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

[Baptiste Jonglez]

Dear Babel hackers,

I'm currently testing my new babel-rtt code, which uses 32-bits
timestamps to achieve a µs-resolution on RTT.

A graph showing the RTT between two directly-connected hosts (Gigabit
Ethernet) can be found in [1].

The two hosts are laptops, directly connected with an Ethernet cable.
Both have Gigabit interfaces.  One has a fast Intel Core2 duo CPU (2.4
GHz), while the other has a slow AMD E2-1800 APU (1.7 GHz).

Do note that the RTT is expressed in µs, not ms.  A few comments,
prompted by a discussion with Juliusz and Matthieu:

- the ping6 result is surprising, as two groups of points clearly
  appear, separated by about 200 µs.  It seems that interrupts might
  be responsible for this, but it's still somewhat unclear.

- 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.  Context-switching and syscalls can probably account for
  these 400 µs: a quick test, on the slower laptop, showed that
  calling the gettimeofday() syscall takes about 100 µs.

Further comments are welcome.

[1] http://ze.polyno.me/babel/32bits-rtt-ethernet.pdf
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