NVChannel Fifo: Graphics Channel Exception… and Plex

For this first post in a long, long time, why not explain another weird problem I had with the iMac ? Yay, let’s.

During the past two weeks or so, the iMac began behaving strangely. The problems were mostly with the graphics, but after a while the system would slow down to a crawl and a reboot was needed. The problem manifested itself through the appearance of visual artefacts: mostly bits and pieces of the interface trailing behind actual windows or menus, but also horizontal lines in Safari or the Finder, or in the worst case entire parts of windows obscured by colored blocks. Once, I exited Firefox but its window stayed on-screen like nothing happened – I had to “erase” the window myself by using a Finder window as some kind of screen wiper. The easiest way to trigger this bug seemed to repeatedly open and close stacks in the Dock: sometimes the stack would appear only half-opened. Dock icons would also become corrupted after a while.

At first, I feared some kind of hardware failure, but I soon found out that after a reboot all these problems disappeared. Everything was working fine and dandy again, no video glitches. I thought it might come from overheating: nope, the temperature was below 50°C all this time, and problems sometimes occurred just as the computer was coming out of sleep (“waking up”, I guess).

A trip to the console enlightened me: the system.log for previous days was filled to the brim with entries more or less like

kernel[0]: NVChannel(GL): Graphics channel exception! status = 0xffff

kernel[0]: NVChannel(GL): Graphics channel timeout!

kernel[0]: NVChannel(GL): Graphics channel exception! status = 0xffff info32 = 0x3 = Fifo: Unknown Method Error

… and so on. I googled the problem, and eventually ended up on this seemingly neverending thread on the Apple Forums. In it, people alternatively blame Leopard 10.5.1, World of Warcraft, the Leopard Graphics Update, Aperture, Google Earth, any software that tries to use OpenGL, etc. I tried panning and zooming like crazy in Google Earth for a while, but no problem whatsoever — this was not it. But it led me in the good direction: video-intensive applications.

What video-intensive application do I use on a regular basis? I don’t play games, I run iPhoto once in a while… but every other day I end up watching a movie using Plex (not too hard to guess if you read the title of this post). So I kept Console opened in the background, launched Plex in windowed mode, opened a movie… and behold! Tons of graphics channel exceptions! Ha-ha!

Needless to say, I trashed Plex and its LaunchAgent right away. I’m pretty sure the issue only arised because I had tinkered with an option I shouldn’t have on my particular hardware; Plex is otherwise a pretty nice piece of software. The weird part is that I replaced Plex with XBMC (which is basically the same thing but less Mac-focused), and there was no Graphics channel exception to be seen. After switching to the MediaStream skin, it was as if nothing had ever happened. Without the graphical problems.

Maybe next time, I’ll write something more interesting, like how the Sarkozy government is slowly dismantling the research structures around here with the help of the AERES, creating inequalities between teaching positions to increase competition and handing  the control of universities to the private sector, all imposed from above, without concertation with the people who do the actual work, by a minister with strong ties to big corporations. Maybe.

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2 Responses to NVChannel Fifo: Graphics Channel Exception… and Plex

  1. Rob says:

    Can you post more of the log (any hex codes saved) and if you are still able to reproduce that hang using Plex ?

    And please drop me an email if you don’t mind..

  2. Pingback: indefinite hiatus | Bloody Fingers

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