[Cerowrt-devel] Proper AQM settings for my connection?

Sebastian Moeller moeller0 at gmx.de
Sat Dec 21 05:32:24 EST 2013

Hector Ordorica <hechacker1 at gmail.com> wrote:
>I'm running 3.10.13-2 on a WNDR3800, and have used the suggested
>settings from the latest draft:
>I have a 30Mb down / 5Mb upload cable connection.
>With fq_codel, even undershooting network upload bandwidth by more
>than 95%, I'm seeing 500ms excessive upload buffering warnings from
>netalyzr. Download is ok at 130ms. I was previously on a 3.8 release
>and the same was true.

So I have been fooled by netalyzr before just as you now. Netalyzr uses a very peculiar probe to measure the depth of the buffers: a totally nonreactive inelastic "flood" of UDP packets of relative short duration. The only real world traffic that looks like this is a denial of service attack on your router. Fq_codel tries very hard to be a good citizen that steers flows gently to their fair share of the bandwidth, in case flows do not react fq_codel will slowly take the gloves of so to say and restrict these flows more aggressively. The netalyzr probe now is too short for fq_codel to actually get serious in its packet dropping. Now real traffic typically, be it TCP or UDP tries to adjust to dropped packets by reducing the transmission rate. In other words netalyzt measures a sort of worst case buffering for fq_codel. Note for pfifo_fast this worst case is actually something you encounter with real traffic as well. So what netalyzr is missing is a report telling you whether the reported buffering will increase the overall latency of the system, or not....
      To summarize unless you see UDP floods as a typical use case for your internet connection, the netalyzr buffering numbers have no great significance for day to day use of your internet connection, if your are using a modern qdisc like fq_codel or pie.

     As Dave taught me in the past, you can easily test this hypothesis by modifying the limit parameter of fq_codel in simple,
.qos or simplest.qos. The larger limit and the slower the link speed in the measured direction the greater the reported buffering.

>With pie (and default settings), the buffer warnings go away:
>And the connection performs very well while torrenting and gaming.
>Should I try new code? Or can I tweak some variables and/or delay
>options in scripts for codel?
>Thanks for your work,
>Cerowrt-devel mailing list
>Cerowrt-devel at lists.bufferbloat.net

Hi Hector,
Sent from my Android phone with K-9 Mail. Please excuse my brevity.

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