Real-time Multiprocessor Scheduling Algorithm Research Articles

Practical performance analysis of real-time multiprocessor scheduling algorithms

This paper presents a practical performance analysing of two real-time multiprocessor scheduling algorithms, namely, Largest Remaining Execution-Time and Local Time Domain (LRE-TL) and Unfair Semi-Greedy (USG). The analysis is intended to reflect the behind-the-scene time overhead incurred by optimal real-time algorithms such as LRE-TL. The overhead is known to be capable of dismissing the actual optimality of such algorithms in practical applications. Here, the time overhead is measured in terms of the number of scheduler invocations and the time required by the scheduling event handlers. In the implementation of the proposed analysis method, the CPU profiler of Oracle JavaTM VisualVM was used to monitor the executions of LRE-TL and USG. The profiler measured the number of invocations of the scheduling event handlers for each algorithm and the total time required for all the invocations. The results revealed that USG outperformed LRE-TL on both measures, indicating that optimal algorithms may prove to be non-optimal in practical applications.

Minimizing scheduling overhead in LRE-TL real-time multiprocessor scheduling algorithm

In this paper, we present a modification of Local Remaining Execution-Time and Local time domain (LRE-TL) real-time multiprocessor scheduling algorithm, aimed at reducing the scheduling overhead in terms of task migrations. LRE-TL achieves optimality by employing the fairness rule at the end of each time slice in a fluid schedule model. LRE-TL makes scheduling decisions using two scheduling events. The Bottom (B) event, which occurs when a task consumes its local utilization thus; it has to be preempted in order to resume the execution of another task, if any, or to idle the processor if none exist. The Critical (C) event occurs when a task consumes its local laxity which means that the task cannot wait any more and has to be scheduled for execution immediately otherwise, it will miss its deadline. Event C always results in a task migration. We have modified the initialization procedure of LRE-TL to make sure that tasks which have higher probability of firing a C event will always be considered for execution first. This will ensure that the number of C events will always be at the minimum; thereby reducing the number of task migrations.

An unfair semi-greedy real-time multiprocessor scheduling algorithm

Most real-time multiprocessor scheduling algorithms for achieving optimal processor utilization, adhere to the fairness rule. Accordingly, tasks are executed in proportion to their utilizations at each time quantum or at the end of each time slice in a fluid schedule model. Obeying the fairness rule results in a large number of scheduling overheads, which affect the practicality of the algorithm. This paper presents a new algorithm for scheduling independent real-time tasks on multiprocessors, which produces very few scheduling overheads while maintaining high schedulability. The algorithm is designed by totally relaxing the fairness rule and adopting a new semi-greedy criterion instead. Simulations have shown promising results, i.e. the scheduling overheads generated by the proposed algorithm are significantly fewer than those generated by state-of-the-art algorithms. Although the proposed algorithm sometimes misses a few deadlines, these are sufficiently few to be tolerated in view of the considerable reduction achieved in the scheduling overheads.

Composition of Schedulability Analyses for Real-Time Multiprocessor Systems

With increasing popularity and deployment of multi-core chips in embedded systems, a number of real-time multiprocessor scheduling algorithms have been proposed along with their schedulability analyses (or tests), which verify temporal correctness under a specific algorithm. Each of these algorithms often comes with several different schedulability tests, especially when it is difficult to find exact schedulability tests for the algorithm. Such tests usually find different task sets deemed schedulable even under the same scheduling algorithm. While these different tests have been compared with each other in terms of schedulability performance, little has been done on how to combine such different tests to improve the overall schedulability of a given scheduling algorithm beyond a simple union of their individual schedulability. Motivated by this, we propose a composition theory for schedulability tests with two new methods. The first method composes task-level timing guarantees derived from different schedulability tests, and the second one derives system-level schedulability results from a single schedulability test. The unified composition theory with these two methods then utilizes existing schedulability tests effectively so as to cover additional schedulable task sets. The proposed composition theory is shown to be applicable to most existing preemptive/non-preemptive scheduling algorithms. We also present three case-studies, demonstrating how and by how much the theory can improve schedulability by composing existing schedulability tests. Our evaluation results also show that the composition theory makes it possible to cover up to 181.7 percent additional schedulable task sets for preemptive fpEDF, preemptive EDF and non-preemptive EDF scheduling algorithms beyond their existing tests.

Capturing urgency and parallelism using quasi-deadlines for real-time multiprocessor scheduling

Recent trends toward multi-core architectures in real-time embedded systems pose challenges in designing efficient real-time multiprocessor scheduling algorithms. We believe that it is important to take into consideration both timing constraints of tasks (urgency) and parallelism restrictions of multiprocessor platforms (parallelism) together when designing scheduling algorithms. Motivated by this, we define the quasi-deadline of a job as a weighted sum of its absolute deadline (capturing urgency) and its worst case execution time (capturing parallelism) with a system-level control knob to balance urgency and parallelism effectively. Using the quasi-deadline to prioritize jobs, we propose two new scheduling algorithms, called EQDF (earliest quasi-deadline first) and EQDZL (earliest quasi-deadline until zero laxity), that are categorized into job-level fixed-priority (JFP) scheduling and job-level dynamic-priority (JDP) scheduling, respectively. This paper provides a new schedulability analysis for EQDF/EQDZL scheduling and addresses the problem of priority assignment under EQDF/EQDZL by determining a right value of the system-level control knob. It presents optimal and heuristic solutions to the problem subject to our proposed EQDF and EQDZL analysis. Our simulation results show that EQDF and EQDZL can improve schedulability significantly compared to EDF and EDZL, respectively.

Reducing Tasks Migration in LRE-TL Real-time Multiprocessor Scheduling Algorithm

In this paper we present a modification of Local Remaining Execution (LRE-TL) real-time multiprocessor scheduling algorithm to reduce tasks migration significantly. LRE-TL, which is based on the concept of fluid scheduling, makes scheduling decisions using two events. The Bottom (B) event occurs when a task consumes its local utilization thus; it has to be pre-empted. The Critical (C) event occurs when a task consumes its local laxity thus; it has to be scheduled for execution. Event C always results in a task migration. We have modified the initialization procedure of LRE-TL to make sure that tasks which have higher probability of firing a C event will always be scheduled for execution first. This will ensure that the number of C events will always be the minimum; thereby tasks migration will be reduced. To verify our work, an independent-samples t-test is used to compare tasks migration using the original LRE-TL algorithm and the modified one. The results showed that there was a significance reduction in tasks migrations when we apply our proposed solution.