Decentralized Self-tuning Relative Delay Guarantee Scheme For Web Server

ABSTRACT:Internet Quality of Service (QoS) could be conspicuously improved with proportional delay guarantee in Web Server. Apache was the first open-source web server software in the world. The software was available for a wide variety of operating systems. Apache Web Server could provide proportional delay differentiated services by modifying its source codes.Single-variable model was generally used for Apache web server. It had poor scalability when was used to handle multiple priorities delay guarantee. A Multi-input Multi-output (MIMO) system model of web server was proposed which decentralized self-tuning controllers based on decentralized control theory and self-tuning control (STC) theory was designed for it. According to decentralized control theory, the multivariable system was divided into interconnected single-variable subsystems. SISO model was obtained by subsystem identification, the model input was the ratio of the number of worker threads which deal with different kind of client connections to the number of all the worker threads created by Apache, the model output was the ratio of the average delay of different kind of client connections to the sum of the average delay of all kinds of client connections. Subsequently, these subsystems were controlled by single-input single-output (SISO) self-tuning controllers in order to handle the non-negligible effects of subsystem interconnections. These controllers could dynamically adjust the number of worker thread which handled different priority requests to ensure, that higher priority requests were processed faster and the average delay ratio of different priority was maintained the same as settings. The parameters of model and controllers were updated according to the result of online identification. WinNT MPM was the only use MPM in Apache under Windows operating system. Sampling mechanism, connection classifier and control thread were added in modified WinNT MPM. A closed loop control system was built inside the Apache.Vmware virtual machine was used to build simulation platform. The results of simulation tests indicate that proportional delay guarantee can be maintained on the Web Server in the closed-loop system even if the number of concurrent client connections changed abruptly under overload conditions.