Swirly Mein Kopf

Saturday, December 24. 2011

First (academic) publication


Nice christmas present: I just got message that the paper “Loop subgroups of Fr and the image of their stabilizer subgroups in GLr(ℤ)” that I extracted from my Diploma thesis in Mathematics, and which I submitted to the Israel Journal of Mathematics in November 2010 was finally published today, in “Online First“ form and under the doi 10.1007/s11856-011-0213-3.

Tuesday, December 6. 2011

Guest lecture on Haskell performance


Today, I had some business in Bonn, so Janis Voigtländer invited me to hold a guest lecture in his advanced functional programming course, maybe telling the students more about “real world” use of Haskell, given that he teaches mostly the theoretical foundations. I chose to discuss an experience I made while implementing SAT-Britney, namely that for the sake of good runtime and memory performance, it may be neither ideal to evaluate a list fully lazily, nor fully strict, but to have something in between. For the talk, I demonstrated this by a small example, showed how to use the GHCi debugger to get some insight in the evaluation order of things, how to benchmark the program and read the ghc statistics and a profiling chart, and finally how to get the most out of GHC’s List Fusion. I wanted to also touch on QuickCheck, but skipped it to finish in time. The full text of my talk (including graphs) is available, but in German; if you want an English version convince your prof to invite me for a guest lecture as well.

Afterwards I sat down with one of the students of the course, Helmut Grohne, who happened to take part in the Debian bug squashing party last weekend and had filed a release critical bug against the Haskell grammar generator frown (no better link available, homepage seems to be down) to debug the problem. It quickly became clear that the bug was introduced by me (can you spot it?) when trying to make it compile with a modern GHC – slightly embarrassing for me.

Saturday, December 3. 2011

Some late recognition of metainit


Four years ago, during DebConf 7 in Edinburgh, I had the idea of creating init scripts not by hand, but rather describe them in a declarative way that handles most common cases and generate the real init scripts from them. Back than, this was partly motivated by the varying quality of init scripts and the advent of an alternative init system, upstart, for which proper service files could be generated as well.

I implemented the idea and uploaded it as metainit to Debian, but it did not take off. Only one package uses it, and this package is not used much either. The project basically fell dormant.

Yesterday, I got this mail:

Hi Joachim,

I'm not very familiar with Debian ecosystem, however our company has recently switched to exclusively using Debian based servers, please forgive me if there exists a better channel for sending patches.

We need (in house) to create a lot of trivial services and the sysv init system seems an ideal way to manage them, and metainit an great way to keep the config simple.

However all our scripts require a heartbeat, the way we normally do this is with a bunch of checkers launched from cron. Metainit was ideal, but lacked the simple 'status' task found in most init scripts.

This small patch adds that task, I hope you and others find it usefull.


together with the mentioned patch. So it turned out that the idea was a useful one after all, although it found its use on our user’s machines, and not within the Debian packages, as I originally anticipated. Nice!

During the recent discussions about the latest init system, systemd, there were more thoughts about generating init scripts/upstart service descriptions/systemd units from a declarative description, and if metainit had taken off back then, things would be easier now. So if anyone feels like reviving the project: Please do.

Thursday, December 1. 2011

Poetry in the problem class


I’m currently running the problem class for the graph theory course at the Karlsruhe Institute of Technology, held by Prof. Maria Axenovich. On one problem sheet, I felt like setting the question in verse, and indeed, two students submitted their solution is in verse form as well. I have assembled the problem and both solution – enjoy!

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