Using Raspberry Pi itself doesn t solve the whole problem of building a massively parallel supercomputer: how to connect all your compute cores together efficiently is a really big problem, which is why supercomputers are specially designed, not just made of commodity parts. That said, research units are really beginning to look at ARM cores as a power-efficient way to bring compute power to bear on exactly this problem: for example, this project that aims to simulate the human brain with a million ARM cores.

http://www.zdnet.co.uk/news/emerging-tech/2011/07/08/million-core-arm-machine-aims-to-simulate-brain-40093356/ "Million-core ARM machine aims to simulate brain"

http://www.eetimes.com/electronics-news/4217840/Million-ARM-cores-brain-simulator "A million ARM cores to host brain simulator"

It s very specialist, bespoke hardware, but conceptually, it s not far from the network of Raspberry Pis you suggest. Don t forget that ARM cores have all the features that JohnB mentioned the Xeon has (Advanced SIMD instead of SSE, can do 64-bit calculations, overlap instructions, etc.), but sit at a very different MIPS-per-Watt sweet-spot: and you have different options for what features are included (if you don t want floating-point, just buy a chip without floating-point), so I can see why it s an appealing option, especially when you consider that power use is the biggest ongoing cost for a supercomputer.

Seems unlikely to be a good/cheap system to me. Consider a modern xeon cpu. It has 8 cores running at 5 times the clock speed, so just on that basis can do 40 times as much work. Plus it has SSE which seems suited for this application and will let it calculate 4 things in parallel. So we re up to maybe 160 times as much work. Then it has multithreading, can do 64 bit calculations, overlap instructions etc. I would guess it would be at least 200 times faster for this kind of work.

Then finally, the results of at least 200 local "neurons" would be in local memory but on the raspberry pi network you d have to communicate between 200 of them... Which would be very much slower.

I think the raspberry pi is great and certainly plan to get at least one :P But you re not going to build a cheap and fast network of them that will compete with a network of "real" computers :P

Anyway, the fastest hardware for this kind of thing is likely to be a graphics card GPU as it s designed to run many copies of a small program in parallel. Or just program an fpga with a few hundred copies of a "hardware" neuron.

GPU and FPU do this kind of think much better then a CPU, the Nvidia GPU s that support CDUA programming has in effect 100 s of separate processing units. Or at least it can use the evolution of the pixel pipe lines (where the card could render mutiple pixels in parallel) to produce huge incresses in speed. CPU allows a few cores that can carry out reletivly complex steps. GPU allows 100 s of threads that can carry out simply steps.

So for tasks where you haves simple threads things like a single GPU will out preform a cluster of beefy CPU. (or a stack of Raspberry pi s)

However for creating a cluster running some thing like "condor" Which cand be used for things like Disease out break modelling, where you are running the same mathematical model millions of times with varible starting points. (size of out break, wind direction, how infectious the disease is etc.. ) so thing like the Pi would be ideal. as you are general looking for a full blown CPU that can run standard code.http://research.cs.wisc.edu/condor/

some well known usage of this aproach are "Seti" or "folding at home" (search for aliens and cancer research)

A lot of universities have a cluster such as this so I can see some of them trying the approach of mutipl Raspberry Pi s

But for simulating nurons in a brain you require very low latency between the nodes they are special OS s and applications that make mutiply systems act as one. You also need special networks to link it togather to give latency between nodes in terms of < 1 millsecond.

http://en.wikipedia.org/wiki/InfiniBand

the Raspberry just will not manage this in any way.

So yes I think people will make clusters out of them and I think they will be very pleased. But I think more university s and small organisations. They arn t going to compete with the top supercomputers.

Saying that we are going to get a few and test them against our current nodes in our cluster to see how they compare with a desktop that has a duel core 3.2ghz CPU and cost £650! I reckon for that we could get 25 Raspberries and they will use much less power, so will be interesting to compare. This will be for disease outbreak modelling.

I am undertaking a large amount of neural network research in the area of chaotic time series prediction (with echo state networks). Although I see using the raspberry PI s in this way will offer little to no benefit over say a strong cpu or a GPU, I have been using a raspberry PI to manage the distribution of simulation jobs to multiple machines. The processing power benefit of a large core will nail that possible on the raspberry PI, not just that but running multiple PI s in this configuration will generate large overheads of waiting for them to sync, data transfer etc. Due to the low cost and robustness of the PI, i have it hosting the source of the network data, as well as mediating the jobs to the Agent machines. It can also hard reset and restart a machine if a simulation fails taking the machine down with it allowing for optimal uptime.

Neural networks are expensive to train, but very cheap to run. While I would not recommend using these (even clustered) to iterate over a learning set for endless epochs, once you have the weights, you can transfer the learning effort into them.

Used in this way, one raspberry pi should be useful for much more than a single neuron. Given the ratio of memory to cpu, it will likely be memory bound in its scale. Assuming about 300 megs of free memory to work with (which will vary according to OS/drivers/etc.) and assuming you are working with 8 byte double precision weights, you will have an upper limit on the order of 5000 "neurons" (before becoming storage bound), although so many other factors can change this and it is like asking: "How long is a piece of string?"

Some engineers at Southampton University built a Raspberry Pi supercomputer:

I have ported a spiking network (see http://www.raspberrypi.org/phpBB3/viewtopic.php?f=37&t=57385&e=0 for details) to the Raspberry Pi and it runs about 24 times slower than on my old Pentium-M notebook from 2005 with SSE and prefetch optimizations.

It all depends on the type of computing you want to do. If you are doing very numerically intensive algorithms with not much memory movement between the processor caches and RAM memory then a GPU solution is indicated. The middle ground is an Intel PC chip using the SIMD assembly language instructions - you can still easily end up being limited by rate you can transfer data to and from RAM. For nearly the same cost you can get 50 ARM boards with say 4 cores per board and 2Gb RAM per board. That s 200 cores and 100 Gb of RAM. The amount of data that can be shuffled between the CPUs and RAM per second is very high. It could be a good option for neural nets that use large weight vectors. Also the latest ARM GPU s and the new nVidea ARM based chip (used in the slate tablet) have GPU compute as well.