The thread scheduler is a fair-share scheduler. This means that it guarantees that partitions receive a defined minimum amount of CPU time (their budget) whenever they demand it. The thread scheduler is also a real time scheduler. That means it's a preemptive, priority-based scheduler. These two requirements appear to conflict, but the thread scheduler satisfies both of these requirements by scheduling through priority at all times so that it doesn't need to limit a partition in order to guarantee some other partition its budget.