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Earliest deadline first (EDF) or least time to go is a dynamic priority scheduling algorithm used in real-time operating systems to place processes in a priority queue. Whenever a scheduling event occurs (task finishes, new task released, etc.) the queue will be searched for the process closest to its deadline.
An important class of scheduling algorithms is the class of dynamic priority algorithms. When none of the intervals overlap the optimum solution is trivial. The optimum for the non-weighted version can found with the earliest deadline first scheduling. Weighted interval scheduling is a generalization where a value is assigned to each executed ...
EEVDF was first described in the 1995 paper "Earliest Eligible Virtual Deadline First : A Flexible and Accurate Mechanism for Proportional Share Resource Allocation" by Ion Stoica and Hussein Abdel-Wahab. [2] It uses notions of virtual time, eligible time, virtual requests and virtual deadlines for determining scheduling priority. [1]
Higher schedulable utilization means higher utilization of resource and the better the algorithm. In preemptible scheduling, dynamic priority scheduling such as earliest deadline first (EDF) provides the optimal schedulable utilization of 1 in contrast to less than 0.69 with fixed priority scheduling such as rate-monotonic (RM). [1]
Thus, during a brief overload of system resources, LST can be suboptimal. It will also be suboptimal when used with uninterruptible processes. However, like the earliest deadline first, and unlike rate monotonic scheduling, this algorithm can be used for processor utilization up to 100%.
But in complex situations it can easily fail to find the optimal scheduling. HEFT is essentially a greedy algorithm and incapable of making short-term sacrifices for long term benefits. Some improved algorithms based on HEFT look ahead to better estimate the quality of a scheduling decision can be used to trade run-time for scheduling performance.
Location of the process scheduler in a simplified structure of the Linux kernel. SCHED_DEADLINE is a CPU scheduler available in the Linux kernel since version 3.14, [1] [2] based on the earliest deadline first (EDF) and constant bandwidth server (CBS) [3] algorithms, supporting resource reservations: each task scheduled under such policy is associated with a budget Q (aka runtime), and a ...
For example, in a real-time scheduling algorithm a deadline could be set to five nano-seconds. In a critical operation the task must be processed in the time specified by the deadline (i.e. five nano-seconds). A task in a real-time system must be completed "neither too early nor too late;..".