Comcast XFINITY → Bufferbloat-long read.

Incorrect, but we're all here to learn.

Awesome, thank you for posting about this! More people need to know.

Need to know what?

Need to know how I greatly increased my Internet service responsiveness by simply preventing my SB6141 cable modem from having its internal buffer filled!

Here is very old document that explains it well:
»www.faqs.org/docs/Linux- ··· WTO.html

OP, I discovered and corrected the same issue you describe. However the fix was specific to the configuration of our CMTS.

Note: I'm not a Comcast employee nor was this on Comcast's network.

To recap, we could max out the upstream qos of a DOCSIS 2.0 modem without affecting the network from surfing web pages, albeit more slowly than normal. However once we did the same test, with same qos, but on a D3.0 modem network connectivity was pretty much nonexistent. Same as you described.

We have a single Cisco UBR 10K driving our DOCSIS plant. The fix was related to rate-adapt and rate-limit config options we had set on our cable interfaces.

I'm not saying that the buffer bloat algorithm wouldn't improve performance, but it's likely not the cause of the issue you're seeing.

I'd try to run this up the chain at Comcast.