Xiao Liu PhD Thesis.pdf - Faculty of Information and Communication ...

8.3 A Novel General Two-Stage Local Workflow Rescheduling
Strategy for Recoverable Temporal Violations
8.3.1 Description of the General Strategy
Given the two fundamental requirements of automation and cost-effectiveness, the
temporal violation handling strategies that we mainly investigate in this chapter are
metaheuristics based workflow rescheduling algorithms. With metaheuristics based
rescheduling algorithms, temporal violations are tackled by rescheduling the current
Task-to-Resource assignment. Note that in cloud computing environments,
computing resources are generally delivered as virtual machines (VMs). Therefore,
in this chapter, Task-to-Resource assignment and Task-Resource list are
interchangeable with Task-to-VM assignment or Task-VM list respectively. For
workflow rescheduling, the following two challenges need to be considered: 1) the
balance between the handling of temporal violations and the on-time completion of
other workflow instances; 2) the suitable size of the Task-Resource list for costeffective
rescheduling. The reason why we do not consider heuristics based
scheduling algorithms in this scenario is mainly as follows:
1) Heuristics based scheduling algorithms can only optimise one objective at a
time. Meanwhile, it is based on local optimisation where it can choose the best
candidates at each local step for the optimisation objective. However, the generated
solution is not necessarily the best solution for the optimisation objective in an
overall sense.
2) From the viewpoint of metaheuristics based scheduling algorithms, the
process of heuristics based scheduling algorithms is to construct valid solutions.
Since metaheuristics based scheduling algorithms can use the valid solutions
generated by heuristic scheduling algorithms as the initial input solution, the
optimisation capability of metaheuristics based scheduling algorithm is theoretically
guaranteed to be better than heuristic scheduling algorithms.
To address the above two challenges, a novel general two-stage workflow local
rescheduling strategy is designed for handing temporal violations. The pseudo-code
for the general strategy is presented in Figure 8.1.
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