[ntp:questions] Detecting bufferbloat via ntp?

Rick Jones rick.jones2 at hp.com
Wed Feb 16 18:18:10 UTC 2011


Dave Täht <d at taht.net> wrote:
> Terje Mathisen <"terje.mathisen at tmsw.no"> writes:

> > Rick Jones wrote:
> >> Kevin Oberman<oberman at es.net>  wrote:
> >>
> >>> No, you probably won't. Both theoretical and empirical information
> >>> shows that overly large windows are not a good thing. This is the
> >>> reason all modern network stacks have implemented dynamic window
> >>> sizing.
> >>
> >>> As far as I know, Linux, MacOS (I think), Windows, and BSD (at least
> >>> FreeBSD) all do this and do it better then it is possible to do
> >>> manually. N.B. Windows XP probably does not qualify as "modern".
> >>
> >> Sadly, I see Linux's dynamic window sizing take the window to 4MB when
> >> 128KB would do.  I'm not familiar with the behaviour of the other
> >> stacks'

> I did a little testing with rick a couple days ago. It turned out his
> problem was not in his end nodes, but somewhere in his path between his
> two sites is something rather bloated.

Or rather, that even after setting the tx queue lengths to 32 packets,
a test between that system and one 7ms away still resulted in 4MB
socket buffers by the end of the test. Ie confirming that the linux
autotuning code was still willing to grow the windows larger than
necessary.

> A *little* is just fine. Bloated buffers - containing hundreds,
> thousands, tens of thousands of packets - which is what we are seeing
> today - is not.

Well, the BDP of a 10GbE link might actually be measured in thousands
of packets or more...  if my systems 7 ms apart were joined by a 10
GbE link, that would be a bit more then 5800, 1500 byte packets.  I'm
thinking that while we may have to configure queues in terms of number
of packets, we shouldn't think of them that way, but as length of
time.

rick jones
-- 
Process shall set you free from the need for rational thought. 
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...




More information about the questions mailing list