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Summary: Q-Cop: Avoiding Bad Query Mixes to Minimize
Client Timeouts Under Heavy Loads
Sean Tozer, Tim Brecht
, Ashraf Aboulnaga
David R. Cheriton School of Computer Science, University of Waterloo
Waterloo, Ontario, Canada
{srtozer,brecht,ashraf}@cs.uwaterloo.ca
Abstract-- In three-tiered web applications, some form of
admission control is required to ensure that throughput and
response times are not significantly harmed during periods of
heavy load. We propose Q-Cop, a prototype system for improving
admission control decisions that considers a combination of the
load on the system, the number of simultaneous queries being
executed, the actual mix of queries being executed, and the
expected time a user may wait for a reply before they or their
browser give up (i.e., time out). Using TPC-W queries, we show
that the response times of different types of queries can vary
significantly depending not just on the number of queries being
processed but on the mix of other queries that are running
simultaneously. We develop a model of expected query execution
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