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

For example, in Figure 8.2, local workflow segment contains activity a p+ 1 to
a p+7 and they are allocated to four different resources R 1 to R 4 . Each resource
maintains a local Task-List by its own scheduler given its input job queue. When
temporal violation handling is triggered, the workflow management system will
acquire the current Task-List of R 1 to R 4 and can automatically combine them into
an integrated DAG task graph which consists of all the tasks, for instance, a total of
n tasks, by assigning a pseudo start activity a Start and pseudo end activity a End .
Therefore, an integrated Task-Resource list L{ ( a , R ) | i = p + 1,..., p + n,
j = 1,2,3,4 }
is built and ready to be optimised.
As shown in Figure 8.1, the strategy has five major input parameters, viz. the
time deficit detected at the checkpoint, the integrated Task-Resource list, the DAG
task graphs which define the precedence relationships between tasks, the normal
distribution models for activity durations, and resources with their execution speed
and the cost per time unit. Besides, some additional information or parameters may
also be required for each individual metaheuristic rescheduling algorithm. The first
stage is to optimise the overall the makespan and cost for the integrated Task-
Resource list through any metaheuristics based scheduling algorithm such as GA
(detailed in Section 8.3.2) and ACO (detailed in Section 8.3.3). The first step is
algorithm initialisation (Line 1) where different metaheuristic algorithms have their
own initialisation process such as chromosome coding for GA and setting of initial
pheromones for ACO. After initialisation, the metaheuristic algorithms are executed
until the predefined stopping condition such as the maximum iteration times is met
(Line 2 to Line 3). During the metaheuristic algorithms based optimisation process,
a best-so-far solution can be produced in each iteration and recorded in the solution
set. The second stage is to search for the best solution from the solution set (Line 5
to Line 7). During this searching stage, the occurred time deficit is first compared
with the compensation time of each solution in the solution set (Line 6). Those
solutions whose compensation time is smaller than the time deficit are discarded
since they cannot handle the current temporal violation. Here, the compensation time
is defined as the difference between the average makespan before rescheduling and
the one after rescheduling. For those remained solutions, the one with the minimum
cost is returned as the best solution (Line 7 to Line 8). Finally, according to the best
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