One of the hard problems in maintaining real-time systems requirements is to keep
track of the impact of resource usage on the applications. 
Often times, it is not sufficient to keep track of upper bounds since requirements
such as jitter are sensitive to both the lower bounds as well as the resource 
scheduling algorithm employed. 
In the case of non-preemptive scheduling, real-time requirements can be missed 
when resource utilization is decreased as in a CPU upgrade, even though there are
no jitter constraints. 
We shall discuss a formal approach to track the sensitivity of real-time 
application requirements with respect to resource usage. 
We shall use as an example the avionics software of the Boeing 777 aircraft.