when using the parallel.for and parallel.foreach is there a way to control priority and max count of the threads it automagically makes?

i have the main thread working on as little as possible, which still keeps it quite busy as most of what can be threaded away has been, then a background thread which handles most of the work includes using parallel tasks to run over (prefered) 6 cores, leaving 1 core to handle most OS/etc

problem is parallel tasks keeps using all 8 cores, which interrupts the main thread and slows it, and i tried setting the background thread to lowest priority but seems no effect when parallel tasks run

ParallelOptions. MaxDegreeOf...