download rate is 300+ Mbit/s and the upload rate is
around 150 Mbit/s for the first few seconds:

http://www.dslreports.com/speedtest/35205027

It can be discussed why they are doing this but my questions are more
related to the impact on the quality of my connection. The ISPs shaper used
to introduce some bufferbloat, especially on the downlink, and I've been
using sqm for a while to mitigate this. But recently they seem to have
changed some configuration since the bufferbloat is now almost zero, except
for some very short spikes which only show up when I check "Hi-Res
BufferBloat" in test preferences (see speedtest above). When I enable sqm
on my router with htb/fq_codel or cake the spikes disappear:

htb/fq_codel:
http://www.dslreports.com/speedtest/35205620

cake:
http://www.dslreports.com/speedtest/35205718

Another difference is that the "Re-xmit" percentage (which I guess is
related to packet loss) is much higher without sqm enabled. Intuitively
this makes sense since temporarily allowing a higher rate should result in
more buffer overflow when the rate is decreased.

So, what do you think:

- Are the latency spikes real? The fact that they disappear with sqm
suggests so but what could cause such short spikes? Is it related to the
powerboost?

- Would you enable sqm on this connection? By doing so I miss out on the
higher rate for the first few seconds. What are the actual downsides of not
enabling sqm in this case?

/Jonas
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