Principle (Non-Linearity in Utilization): WIP and flow time increases non-linearly in utilization.
The principle implies that as we approach the capacity of the system,
both WIP and flow time increase rapidly, as shown in the figure below.
The reason for this is the following. All systems have inherent variability
in them. As we work closer and closer to the capacity of the system the
variability in the system will cause the bottleneck to starve. The only
way to prevent this starvation is to have more WIP in the system.
But even in the system with more WIP, a bad enough sequence of events will
occur to starve the bottleneck. To achieve higher and higher utilization,
we must prevent rarer and rarer sequences of events, which requires more
and more WIP. In a system with variability, it is impossible to prevent
every starvation event at the bottleneck, so the WIP required to achieve
100 percent utilization is infinite. By Little's
Law, the flow time also blows up as utilization approaches 100 percent..
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