Emmet Caulfield: April 2008 Archives

On Ubuntu 8.04 “Hardy”, the ATI Catalyst 8.4 Driver .deb packages for amd64 will build from the ATI download, but one of the four packages, xorg-driver-fglrx_8.476, will not install out of the box when you try to install the packages in the usual way.

The problem is an error in the preinstall script for the package. In a Debian package, one of the things that a package script sometimes does is “divert” files. The idea is that a package can specify that any attempt by a future package to install a file having a particular name is to be handled by renaming the file to something else, or diverting it. In this way, a package can protect files from being overwritten without causing another package to fail to install. If you have customised a package-supplied file, which might be overwritten by a package upgrade, you can use dpkg-divert to protect your version.

The ATI xorg driver package uses this facility to stop MesaGL, a software Open GL renderer, from overwriting the ATI hardware drivers. That's a good thing, but the problem is that the preinstall script diverts two files to the same target. In other words, it says “instead of installing /foo/bar, rename it /foo/quux” then a second later, “instead of installing /foo/baz, rename it /foo/quux as well”. That's not allowed (because it's obviously stupid).

This accounts for the seemingly bizarre error-message that xorg-driver-fglrx_8.476 is trying to divert to a file target that it, itself, has already diverted another file to!

I suspect that the person who wrote the script copy'n'pasted the line, remembered to edit the filename to divert, but forgot to change where it was diverted to. An easy, if silly, mistake to make. ATI have made a lot of progress in providing decent Linux driver, maybe we shouldn't be too hard on them for lousy QA.

The fix follows below the fold…

In reading Nick Trefethen's Group velocity interpretation of the stability theory of Gustafsson, Kreiss, and Sundström and Group velocity in finite difference schemes, the thought occurred to me that if instability is a result of reflections at boundaries, can the theory be extended to meshes? This is somewhat analogous to reading a wave-theoretical exposition of Snell's Law and asking if the theory can be extended to anisotropic inhomogeneous media. I suspect that it can, but I don't know if it's worthwhile.

Now, the velocity of propogation of light in a medium is determined by the permittivity and permeability of that medium. From these quantities, we can also calculate the characteristic impedance of the medium. We generally think of media as linear, isotropic, and homogeneous, but this is not the case in general. We often think of monochromatic light but, again, this is not the case in general. When light passes from one medium to another, a difference in the characteristic impedance of the media leads to reflections at the boundary.

What Trefethen started me thinking was, is there a kind of “permittivity” of a grid and a “permeability” for a method which would give a wavenumber-dependent “characteristic impedance” for the grid/method combination and a different characteristic impedance for boundaries, owing to either imposed or numerically necessary boundary conditions, and allowing instability to be determined easily?

Generalising to an irregular mesh, the mesh analogue of permittivity would be a wavenumber-dependent discrete tensor. However intimidating it might sound, and however intractable it might be for a human to calculate, it would surely allow a computer to predict, given a mesh and a method, what Fourier modes would cause instability and, perhaps more importantly, where in the mesh this might arise. A potential end-product might be a software tool which takes a mesh from, say, COMSOL Multiphysics, and identifies “hot spots” (analogous to “shiny lumps” in an optical medium) where particular Fourier modes would reflect, giving rise to instability. Given the Fourier transform of a particular initial condition or forcing function, instability might be predicted and avoided by mesh refinement.

Now, it may be that this analysis would end up being equivalent to solving the problem on the mesh in full generality, or be obscenely computationally costly for some other reason, in which case, I admit, the entire idea is utterly worthless, but there's no harm in asking, right?

Someone mentioned the idea of handing out flyers outside screenings of Expelled: No Intelligence Allowed, so I drew up a list of bullet-points that I'd like to include. Of course, it's too long for a flyer, but maybe it might give someone ideas or they might pick out a few of them.

I intend to annotate this with hyperlinks to relevant online documents and other media. If you can suggest links, please do. I'm particularly interesting in a video of one of Stephen Meyer's lectures: in the Q&A at the end, an evolutionary biologist makes a fool of Meyer by describing a half-dozen precursor structures to the rotary bacterial flagellum beloved by cdesign proponentsists.

This post is dedicated to the public domain. Do with it as you wish.

For Europeans engaging in online political discussion with Americans, it is crucial to understand American political terminology. Provided that you know the political disposition of your correspondent, this is quite easy. Using the following table, look up the term in column 1 (Republican) or column 2 (Democrat), depending on the political disposition of your correspondent. You can then conveniently find the equivalent European term in column 3.

OK, I've finally succumbed. Movable Type is just so damn easy to install and use that I can't really justify maintaining my personal website with hand-crafted XML, XSL-T, and Perl any more.

Eventually I will tart it up, customise it, and move stuff over from emmetcaulfield.net, but until I find my way around Movable Type, this default skin will have to do.

Jag bor i Uppsala, men efter halvannan år, min svenska är ännu jätte dalig. Ibland ska jag skriva nagonting på svenska också och jag hoppas att det skulle hjälpa mig lite när jag försöka att förbättra min svenska. Just nu kan jag läsa ganska bra, men min riktig problemerna med språket är att förstår talat svenska och att skriva. Om du ar svensk, inte vara buskablyg, och snälla hjälp mig: korrigera felen i texten!

De temps en temps, c'est possible que je vais écrire quelquechose en Français. J'espére que ça m'assisterai pendant que j'améliore mon Français, mais maintenant, j'habite en Suede et je veux apprendre la Suedois surtout. Si vous êtes un francophone, s'il vous plaît corrigez mon Français.

Anois is arís, b'fhéidir go scríofaidh mé cúpla focail as Gaeilge. Chaith mé trí mhí sa ghaeltacht Chonamara nuair a bhí mé óg, ach rinne mé dearmad ar an cuid is mó den ghaeilge a bhí agam san am sin. Tá fhios agam nach bhfuil mo gramadach lán ceart. Tá spéis agam mo chuid Ghaeilge a chur ar fheabhas, ach mar tá cónaí orm i tSualainn anois, táim chun an Sualainís a fhoglaim mar tosaíocht. Anseo, do chuala siad faoi teanga "Éireannach", ach ceapann an tromlach dóibh gur saghas Béarla é: bíonn iontas oraibh nach bhfuil.