[Babel-users] us vs ns resolution in babel rtt branch

[Baptiste Jonglez]

On Sun, Mar 15, 2015 at 03:00:12PM -0700, Dave Taht wrote:
> I still kind of like getting down to ns resolution here. A single 64
> byte packet at 10gigE takes like 64ns. I have discussed elsewhere my
> hopes for using babel messages to determine the capacity and
> utilization of a given network interface before, depending on
> fq_codel's behavior as a substrate....

I still believe other factors dwarf these 64 ns...

I had done some measurements with the µs-precision code, on a direct
gigabit link between two hosts (no switch):

  http://files.polyno.me/babel/evalperf/figures/32bits-rtt-ethernet-thinkbad-gilead.svg

As you notice, ping6 reports 400 µs, babeld reports 800 µs, while the
theoretical latency is 512 ns for a 64-bytes packet and 12 µs for a full
1500-bytes packet.  I am neglecting propagation delays (that would amount
to about 50 ns for a 10-meter cable).

So, the actual latency (measured either with ping or babel) is 3 orders of
magnitude higher than the theoretical latency.  Do note that my tests were
done with low-end hardware, so it might be better with high-performance
NICs.

Do you have latency measurements on 10G links?  If so, what tool do you
use?  I believe both ping and babel are not very accurate at these
timescales.

Baptiste
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