iMac Unresponsive - Requires Reboot - Logs Vague

The graphics processor is faulty.

Make a "Genius" appointment at an Apple Store to have the machine tested.

Back up all data on the internal drive(s) before you hand over your computer to anyone. If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to bare metal from any of them.) Don’t erase the recovery partition, if present.

*An SSD doesn't need to be zeroed.

I have nothing to add to what I wrote earlier. Good luck.

Hello Brian.

I think Linc (who is something of a legend in Apple discussions) has given the simplest and most effective answer: get your graphics card replaced.

However, having expereienced the same issues as you, I think I can shed some light on it and suggest a workaround.

GPU Debug issue solved? (Sort of... )

I had irregular GPU freezes on my 2010 iMac / Snow Leopard / ATI Radeon 5670 starting around mid 2012 becoming increasingly frequent to the point of twice a week in April 2013 always with the same Kernel log: GPU Debug etc. Now they have stopped. Maybe this helps you too.

If my analysis is correct, this solution will apply to any modern Mac and any OS. See below.

First, the analysis:

• Always the same Kernel log after the crashes
• kernel[0]: ** Device in slot: SLOT--1 **
• kernel[0]: ** GPU Debug Info Start **

• Only the user interface / GPU freezes. The mouse moves but no interaction is possible
• I can log in to my frozen iMac from another Mac via SSH / Terminal and see all processes running.
• I was able to terminate processes via SSH which would not take kindly to a forced shutdown and ultimately restart the machine without pressing the power button.

• Restarting the WindowServer via SSH does not restore functionality to the affected Mac
• This suggests the problem is not software / OS related in origin.

• My Mac is 100% up to date for firmware, software and OS (albeit with OS X 10.6)
• The GPU is not overheating. GPU is operating within specified temperature parameters

• My crashes always occurred when using a Mozilla browser to open several tabs at once (never Safari).
• Others apparently experience it when playing Flash videos or using Aperture.
• My logs sometimes point towards a problem with a browser plugin process

• The common element?
• Hardware acceleration - using the GPU to do things it is capable of but wasn't necessarily designed for.

The Workaround

Turn off hardware acceleration

Er, that's it!

In any Mozilla browser Go to Preferences / Advanced / General and de-select the option: "Use hardware acceleration when available"

Sorted!

From now on, webpages may load slower, flash video may take a while to start and you'll see your CPU usage increase when opening lots of browser pages such as restoring a Firefox browsing session. I will gladly live with that to stop my computer UI freezing.

I have gone from two GPU crashes a week down to none at all in one month.I'm keeping my fingers crossed.

Caveat

If my expereiences don't match yours, you may have a different app that uses hardware acceleration to render images, like Aperture. I cannot advise or comment on turning off harware acceleration in Aperture as I have never used it.

Of course this isn't really a solution but a workaround, however if you can save the cost of a new motherboard, the minor inconveniences may be a price worth paying.

The Fallout

My experience suggests 3 possible source issues:

1. Premature failure in a wide variety of GPUs when running hardware accelerated apps
2. Badly written GPU acceleration protocols in Mozilla / Flash / Aperture etc
3. Apple driver / firmware issues for GPU hardware acceleration

- or a combination of the above.

Good luck

I'd value comments, critique and observations.

Please post back here if this helps you (or not) as it will add to a body of evidence.