Comments on “IPPTS: An Efficient Algorithm for Scientific Workflow Scheduling in Heterogeneous Computing Systems”

Abstract

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IPPTS</i> (Improved Predict Priority Task Scheduling) is a list scheduling algorithm that schedules task graphs on fully connected heterogeneous distributed systems, with an objective of minimizing the overall makespan (i.e., schedule length). With respect to the literature on list scheduling techniques for task graphs, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IPPTS</i> improves the task prioritization by considering the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">“out-degree”</i> of a task. However, we have observed that the IPPTS algorithm contains an ambiguity which introduces <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">the possibility of assigning higher priority to a task compared to its predecessors in a task graph</i> . This priority inversion <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">may lead to the generation of an incorrect schedule</i> due to the violation of precedence-constraints among tasks. In this note, we first highlight this issue using a counter example. Then, we discuss two possible ways to fix the ambiguity in the algorithm.