Optimal Semi-Partitioned Scheduling in Soft Real-Time Systems

Abstract

Semi-partitioned real-time scheduling algorithms extend partitioned ones by allowing a (usually small) subset of tasks to migrate. The first such algorithm to be proposed was directed at soft real-time (SRT) sporadic task systems where bounded deadline tardiness is acceptable. That algorithm, called EDF-fm, is able to fully utilize the underlying hardware platform's available capacity. Moreover, it has the desirable practical property that migrations are boundary-limited, i.e., they can only occur at job boundaries. Unfortunately, EDF-fm requires restrictions on pertask utilizations, and thus is not optimal. In this paper, a new boundary-limited, semi-partitioned algorithm is presented for SRT systems that is the first such algorithm to be optimal. This algorithm, called EDF-os, is similar to EDF-fm but utilizes several new mechanisms that obviate the need for per-task utilization restrictions. Experiments presented herein show that, not only is EDF-os provably better than EDF-fm with respect to schedulability, tardiness bounds under EDF-os are often much lower as well.

EDF-hv

“…al. in [1,2]. The work in [1] presents EDF-fm, a semi-partitioning algorithm that guarantees feasibility up to 100% of the system capacity with the restriction that no single task can have a utilization greater than 50% of a single core.…”

“…The work in [1] presents EDF-fm, a semi-partitioning algorithm that guarantees feasibility up to 100% of the system capacity with the restriction that no single task can have a utilization greater than 50% of a single core. In [2], EDF-fm was extended to EDF-os, which removes the per task utilization requirements, i.e., tasks can have a utilization up to 100% of a single core. Neither of these semi-partitioning heuristics can guarantee deadlines; however, both have a bound on how late the tasks can be, referred to as bounded tardiness.…”

“…Further, in both EDF-fm and EDF-os, to reduce migration overhead, a migrating task can only migrate from one core to another after a job/iteration of the task has fully completed, a property referred to as boundary-limited. EDF-os [2] is designed for periodic tasks on a homogeneous multi-core system. As a semi-partitioning algorithm, task are either fixed (i.e., the task is assigned to execute on only one core) or migrating (i.e., the task is assigned to execute a defined share of jobs on a subset of the cores).…”

“…Additionally, on cores that have two migrating tasks, one migrating task has a fixed priority over the other. Due to the method in which tasks are assigned to the cores, Anderson et al were able to derive tardiness bounds for the tasks [2].…”

Reducing the Interprocessors Migrations of the EKG Algorithm

“…To overcome the problem of the partitioned approach and increase the utilization rate of the system, recent works [6,7,8,9,10] have introduced the semi-partitioning scheduling in which most of tasks are assigned to particular processors as the partitioned scheduling, but the remaining tasks (unasigned tasks) are allowed to migrate between processors. In other words, each remaining task is splitted into a set of sub-tasks and each one of them is affected to a processor.…”