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Archive for December 20th, 2010

An Atom for the Sherline Milling Machine

Somewhat against the recommendations of the experts on the EMC2 mailing list, I bought a Foxconn R30-D2 with an Intel Atom D520 from Newegg during a sale: add 2 GB of memory from Crucial, a spare SATA drive from my collection, and it’s ready to go. I also bashed a spare parallel printer port card into the box, although it isn’t really needed right now: unlike the Intel system board, Foxconn brings the on-board parallel port directly to the back panel.

The Foxconn support site is a nightmare and was, AFAICT, dead for the first few weeks I had the box. The key fact to remember is that the -D2 part of the number specifies the system board / CPU, so the same downloads / BIOS updates apply to the R10, R20, R30, and R40 models. There is no new BIOS available to fix the “fan runs all the time” problem reported by so many people.

I installed Ubuntu 10.04 LTS from the distro CD, then ran the EMC2 installation script. All that is routine, as described there. You probably want to install the EMC2 Live CD, though, and to get much the same result with less fiddling.

Turn off Hyperthreading in the BIOS, which seems to make the RTAI real-time hypervisor happier. Under those conditions, the default install has an interrupt latency of about 13 µs.

The Atom D520 is a dual-core processor and you can devote one core to EMC2’s real-time functions, thus eliminating much of the usual contention and interrupt latency. That works surprisingly well and is completely automagic after you add the isolcpus=1 kernel option to the appropriate line in /boot/grub/grub.cfg. Thusly:

linux	/boot/vmlinuz-2.6.32-122-rtai root=UUID=57fe2b04-ffe4-4de3-a597-89bd4ed01018 ro  vga=758  noquiet nosplash isolcpus=1

With that done, the latency drops down under 8 µs, which is entirely satisfactory. I can push it to 10 µs by doing stupid things: scrubbing a glxgears window over a Flash video in Firefox, for example.

The catch is that the wonderful new grub2 bootloader rewrites its boot configuration file on the fly, based on a set of rules that, evidently, cannot apply different kernel configuration parameters to different kernels within the same partition. As a result, you must choose between:

  • Running stock Ubuntu on one core
  • Manually tweaking grub.cfg after every kernel update

Given that an Atom isn’t exactly a blinding flash and a deafening report in the performance department, I opted for the second method. If Ubuntu was still using Legacy Grub, then I’d just tweak menu.lst and be done with it. This is, I suppose, progress.

Memo to Self: Adjust grub.cfg every mumble time.

[Update: There’s now a fix for that, as described in the EMC2 wiki. Go for it!]

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