Chapter 4 - DSpace at Waseda University

Appendix
critical task and non-critical task. Moreover, according to whether CPU is conceded or not
when high priority task arrives, it can be divided in to preemptive task and non-preemptive
task. Common task concedes CPU to high priority task, which is preemptive task. However,
when task is processing critical section, it cannot be prior occupied.
Methods of giving priority to task are static priority method and dynamic priority method.
Static priority is a method that never changes priority that was assigned from scheduler and
dynamic priority means method that changes dynamically according to process that will be
occurred. The dynamic priority method can be scheduled more effectively more than static
priority. The dynamic priority algorithm mostly used in commercial real-time OS, such as
VxWorks or PSOS, are fixed priority scheduling and Earliest Deadline First (EDF) method.
Both of algorithms are preemptive scheduling method and they are algorithm that assumes
single processor.
Firstly, as shown in the name, fixed priority scheduling is a method that gives fixed priority
to every task. It was a problem about how to grant fixed priority. In 1973, Liu and Laryland
proposed algorithm called rate-monotonic scheduling. This method proposed to grant high
priority to task with short period (premise: every task is periodic task and deadline of task is
equal to period of task). Due to simplicity of rate-monotonic scheduling and its mathematical
characteristic which makes it possible to schedule anytime Utilization Factor (UF) is under
0.67, it was often used for these few decades.
EDF is an algorithm that grants higher priority to task that is shorter to deadline. For this
reason, it is also called as deadline driven algorithm. This algorithm can be used to schedule
not only periodic task but also all other tasks on single processor architecture. It was proved
that it is most optimal mathematically.
86