Comment by snvzz

The most important is to set the policy, described in sched(7), rather than the priority.

Notice that without setting the priority, default policy is other, which is the standard one most processes get unless they request else.

By setting priority (while not specifying policy), the policy becomes fifo, the highest, which is meant to give the cpu immediately and not preempt until process releases it.

This implicit change in policy is why you see such brutal effect from setting priority.