Parallel computing is the next paradigm shift, everybody knows this, but not everyone is taking the proper action to face it adequately. One thing to do is to read on the subject and force oneself to code using threads and various degrees of parallelism; and that’s pretty easy now that a quad core machine doesn’t cost all that much. But the next step, distributed computing, necessitates, well, more than one machine, and if you have different levels of memories and communication channels, all the better.

So out of a bunch of old x86 PCs, I’ve decided to build my own portable mini-cluster with 8 nodes. Nothing all that impressive, but still plenty of fun to build.

Getting the hardware. I got the computers (iPaqs) from my friend Nicolas for next to nothing. He got them from eBay, nicely delivered to his home on a crate or something. For the next generation, if there’s one, I will consider getting a batch of identical computers from a shop like Insertech, they often have deals like Pentium4s for 50$ each. In either cases, I don’t think you’ll have to resort to dumpster diving to find inexpensive, yet working, computers.

I also got a CRT screen (I had no LCD to spare), a keyboard, and 10 6′ Ethernet cables from Insertech and a local shop to complete the hardware. I had a 16-ports 100-mbits Ethernet switch, so I’ll reuse it as the cluster switch (these computers do not handle gBit Ethernet anyway).

Disassembling the iPaqs. These machines contain an inordinate amount of junk. This is what an assembled iPaq looks like:

and that’s what it looks like once taken apart

You don’t need much for that step, except a set of torx screwdriver bits—just a set with the “security” ones.

You cannot not be shocked by the amount of steel and plastic such a computer can hold. I think more than half of the weight of the computer is junk. Good thing computers usually last rather long, because that’d be quite wasteful to just throw all that junk to the curb. In fact, I made sure all parts were sent to the right recycling facilities.

It takes about 20 minutes to disassemble one of these computers completely, as shown in the above picture. So, about two and a half hours’ work and you’ve disassembled the eight computers.

Testing the hardware. Before using of a computer in the cluster, I made sure they were fully functional: USB ports, networks, IDE controller, and especially memory. Memory is especially important to test, because bad memory chips will result not in outright crashes, but in a series of unpredictable symptoms. If your machine acts like it’s possessed by a spawn of Cthulhu, despite being virus-free, take an hour to run a memory test. Fortunately, it’s most easy using memtest86+, conveniently bundled as a boot image on Ubuntu (either from the hard drive or from the live CD, anyway you can even make a boot floppy with it).

Testing memory takes about 30 to 45 minutes on those machines, so I had plenty of time to do other stuff in the mean time, like sketching the final assembly. Turns out, however, that the memory of the machines was mostly OK, except for one 64MB stick with a sticky 1-bit. As I got ten machines, I got a spare from one of the two other machines, so everything’s good.

Assembling the cluster. I used 36″ gauge 10 screw rods, with 24 threads per inch. These can be found in any home hardware/improvement centers for something like 2$ each, they’re really inexpensive. I also got a box of 100 matching hex-nuts, also quite inexpensive (4$ each or so). The four rods (I ended up using only three) plus two 100 hex-nuts boxes cost less than 25$, tax included. Since they didn’t have the right size to fit directly into PC-boards screw holes, I used a titanium drill bit to widen these a tiny bit, to make sure the threaded rods would glide in nicely.

For the first board, I left about 1.5 inch of threaded rod under the board, so I would be able to insert the rod into the side panels that would eventually hold everything together. The rest is simply a question of patience: you slide a board in on top of the hex-nuts, you screw three more hex-nuts to fasten the board into place, then three more to support the next stacked board, about 4″ apart. You also have to add two extra nuts to hold the power switch board.

So screwing those may take a long time, but I eventually used a drill and a rubber band to act as a transmission strap wrapped around an hex-nut to screw this a bit faster. Since this is an type job, it gets faster and faster as you reach the top of the stack. For the last board, that is, computer0 , I left a bit of extra space to mount the USB and audio plugs board, as well as a small hard-drive that will later serve as the dhcp/boot-p image server.

Cutting MDF is easy, but incredibly messy. I did that a few hours before starting assembling the boards onto the rod and left the dust in my shop clear out a bit. MDF is versatile, rather strong, and about as hard to work as cardboard, which made piercing holes, handles (the big round holes) and sanding corners quite easy. For all holes, I used a drill press. It ensures, for one thing, that the holes are indeed perpendicular to the wood (or MDF) sheet’s plane.

The rest is assembling the whole thing: first the bottom with a side (using screws and steel right-angle joints, not visible on the pictures as under the cabinet) sliding in the rods with the motherboards, fastening the PSUs, then the other side panel. And you’re done. So I thought.

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Turns out that even if the rods and motherboards assembly is rather light—a few pounds—the 10-gauge rods themselves aren’t strong enough to hold all this without bending, something I didn’t foresee. I added an extra MDF board (shows only a little on the picture) to lift the boards and make sure it shows a straight back.

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I’m thinking of several names for this first experiments. Surely, CFM-00 would be appropriate (I’ll let you make out what the letters stand for). Maybe MDF-00 would work out as well.

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All in all, it’s a 15 to 20h job. About 3h to disassemble the computers, a few minutes to cut the MDF, a few hours to stack and fasten the motherboard together and about 10 to test the individual computers. It’s basically a week-end project but for many reasons I completed it over a period of six months, from acquiring the computers from Nicolas to fastening the last hex-bolt.

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The next step is to figure out what software to use. Bootp is a must in this case, and I am considering using Mosix2. So, that’s were I’m at with this project. Feel free to comment or suggest parallel and distributed processing software and frameworks (but I’d like to stay in the Linux/open source family of solutions).

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