Category Archives: School

Lab Fridge Repair: Thinking with 3D Printing

I’m posting this because it’s such a nice example for the standard “What are 3D printers really good for?” question.

When I got to the lab today, I was told the can chute in our mini-fridge was broken. Inspection showed that too many of the little plastic inserts/bushings that retain the bars were missing and/or broken. This is a years-old cheap GE minifridge, so it isn’t even worth looking for OEM replacements.

Now we get to the “Thinking with 3D printing” part: I plucked one of the remaining ones, went over it with some calipers, transferred the measurements into OpenSCAD, and printed one off to test fit. The ID was a little tight, so I adjusted the model, printed 6 more, and fixed the problem.

In case the model is useful for anyone else: OpenSCAD and STL.

Important Details:

  • This took like an hour from start to finish, and wasn’t the only thing I was doing at the time. The printing itself was around 1 minute per insert.
  • The new inserts are better than the originals. Not quite as pretty in some ways (though they are blue and glow-in-the-dark, because that’s our current junk filament), but the fit is considerably better.
  • That “iterate” step in the middle, where you just try it and adjust if needed is among the most beautiful things about 3D printers.
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Spring 2014 Semester Retrospective

Continuing my habit of posting before and after notes on my courses, after notes for Spring 2014.
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My Instructional Technology class asked us to play with Prezi, Some thoughts:

  1. I don’t think I’ve ever seen a presentation using Prezi that wasn’t made worse by it. Sometimes because the internet connection wasn’t good enough to support it, and it wasn’t installed locally. Usually because the transitions were distracting. Sometimes because there were better less spatial schema available for the information.
  2. I don’t think this is necessarily a general problem with Prezi, I think it is just that we have decades of experience designing slides, and (at least in principle) roughly know how not to suck at it, while no such body of knowledge exists for zooming presentations. Especially because it plays on our tendency to use novel features because they are there, just like we’ve slowly learned to exercise restraint with the awful things that can be done with fonts and colors in most slideware.
  3. Prezi’s designers clearly read Jef Raskin’s The Humane Interface and took it as gospel. It looks like a subset of a ZUI (Zooming User Interface) that he advocated for, unselfconsciously ignoring subsequent criticisms.
  4. Building something like that on top of Flash in this day and age is utter, inexcusable insanity. I went through four OS+Browser combinations before I found an environment where it didn’t crash Flash immediately.
  5. Their motion interpolation is the most annoying, nauseating motion in a world of annoying, nauseating smooth scrolling schemes.
  6. Sign up with your .edu address through the “for education” link if you have one. You basically get the small paid plan for free.
  7. By default, they appear to send you email every fucking day, and have one of those intentionally obtuse tools (not accessible except for via the unsubscribe link in the email footer, unsubscribe button set up to look like it might delete your whole account, etc.) to fix it.

Bah. 99 times out of 100, I’m sticking with Beamer.

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WNR3500L Repair

Some years ago the KAOS lab bought a number of Netgear WNR3500L routers to use as network glue. A pair of them have been in constant service since 2011 in the lab and machine room, and both of those died in the last two weeks. I’ve finally satisfactorily tracked down the culprit, and figure it’s worth a quick write up.

The WNR3500L is a bit of a false-promise machine; it was sold as an “open router” with excellent community firmware support, but it has some pretty extensive blobs that mean it really only runs versions of DD or Tomato with a small range of 2.6 kernels.

I initially just swapped the first one to die out for a spare (which happened to be flashed with Tomato instead of DD, it was an experiment when they were first set up), and reloaded most of the settings by hand (we failed to save or document many of the settings elsewhere. Our fault.) until I had time to investigate. When the second one died, it became urgent.


Conveniently, the WNR3500L has a 3.3V RS-232-like serial port (115200/8/n/1) under an easily removed panel, retained by one small torx screw/pin thing (Both are out in the picture while I was figuring out which side was which).


This leads to another victory for the Bus Pirate. Any 3.3V compatible UART adapter would work, but the bus pirate has much nicer cables for this sort of thing. In retrospect, those headers are correctly sized to just plug the female harness ends directly, but I used clips out of habit.

It turns out they were booting right up with only warnings, but the configuration was sufficiently garbled that neither the WLAN nor the Ethernet ports were coming up. I blew the first one’s settings away (note: at least on the old version these were running, reboot TWICE after clearing the NVRAM to get back to a consistent state: the first time it writes default values, the second time it boots cleanly). With the software-only nature of the problem established, I took more time to investigate the second one, at which point…

nvram show
[1310 lines excluded]
size: 32782 bytes (-14 left)

Well, There’s Your Problem.

Turns out the tiny little log fields for monthly traffic statistics filled up the nvram and were causing configuration corruption, and thus failures.

There are some forum threads and bug reports on the matter, and it was fixed about two years ago with a test to prevent writing past the end of the file, a default setting to automatically delete traff logs after 12 months, and various other enhancements. Now that we’re up and running again it isn’t desperate, but it looks like there is a DD release from mid-2013 with the fixes applied that still supports these things, and they are probably due to be re-flashed to keep this from happening again.


  • My home OpenWRT on a TP-Link 1043ND uses a connected USB key for logging and such. This is a much better behavior (like most things in OpenWRT: DD-WRT appears to be kind of an unmaintainble mess).
  • Keep an eye on appliances like routers, they’re still just computers.
  • Back up your damn configurations, or at least retain copies of the information required to regenerate them. Regenerating settings, especially when it involves manual tasks like MAC address hunting, sucks.

Other notes:

  • These things’ bootloaders appear to always look at through the non-WAN ports for a tftp payload named vmlinuz. There may be a way to disable that, but I think I’d prefer to leave it on, since it only enables local attacks and provides a rescue mechanism.
  • The WNR3500L has a documented weakness about WAN-LAN throughput with various firmwares. Our uplink is actually good enough for it to matter, so perhaps one of the newer versions will improve that.
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Spring 2014 Impressions

I’ve been terrible about writing things up lately, but want to at least put my semester impressions on the head of the chain of such posts.

One of my overall experiences is that teaching (and also, taking classes outside my typical discipline) has made me much more talky in class.

CS541: Compiler Design/Finkel
It’s a core class in the CS program, but I’m taking it this semester because Dr. Finkel is teaching it, and his classes are always excellent. I believe this leaves only one compilers/programming languages class offered at UK I haven’t taken, and that one is in the math department and entirely not my thing. I’m just shy of qualified to teach this material, so there’s a slightly odd dynamic in which I wait before delivering answers, and Raphi is frequently imposing a cooldown on my responding in class anyway.

We’re building a compiler for a cleaned up, stripped down C-Like language in Java, basically following Crafting A Compiler. I’ve used an old version of Fischer LeBlanc as reference for compiler material and like it better than the others I’ve used, so I figure this should work out well. Thus far, I’m having more trouble getting myself back into the Java mindset (So OOP. Much Type System. Wow.) than with the Compilers material. High expectations based on experience.

CS621: Parallel and Distributed Computing/Zhang
It’s basically a class in MPI. I’m hopeful that having some nice assigned MPI projects to work on will be good for me. There are two problems though: first, I’m afraid it’s going to turn into a linear algebra class that just happens to take place on parallel supercomputers. Worse, the lectures are terrible. The organization is bad. The slide decks are bad. The Englishrish is bad. The quality of response to questions is bad. It may actually be worse than the terrible Differential Equations with Jar Jar Xin class I took as a freshman. However, it is of the form “CS6xx,” and the content is at least in principle something I want, so as long as the projects and grades turn out, I’m happy. Kind of pissed that the crap lecture overlaps the Friday CS BoF and associated conversation.

GS630: Instructional Technology/Rice
I’m having so much fun in here. It basically consists of a bunch of jaded grad students discussing instructional technology for college teaching, largely from experience, lead by a specialist in the area from CELT. Some of the discussions basically go like this. I’m afraid I’m becoming “that guy” in here because I talk disproportionately and occasionally convivially argue with or reference outside material to the instructor, but again, I’m having a ton of fun, and it counts toward my PFF certificate.

GS610: College Teaching Seminar/Worley
I typically don’t post up my beginning of semester impressions post until all my classes have met, but this one doesn’t start until well into February. At some level, I’m pleased that it’s a general college teaching course instead of a department-specific one, I have trouble imagining a full semester in-discipline teaching class that didn’t turn into a myopia reinforcement program. It should be fun, all the GS classes thus far have been.

I’m not teaching this semester, and hopefully will be getting to more research projects with the freed up time.

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Fall 2013 Semester Retrospective

I noticed while writing the pre- post for the new semester that I lost track of my customary semester review for the past semester while I was moving hosting. Pushing it up now.
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SC13 Retrospective

Posting up my notes from SC13 is another thing I didn’t get to during the end of the semester. Remedying now.

The main takeaway sequence from conversations on the floor is as such:

  1. The era of single-core performance gains is already over.
  2. Furthermore, the era of usable single-die performance for MIMD machines is coming to an end.
  3. Therefore, big machines are going to be getting physically bigger… to the point where connection lengths are a problem (everything is Infiniband, and Infiniband doesn’t tolerate long runs well)
  4. There is a LOT of cooling effort to make the necessary density happen – central large fan systems, immersion cooling, closed-circuit water gear, etc.

The other really exciting thing that it seems AMD is going to make it, and more. Their lean period finished when the payoff on the XBone/PS4 came in, and they have a VERY good plan for the next >2 years. It works with the premise above about single-core/die MIMD performance ending, and points in the HSA direction – this is the crazy parts with MMUs so a CPU and GPU can share memory without skew penalty and such. ARM and partners are also generally pointed that way, and have been for some time, though apparently AMD isn’t getting out of the x86 game, but it does look like they are getting out of the fat core game.
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Heterogeneous System Architecture

Post-SC, I sat down to do some deeper reading on the HSA (Heterogeneous System Architecture) stuff. This is AMD/ARM (and many friends)’s plan for the future, and it is pretty fucking exciting (in an obscure technical sort of way).

The best starting point I found is this year old whitepaper [PDF warning]. They’re using slightly odd terminology, the important bits are LCU = Latency Compute Unit = Conventional MIMD CPU Core, TCU = Throughput Compute Unit = Accelerator, typically SIMD-engine-ish like a GPU, HSAIL = HSA Intermediate Language = IR that can be compiled at install/run time to accelerator’s ISAs. The hardware-side implementation details are nowhere to be found, but there are a lot of seriously exciting model-affecting things detailed on the software side. The general model, with things broken into grid, work group, work item, wavefront is FAR more sane than most of the parallel schemes (I’m thinking specifically of the awful CUDA nomenclature). Internally, the exciting stuff includes requiring a limited sort of preemption on the accelerators, a relaxed consistency model across memory shared over a whole system (nice thread-like shared memory), an intermediate low level language/VM for portability, and assurances about barrier capability in the TCU. The actual objects are basically FAT ELFs with a complete copy of the program for the LCU, plus the HSAIL representation for the parts that can be shipped to TCUs. I’m pleased that there seems to be a clever run-time that does a bunch of platform enumeration and controls where parts run in a rule-automated-but-overrideable way.

I had some folks at SC tell me they’d try to get me a more implementation-focused whitepaper on the hardware side at AMD but they weren’t sure if/when details would be clear for distribution. On the software side, the details are in a published draft of the ISA/Model/Compiler Writer’s Guide that I browsed around in a bit and found very enlightening. The reference tool-chain seems to be mostly built on LLVM and OpenCL.

I have some other SC-related thoughts to share, but I want to get them a little bit more refined (and decide which are for public consumption) before I post.

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I will be at SC’13 November 16-21 with the of Kentucky research exhibit again this year in booth 629. Media and impressions should appear somewhere in my ‘net presence during and after the conference, it is always a good show.
Edit:Pushing photos from the show floor into this album.

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chdk-ptp PKGBUILD

In another episode of fixing things for the Cameras as Computing Systems class I’m taking, I made a PKGBUILD that apparently correctly builds and installs chdk-ptp on Archlinux systems. Chdk-ptp is a tethered-control application for Canon cameras running CHDK, that hooks a variety of custom extensions to the ptp protocol. Their build system is a little lackluster, is missing things like an install rule, and requires a helper script be installed to do some path munging before running the binary, but the documentation is good enough to sort it out, and the program itself seems to work. My package depends on iup-all-bin from the AUR which also provides the cd dependency (not marked in its provides array, though there isn’t an official package to conflict with). I tried to use the built-from-source AUR packages for cd (Seriously, who thought that was a good name for a piece of software?) and iup, but iup was giving me a hassle and the chdk-ptp documentation suggests the binary distribution will be less trouble anyway.

The PKGBUILD format has changed a bit since I was last making my own- I like most of the changes in terms of clarity and modularity, but it does require a bit of re-learning. It also means I have a couple of pet packages that probably need attention.

The build is a little janky so despite it passing through namcap with only one expected warning, I don’t want to put it in the AUR until I’ve tested it enough to be reasonably sure it works as intended. I expect I will get around to that eventually.

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