[Beowulf] A cluster of Arduinos

Vincent Diepeveen diep at xs4all.nl
Wed Jan 11 19:36:37 EST 2012


Yes this was impossible to explain to a bunch of MiT folks as well,
some of whom wrote your book i bet - yet the slower the processor,
the more of a true SMP system it is.

It's obvious that you missed that point.

Writing code for a multicore is tougher, from SMP constraints viewpoint,
than for a bunch of 70Mhz cpu's that have a millisecond latency to  
the other cpu's.

So it's far from demonstrating clusterprogramming. Lightyears away.

Emulation at a simple quadcore is in fact better representative than  
this.

If you want to get closer to clusterprogramming than this, just buy  
yourself off ebay
some barcelona core SMP system with 4 sockets. Say with energy  
efficient 1.8Ghz CPU's.

So with one of the first incarnations of hypertransport, as of course  
later on it dramatically improved.

Latency from cpu to cpu is some 300+ ns if you lookup randomly.

Even good programmers in game tree search have big problems working  
with those latencies.

Clusters are having latencies that are far worse than that. Yet as  
cpu speeds no longer increase much
and number of cores doesn't double that quickly, clusters are the way  
to go if you're CPU hungry.

Setting up small clusters is cheap as well. If i put in the name  
'mellanox' in ebay i see bunches of
cheap cards out there and also switches.

With a single switch you can teach half a dozen students. You can  
just connect the machines you already
got there onto a few switches and write MPI code like that.

Average cost per student also will be a couple of hundreds of dollars.

Vincent



On Jan 12, 2012, at 12:24 AM, Lux, Jim (337C) wrote:

>
>
> -----Original Message-----
> From: beowulf-bounces at beowulf.org [mailto:beowulf- 
> bounces at beowulf.org] On Behalf Of Vincent Diepeveen
> Sent: Wednesday, January 11, 2012 2:47 PM
> To: Beowulf Mailing List
> Subject: Re: [Beowulf] A cluster of Arduinos
>
> Jim, your microcontroller cluster is not a rather good idea.
>
> Latency didn't keep up with the CPU speeds...
>
> --- You're missing the point of the cluster.  It's not for  
> performance (where I can't imagine that the slowest single CPU PC  
> out there wouldn't blow the figurative doors off).  It's to provide  
> a very inexpensive way to experiment/play/demonstrate loosely  
> coupled multiprocessor systems.
>
> --> for example, you could experiment with redundant message  
> routing across a fabric of nodes.  The algorithms are fairly  
> simple, and this gives you a testbed which is qualitatively  
> different than just simulating a bunch of nodes on a single PC.   
> There is pedagogical value in a system where you can force a link  
> error by just disconnecting the cable, and your blinky lights on  
> each node show what's going on.
>
>
> There is still too much years 80s and years 90s software out there,  
> written by the guys who wrote books about how to parallellize,  
> which simply doesn't scale at all at modern hardware.
>
> -->  I think that a lot of the theory of parallel processes is  
> speed independent, and while some historical approaches might not  
> be used in a modern system for good implementation reasons,  
> students and others still need to learn about them, if only as the  
> canonical approach.    Sure, you could do a simulation on a single  
> PC (and I've seen them, in Simulink, and in other more specialized  
> tools), but there's a lot of appeal to a hands-on-the-cheap- 
> hardware approach to learning.
>
> --> To take an example, if you set a student a problem of lighting  
> a LED on each node in a specified node order at  specified  
> intervals, and where the node interconnects are not specified in  
> advance, that's a fairly interesting homework problem.  You have to  
> discover the network connectivity graph, then figure out how to  
> pass the message to the appropriate node at the appropriate time.   
> This is a classic "hot plug network discovery" kind of problem, and  
> in the face of intermittent links, it's of great interest.
>
> --> While that particular problem isn't exactly HPC, it DOES relate  
> to HPC in a world where you cannot assume perfect processor nodes  
> and perfect communications links.  And that gets right to the whole  
> "scalability" thing in HPC.  It wasn't til the implementation of  
> Error Correcting Codes in logic that something like the Q7A  
> computer was even possible, because it was so large that you  
> couldn't guarantee that all the tubes would be working all the  
> time.  Likewise with many other aspects of modern computing.
>
> --> And, of course, in the spaceflight world, this kind of thing is  
> even more important.  A concept of growing importance is the  
> "fractionated spacecraft" where all of the functions that would  
> have been all in one physical vehicle are now spread across many  
> smaller pieces.  And one might reallocate spacecraft fractional  
> pieces between different virtual spacecraft.  Maybe right now, you  
> need a lot of processing power to do image compression and  
> analysis, so you want to allocate a lot of "processing pieces" to  
> the job, with an ad hoc network connection among them.  Later,  you  
> don't need them, so you can release them to other uses.  The pieces  
> might be in the immediate vicinity, or they might be some distance  
> away, which affects the data rate in the link and its error rates.
>
> --> You can legitimately ask whether this sort of thing (the  
> fractionated spacecraft) is a Beowulf (defined as a cluster  
> supercomputer built of commodity components) and I would say it  
> shares many of the same properties, especially in the early Beowulf  
> days before multicores and fancy interconnects were fashionable for  
> multi-thousand processor clusters.  It's that idea of building a  
> large complex device out of many basically identical subunits,  
> using open source/simple software to manage it.
>
>
> -->> in summary, it's not about performance.. it's about a teaching  
> tool for networking in the context of cluster computing.  You claim  
> we need to cast off the shackles of old programming styles and get  
> some new blood and ideas.  Well, you need to get people interested  
> in parallel computing and learning the basics (so at least they  
> don't reinvent the square wheel).  One way might be challenges such  
> as parallelization of game play; another might be working with  
> parallelized database; the way I propose is with experimenting with  
> message passing parallelization using dirt cheap hardware.
>
>
>
>
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