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

violation handling strategies in the scientific workflow systems to address the
violations of QoS constraints [100, 101]. Workflow local rescheduling is to
compensate for the time deficit by rescheduling some subsequent activities after the
handling point in a local workflow segment to reduce their scheduled execution time
(detailed in Section 8.3). In this chapter, since we focus on checkpoint selection and
temporal verification, instead of applying a specific workflow rescheduling strategy,
a pseudo-rescheduling strategy with a reasonable average time compensation rate
(i.e. the reduced rate of the scheduled execution time for the subsequent activities) of
50% is applied to represent the average performance of representative rescheduling
strategies [56, 100] (detailed in Section 8.5). The size of rescheduled workflow
activities, i.e. the number of subsequent activities which need to be rescheduled, is
randomly selected up to 5 which is normally large enough to compensate for time
deficit with the given compensation rate. In the real world, not every workflow
rescheduling can be successful (for example, when the current workload is
extremely high). Therefore, we set 80% as the success rate for temporal violation
handling.
CSS TD is applied with default settings as defined in [22].
6.3.2 Experimental Results
Here, we demonstrate the experiments on checkpoint selection. The number of
selected checkpoints is recorded for every experiment. However, for the ease of
discussion without losing generality, we will only demonstrate the results in
COM(1.28), i.e. with 90% initial probability temporal consistency state. More
experimental results and supplement material could be found online 5 . Since the
necessity and sufficiency of our checkpoint selection strategy have already been
proved in Section 6.2.3, the experiment here is mainly to verify the effectiveness of
our strategy in different system environments. Therefore, additional experiment
settings have been implemented. Firstly, we demonstrate the results with different
noises. Embedding noises is to artificially increase the duration of a randomly
selected activity. The purpose of embedded noises is to simulate the system
environments where unexpected delays often take place. Meanwhile, noises can also
simulate estimation errors such as those brought by the two assumptions (e.g. non-
5 http://www.ict.swin.edu.au/personal/xliu/doc/CheckpointSelection.rar
100