Headroom is the difference between the average utilization on a system
and the maximum available headroom. 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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