  We study the necessity of predictive information in a class of queueing
admission control problems, where a system manager is allowed to divert
incoming jobs up to a fixed rate, in order to minimize the queueing delay
experienced by the admitted jobs.
  Spencer et al. (2014) show that the system's delay performance can be
significantly improved by having access to future information in the form of a
lookahead window, during which the times of future arrivals and services are
revealed. They prove that, while delay under an optimal online policy diverges
to infinity in the heavy-traffic regime, it can stay bounded by making use of
future information. However, the diversion polices of Spencer et al. (2014)
require the length of the lookahead window to grow to infinity at a non-trivial
rate in the heavy-traffic regime, and it remained open whether substantial
performance improvement could still be achieved with less future information.
  We resolve this question to a large extent by establishing an asymptotically
tight lower bound on how much future information is necessary to achieve
superior performance, which matches the upper bound of Spencer et al. (2014) up
to a constant multiplicative factor. Our result hence demonstrates that the
system's heavy-traffic delay performance is highly sensitive to the amount of
future information available. Our proof is based on analyzing certain excursion
probabilities of the input sample paths, and exploiting a connection between a
policy's diversion decisions and subsequent server idling, which may be of
independent interest for related dynamic resource allocation problems.
