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Plug the modem - get the problems...Plug the modem - get the problems...Posted Dec 12, 2011 15:46 UTC (Mon) by khim (subscriber, #9252)In reply to: Bufferbloat: Dark Buffers in the Internet (ACM Queue) by marcH Parent article: Bufferbloat: Dark Buffers in the Internet (ACM Queue)
This keeps coming... why would the 1Gb/s link hold the data on behalf on the 1Mb/s link? Buy modem, attach to computer, get the problem. Computer only know about 1GBit link to the modem and so allocates 1MB buffer, router only gets 1Mbit because your line is not ideal... instant 1000x impedance mismatch.
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Plug the modem - get the problems... Posted Dec 12, 2011 15:55 UTC (Mon) by marcH (subscriber, #57642) [Link]
> instant 1000x impedance mismatch.
Not the problem.
What's happening here is bufferbloat inside *the modem*; NOT inside the computer. Make the modem adjust its queue size depending on the speed of each outgoing link and your problem is solved.
The problem is the modem having the same buffer size on every link (the buffer is probably even shared across the links). Simple laziness from the designers.
Plug the modem - get the problems... Posted Dec 12, 2011 15:57 UTC (Mon) by marcH (subscriber, #57642) [Link]
> What's happening here is bufferbloat inside *the modem*; NOT inside the computer.
... unless you have Ethernet flow control enabled, in which case you might have bufferbloat in BOTH places because of backpressure! Disable flow control right now since it's not compatible with Van Jacobson congestion control.
Plug the modem - get the problems... Posted Dec 16, 2011 2:47 UTC (Fri) by quanstro (guest, #77996) [Link]
ethernet flow control, at least through switches, makes
tcp-style flow control work better! at least that's been my experience.
ethernet flow control Posted Dec 16, 2011 16:57 UTC (Fri) by marcH (subscriber, #57642) [Link]
Your mileage may vary. The effect of Ethernet flow control depends on a wide range of parameters.
Ethernet flow control is effectively chaining queues across devices. Since the aggregated queue is bigger I can see how it *may in some cases* enhance TCP throughput. But it will obviously make any existing bufferbloat even worse.
Most importantly, Ethernet flow control will create HOL blocking.
Your mileage may vary.
Some old musings with Ethernet flow control: http://marc.herbert.free.fr/noq/
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