Headroom is the difference between the average utilization on
a system and the maximum available capacity. Optimum headroom
varies depending on size of system. While a single processor
system might require 50% headroom to preserve reasonable response
times, a 16-way system might have reasonable response times
when loaded at 80%.
Adequate headroom can also depend heavily on the characteristics
of the loads; highly interactive systems require much more headroom
than those that can tolerate delays in response time; batch
systems may get by with very little headroom at all.
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