Improved response time analysis of sporadic dag tasks for global FP scheduling

One of the major sources of pessimism in the response time analysis of globally scheduled real-time tasks is the computation of the upper-bound on the inter-task interference. This problem is further exacerbated when intra-task parallelism is permitted, because of the complex internal structure of parallel tasks. This paper considers the global fixed-priority scheduling (G-FP) of sporadic real-time tasks, each one modeled by a directed acyclic graph (DAG) of parallel subtasks. We present a response time analysis (RTA) technique based on the concept of problem window. We propose two novel techniques to derive less pessimistic upper-bounds on the workload produced by the carry-in and carry-out jobs of the interfering tasks, by taking into account the precedence constraints between their subtasks. We show that with these new upper-bounds, the proposed schedulability test does not only theoretically dominate state-of-the-art techniques but also offers significant improvements on the schedulability of DAG tasks for randomly generated task sets.