Occasionally I get multi-threaded tasks which use all the cores allocated to BOINC and they take ages but don't get swapped out (usually because they're near or past their deadlines), and as a result, don't give other tasks execution time.

Is there any way to limit the number of cores any one task can use?

Say, for example, that BOINC is allowed 9 cores to work with (75% of a 12-core system). The way it's acting now, I get one task using 9 cores and tying them all up and nothing else running. I'd like to be able to tell BOINC to cap the number of cores per task, so that if, for instance, the limit was set at 50%, no one task would use more than 4 cores (assuming rounding down) of the 9 allocated to BOINC, ensuring that there would be at least three tasks running at all times.

No, there is no such setting available. And besides, that wouldn't work with multi-threaded applications, as they assume that they will be able to use either all CPU cores in the system, or at least all of those appointed to BOINC. The only solution around that is not to run the MT application, you should be able to select that through the project preferences.

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Depending on the project/application and the multi-threading technology used, it can be done through anonymous platform (app_info.xml) - though that needs some care and knowledge, and isn't recommended for a general user.

I did ask David Anderson to extend the app_config.xml tool-set to include this, but he refused.

It's a shame to hear there's no safe way to do it at the moment.

Thanks, anyway, guys.

Depending on the project/application and the multi-threading technology used, it can be done through anonymous platform (app_info.xml) - though that needs some care and knowledge, and isn't recommended for a general user.

I did ask David Anderson to extend the app_config.xml tool-set to include this, but he refused.

And now this poor decision is proving to be a problem over on MilkyWay. ;-)