The counter-intuitive truth about efficiency. Watch queues grow infinitely when you maximize utilization.
See It In ActionAt 100% utilization with variability, the queue never stabilizes - it grows forever
"We should maximize utilization. Idle resources are wasted resources. 100% utilization = maximum efficiency."
At 100% utilization with any variability, queues grow without bound. There's no slack to absorb the inevitable timing mismatches.
Aim for 80-85% utilization. The remaining 15-20% isn't waste - it's the buffer that keeps your system stable and responsive.
At high utilization, wait time increases exponentially, not linearly. Going from 80% to 90% utilization doubles wait time. Going from 90% to 95% doubles it again.
| Utilization | Relative Wait Time | Status |
|---|---|---|
| 50% | 1x | Responsive |
| 70% | 2.3x | Good |
| 80% | 4x | Target zone |
| 90% | 9x | Caution |
| 95% | 19x | Dangerous |
| 100% | ∞ | System failure |
For most operations with variability, 80-85% utilization provides a good balance between efficiency and responsiveness. Critical systems (ERs, production lines) often target even lower to ensure capacity for surges.
With variability in arrivals and service times, temporary demand spikes are inevitable. At high utilization, there's no slack capacity to absorb these spikes, so queues form and persist.
Slack capacity isn't idle - it's actively providing responsiveness and stability. Think of it like the empty space in a parking lot: without it, new arrivals have nowhere to go.
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