65-node simple mesh test (and counting... ;-)

Kim Quirk kim at laptop.org
Sat May 10 08:51:00 EDT 2008

This looks great, Poly. Can you please put this test plan and results into a
wiki page, preferably linked somewhere off the main 'Testing' page.


On Fri, May 9, 2008 at 3:29 AM, Polychronis Ypodimatopoulos <ypod at mit.edu>

> Dear devel,
> Here are the latest results from Cerebro's (http://cerebro.mit.edu)
> scaling properties. A 65-node testbed was used (703, Q2D14). The
> NetworkManager had to be disabled in order to stabilize the behavior of
> each XO's wireless interface. Unfortunately, the difficulty and time
> necessary to manage increasingly more nodes is linear (given that the
> NetoworkManager is disabled ;-), but increases steeply.
> ** Test plan:
> Cerebro was started on all 65 laptops almost at the same time. We
> attempted to emulate the "65 children turn on their laptops in class at
> the same time" scenario. With Yani's help, it took about 5 seconds for
> both of us to press 'enter' on all laptops. Each XO would discover each
> other, exchange profile information and keep exchanging
> presence/discovery information.
> ** Measurements:
> Quantitative:
> According to the protocol, presence (mac address) arrives about other
> XOs first, then the profile for the newly arrived mac address is queried
> and finally the profile is cached. We assume that initially each XO has
> no cached information about other XOs. As a result, every XO will query
> everyone else.
> We measured the time it took for each XO to discover and exchange
> profile information with everyone else, bandwidth usage at all times
> (during profile exchange and after the network stabilized when all
> profiles were received everywhere)
> Qualitative:
> Collaboration was tested on all 65 nodes: one shared a chat session,
> everyone else joined. The chat session was based on Cerebro's
> collaboration model.
> ** Results:
> Discovery and profile information:
> The following graph shows arrival of profile information at each XO from
> other XOs a function of time. Each bar is a 3-second bucket representing
> the average number of profile arrivals during this 3-second period. The
> standard deviation is shown with the blue lines.
> http://wiki.laptop.org/images/a/af/65-arr-1.png
> The following graph is the cumulative distribution function. It shows
> that, on average, each XO has received about 95% of the profiles of the
> rest of the nodes within just 20 seconds. This performance boost is due
> to the fact that each XO queried for its profile, responds by
> broadcasting the profile, instead of unicasting it to the requester. As
> a result, the other nodes receive the profile too and the next node is
> queried, yielding a linear cost, instead of a quadratic one.
> http://wiki.laptop.org/images/7/72/65-cdf-1.png
> Bandwidth usage:
> The following wireshark snapshot shows bandwidth usage that peaks
> momentarily at about 60kbytes/sec. The snapshot is also in accordance
> with the first graph above, showing that after about 55 seconds the
> network stabilizes. After the network stabilizes, bandwidth usage drops
> to 1 packet every 3 seconds (less than 500bytes/sec), as the arrival
> rate adapts to the density of the network.
> http://wiki.laptop.org/images/5/51/Bandwidth-presence-info-1.png
> Chat session:
> Before the experiment was started, a node shared a chat session and all
> 64 nodes joined consistently. I sent a few chat messages from a couple
> of XOs and were received on all other XOs.
> ** Other notes
> After about 6.4 hours of continuous operation on all 65 nodes, Cerebro
> shows stable memory usage (<10MB) and consistent CPU usage (83 minutes
> of CPU usage in 'top').
> Comments/suggestions?
> Pol
> --
> Polychronis Ypodimatopoulos
> Graduate student
> Viral Communications
> MIT Media Lab
> Tel: +1 (617) 459-6058
> http://www.mit.edu/~ypod/ <http://www.mit.edu/%7Eypod/>
> _______________________________________________
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