In one of my PCs with two GTX980ti inside, at the moment I can only process GPU tasks with 1 of the 2 GPUs, since one of the fans of one card is defective.
Until I will renew this card, as a workaround I have adapted the BOINC cc_config.xml, so that all projects which I currently participate in will not use the defective GPU (i.e. only GPU1 crunches, NOT GPU0).
e.g. for Primegrid, the entry in reads as follows:
<exclude_gpu>
<url>http://www.primegrid.com/</url>
<device_num>0</device_num>
</exclude_gpu>

In order to be able to crunch Asteroids on this PC, I added another entry which reads as follows:
<exclude_gpu>
<url>https://asteroidsathome.net/</url>
<device_num>0</device_num>
</exclude_gpu>

However, this does not work. Still both GPUs are crunching Asteroid tasks.
I tried to change the wording of line 2, like adding "www", or "http" instead of "https", and some other variants. Nothing works.
Anyone any idea what the wording of line 2 should correctly be so that BOINC realizes that only GPU 1 should process Asteroid?

post the entire cc_config file so it can be inspected for typos or other mistakes. i assume these entries are in the <options> section.

did you restart BOINC?

changes to cc_config often require a full BOINC recycle. not just a re-read config files.

edit, barring any other typos or mistakes, the likely problem is that you've got the master url for the project wrong.

the proper URL is: https://asteroidsathome.net/boinc/

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the proper URL is: https://asteroidsathome.net/boinc/

thanks a lot for providing the master URL. In all cases I tried I omitted "/boinc/" - since this is not part of the URLs of all the other projects which I included in this cc_config.xml
And yes, I know that after such a change, BOINC needs to be restartet.
I am not at home at the moment, but will try to make the correction later this evening.
Thanks again for your help.

unfortunately, editing the Asteroid Master URL as suggested did not deliver the desired result either.

Here is the complete cc_config.xml. No idea where there might be a typo in the Asteroid section (towards the bottom) - I didn't catch any:

<cc_config>
<options>
<fetch_on_update>1</fetch_on_update>
<max_file_xfers_per_project>6</max_file_xfers_per_project>
<report_results_immediately>1</report_results_immediately>
<exclude_gpu>
<url>http://www.gpugrid.net/</url>
<device_num>0</device_num>
</exclude_gpu>
<exclude_gpu>
<url>http://www.worldcommunitygrid.org/</url>
<device_num>0</device_num>
</exclude_gpu>
<exclude_gpu>
<url>http://einstein.phys.uwm.edu/</url>
<device_num>0</device_num>
</exclude_gpu>
<exclude_gpu>
<url>http://www.primegrid.com/</url>
<device_num>0</device_num>
</exclude_gpu>
<exclude_gpu>
<url>https://asteroidsathome.net/boinc/</url>
<device_num>0</device_num>
</exclude_gpu>
</options>
</cc_config>

As mentioned earlier, with all other projects contained in the cc_config.xml, the exclusion of device_0 works fine.

anyone any idea what I could still try to get the cc_config.xml work the way it's supposed to ?

Basically in such situation I would go with a global settings 'ignore_nvidia_dev'. Aside of that inside 'exclude_gpu' structure you may need to specify 'type', 'app', etc.
How BOINC client behave with different projects could differ by so many reasons, just the fact that all those project use different version of BOINC Server will make irrelevant any comparisons. Then the project settings come into account. Then...

This is BOINC issue, so please take a deeper look at the documentation, and follow the instructions. If "Update" or "Reset" of the project does not fix that, you may try reinstall or upgrade your BOINC client.
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Client configuration

'exclude_gpu'
Don't use the given GPU for the given project. device_num specifies the number of the GPU to exclude (0..63). If not given, exclude all GPUs of the given type. type is required if your computer has more than one type of GPU; otherwise it can be omitted. app specifies the short name of an application (i.e. the name element within the app element in client_state.xml). If specified, only tasks for that app are excluded. You may include multiple exclude_gpu elements. If you change GPU exclusions, you must restart the BOINC client for these changes to take effect. If you want to exclude the GPU use for all projects, look at the ignore_ati_dev, ignore_nvidia_dev and ignore_intel_dev options further down. Requires a client restart.

Hopefully someone will come up with a solution meanwhile.

Cheers!

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“The good thing about science is that it's true whether or not you believe in it.” ― Neil deGrasse Tyson

I updated BOINC to the latest version - and now the exclusion of GPU device_0 works as desired :-)