Controlling the Behaviour of Database Servers with 2PAC and DiffServ

Abstract

In order to avoid stress conditions in information systems, the use of a simple admission control (SAC) mechanism is widely adopted by systems’ administrators. Most of the SAC approaches limit the number of concurrent work, redirecting to a waiting FCFS queue all transactions that exceed that number. The introduction of such a policy can be very useful when the most important metric for the system is the total throughput. But such a simple AC approach may not be sufficient when transactions have deadlines to meet, since in stressed scenarios a transaction may spend a lot of time only waiting for execution. This paper presents 2 enhancements that help keeping the number of transactions executed within the deadline near to the throughput. The enhancements are DiffServ, in which short transactions have priority, and a 2-Phase Admission Control (2PAC) mechanism, which tries to avoid the previousmentioned problem by limiting the queue size dynamically using informations provided by a feedback control. It also introduces the QoS-Broker – a tool which implements both SAC and 2PAC – and uses it to compare their performances when submitted to the TPC-C benchmark. Our results show that both total throughput and throughput within deadline increase when the 2 enhancements are used, although it becomes clear that 2PAC has a much bigger impact on performance than DiffServ.