Re: [Babel-users] Follow-up on the new babel-rtt branch: µs resolution

[Dave Taht]

On Wed, Oct 30, 2013 at 5:15 PM, Juliusz Chroboczek
<jch at pps.univ-paris-diderot.fr> wrote:
> Baptiste:
>
>> - Babel sees a RTT that is 400 µs higher than ping6.  The babel-rtt
>>   implementation timestamps outgoing message as late as possible, and
>>   timestamps incoming messages as early as possible, but it's not
>>   perfect.
>
> The good news, of course, is that the offset is pretty constant, so
> the samples computed by babeld are good enough to be input to the
> metric computation.  The smoothed samples would appear to have
> a precision of roughly 200us, which should be good enough for almost
> any application.

I will try to get an equivalent measurement on vastly weaker hardware
(ar71xx/ath9k) on my next set of builds (after ietf), ~2 weeks.

I would like measurements under various loads...

> Baptiste, I'm wondering if you can count the number of Ethernet
> switches that way.

In cut-through mode this switch claims 400ns or so latency.

http://ark.intel.com/products/76302/Intel-Ethernet-Switch-FM5224

Older models of this were better - 300ns or less.

>
>> I'm delighted to see this measurement as collected by babel
>
> I knew you would be :-)
>
>> my sekret plan was to be able to measure heavy traffic benchmarks vs
>> various qdiscs like the new "fq" and older fq_codel ones...
>
> Well, the reason we decided it's worthwile to improve the precision of
> our measurements is that, according to Jim, you had a secret plan to
> use RTT to measure link-layer congestion in Wifi meshes.

Yes, ~200us precision is nicely less than a txop. This will help.

> How many
> more secret plans do you have?

One or two. ;)

> -- Juliusz
>
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-- 
Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html