Devel Digest, Vol 27, Issue 59

Kim Quirk kim at
Mon May 12 09:59:35 EDT 2008

The work that Pol is doing on mesh is all in development now... nothing that
has been released in a signed build. So I would NOT recommend that many
people try to upgrade to this. Plus I believe the work he is doing requires
api changes, so other activities probably won't work collaboratively at all
(Pol, you can confirm this).

We should be careful to only recommend development builds to people
technical enough to understand how to downgrade if they get into trouble and
fully understand that they will probably lose all their data, etc. (like
developers). Probably there are very few teachers who have enough technical
understanding to be able to support a classroom of different builds.

Other thoughts on this?

On Fri, May 9, 2008 at 9:01 AM, Greg Smith (gregmsmi) <gregmsmi at>

> Hi Polychronis,
> Thanks for sharing the results. Did you use a wireless AP or active
> antenna? If you can include a few details on that it will help. Can you
> also include the XO build # and XS build and config if relevant?
> Would you say that this test passed? That is, can we recommend that
> schools use the chat activity with one chat session which all join?
> Lastly, can you tell us what kind of testing time and focus you will
> have in the near future? I believe there is a mesh test lab coming up at
> Nortel in Ottawa as well. Any feedback on test capacity and plans there
> is appreciated too.
> I ask because there is recent feedback on mesh issues from a teacher at
> Lambayeque, Peru and
> a teacher in Uruguay has asked about supported Mesh features too. The
> Lambayeque page says: they wish they knew in advance that Acoustic
> Measure Activity would not work with 6 groups of two students each.
> That's mostly an issue with activity design and our communication about
> what activities support but it does raise a good test case (6 groups of
> 2 sharing a single activity).
> I think both (Peru and Uruguay) teachers can help define meaningful mesh
> use cases which will be applicable in many schools. I want to set the
> right expectation on our capacity before I ask them to spend a lot of
> time working with us.
> I can start by telling them that chat as you describe above will work
> well, if you agree. Then we can follow up to gather more details on how
> they want to use the mesh.
> The good news is they are motivated to use the mesh which helps validate
> one design goal of the XO. Now we just need to understand how they want
> to use it :-)
> It looks like you are focused on finding the maximum scale of Xos which
> can be in a mesh. That's clearly important info too. I'm just checking
> if you have capacity to look at a few other test scenarios as well.
> Thanks,
> Greg S
> ****************************
> Message: 5
> Date: Fri, 09 May 2008 03:29:51 -0400
> From: Polychronis Ypodimatopoulos <ypod at>
> Subject: 65-node simple mesh test (and counting... ;-)
> To: OLPC Development <devel at>, Sugar ml
>        <sugar at>,     viral at
> Message-ID: <4823FD6F.9090209 at>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Dear devel,
> Here are the latest results from Cerebro's (
> 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.
> 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.
> 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.
> 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
> <>
> _______________________________________________
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> Devel at
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