43
3.6
Conclusions
In this chapter, we have investigated the impact of dynamic coscheduling algo 
rithms on cluster based Web server performance. We have modified a distributed Web
server model, called PRESS [23], to implement coscheduling techniques for remote cache
accesses to minimize the response time. We have developed a flexible and comprehensive
simulator testbed for evaluating different server architectures. The simulator is compre 
hensive in that it captures the characteristics of any user level communication at the
network level, models the behavior of a coscheduling algorithm at the kernel level, and
models the Web server details at the application level. The simulator is flexible enough
to analyze performance tradeoffs with various systems and workload parameters.
We compared four server designs (PRESS over VIA, PRESS with DCS, cosched 
uled PRESS with DCS and blocking, and Adaptive) on a 32 node cluster environment.
The simulation results indicate that it is possible to minimize the average response time
of client requests by coscheduling the communicating processes at remote nodes. In par 
ticular, the use of the DCS coscheduling scheme reduced the latency up to four times in
lightly loaded servers compared to that with the original PRESS model. However, the
performance of the dcs model degrades as the probability of coscheduling communicat 
ing processes decreases due to competition among the remote cache read requests. Thus
we proposed the adaptive model. The adaptive model can provide a stable performance
benefit by employing a coscheduling algorithm selectively depending on the systems load.












  

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