The low cost and availability of clusters of workstations have lead 
researchers to re-explore distributed computing using independent workstations. 
This approach may provide better 
cost/performance than tightly coupled multiprocessors.
In practice, this approach often utilizes wasted cycles 
to run parallel jobs.
In this paper we address the feasibility of such a non-dedicated 
parallel processing environment assuming workstation
processes have preemptive priority over parallel tasks.
We develop an analytical model to predict parallel job response times.
Our model provides insight into how significantly
workstation owner interference degrades parallel program performance.
A new term {\bf task ratio}, which relates
the parallel task demand to the mean service demand of non parallel
workstation processes, is introduced.
We propose that {\bf task ratio} is a useful metric for determining 
how large the demand of a parallel applications must be 
in order to make efficient use of a non-dedicated distributed system.