Hi,
it's been a while since I last let one of my machines do some work for BOINC (primarily SETI@Home), so I may not be quite up to date, but..
as I see that, for me, both, 7.0.26 as well as 7.0.64 cause some "Computation errors" at the end of each WU and another thread (the sticky one) explains that basically any(?) GPU computation is currently bugged in some way, I wonder about two things:

1. how can I completely disable GPU computing?
I have set-up my BOINC to not use my GPU in every way I could find as well as enabled it to compute at any time, any day, even when the CPU isn't idle and I'm working on something else. Yet, the status messages all say "GPU suspended - user request (0.798 CPUs + 1 NVIDIA GPU)" and while it's true I requested the GPU to be suspended, computation never starts on anything unless I enable BOINC to utilise the GPU. From my understanding, when I enable computation in very general and suspend GPU, computation should still run on CPU. There has to be a setting or a combination of which, that I'm missing and that would have BOINC do its thing nonetheless.

Even though I don't expect this to matter really, the CPU is an Intel i7-3630QM and the GPU is an Nvidia GTX 680M. (Yes, it's a notebook.) OS is MS Windows 8 x64.

2. how DO people actually use their GPUs for BOINC?
If I'm not totally mistaken, GPU computation with the official BOINCs has been bugged for at least a year? As I mentioned above, 7.0.26 (which is rather old) as well as 7.0.64 (which was just released 3 days ago) don't seem to do it. Thus, I wonder where those GPU-powered WUs people seem to be able to do are actually coming from. Do they only originate from certain projects and is it the SETI@Home executable that is actually faulty at GPU processing or what else..?
Looking for options here, as I'd really like to crunch a few WUs now and then, even it isn't much (especially without GPU). Please advise.

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1. how can I completely disable GPU computing?

You disable the use of the GPU by going to Your Account->then edit the Project preferences and uncheck "Use Nvidia GPU". This will have BOINC stop asking for work for the GPU, it will not affect any GPU work already in cache. That will have to run to completion, or be aborted.

2. how DO people actually use their GPUs for BOINC?

It's not BOINC that is at fault here. BOINC detects your GPU perfectly fine, but it doesn't do any calculations. That's done by the project science applications. So it's as you assume, the Seti application is the problem here. Seti has at the moment 3 CUDA applications, one for just plain CUDA (old cards, early drivers), CUDA2.3 (middle card, middle drivers) and CUDA-Fermi. The latter is for Fermi style GPUs, the GTX 4xx and 5xx GPUs.

The GTX 6xx GPUs differ in architecture, and are not backward compatible with any of the previous applications. And thus any work that is run on them without any of the 3 workarounds given in the sticky thread, will result in error.

So the only thing you have to do to get things working right, is to just follow the setting of the environment variable and your Kepler GPU will work without trouble. It's been tried and tested by many. They can't all be wrong.

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Thanks,
a few days after my initial post, it has all sort of worked out itself.
The computing and downloading seem to need some time to adapt to new settings even after restarting and forcing updates - and while I had turned off GPU computing in the BOINC client, I completely forgot about the web-based project-specific preferences.
It even runs fine now with the GPU enabled (and that variable in place), although it's obviously very conservative at using what's left available to it, as it rarely goes above 2 or 3 percent GPU usage. Hope is the people at Berkeley working on SETI@Home may eventually come up with a Kepler-compatible client, so I can push this little machine to a few more challenges.
~harl

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... it rarely goes above 2 or 3 percent GPU usage

This is normal with new NVIDIA drivers (like yours 314.22 ; IIRC drivers > 270 act like this)

(Which programs do you use to monitor GPU load? E.g.:
TechPowerUp GPU-Z
http://www.techpowerup.com/gpuz/

SIV - System Information Viewer
http://rh-software.com/
http://rh-software.com/downloads/siv.zip
)

You need to free a CPU core to fix low % GPU usage

I think you use Local preferences (in BOINC Manager):
http://boinc.berkeley.edu/wiki/Local_preferences#Advanced_view

so set:
On multiprocessor systems, use at most 99 % of the processors

This will free one CPU core (one less CPU task will be run)

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- ALF - "Find out what you don't do well ..... then don't do it!" :)