Os examples scheduling
- 1. Operating systems By : Dana Dia'a
- 2. Linux scheduling In Linux, tasks are divided into classes and the processorchooses the algorithm based on the priority assigned foreach class. The aspects in which processorschedule tasks may be differentbased on priority. The scheduling criteria, for example: Android is based on Linux differs from Windows which is based on Linux, too, and they all are differentfrom Linux-based servers.To determine the next task to be running, the scheduler selects the highest priority in the highest -priority scheduling class. In Linux, there are two scheduling classes:default scheduling class; using completelyfair scheduler(CFS), and a real-time scheduling class. According to CFS scheduling algorithms, CPU process time is divided upon tasks and the proportion is based on the nice value (lower value is higher relative priority; which means higher proportionof CPU processtime). Priority does not assign directly by CFS scheduling; instead, time is recorded by the virtual-run time.
- 3. Windows scheduling Windows threads use a priority-based, preemptive scheduling. Highest priority runnable thread always runs. Windows dispatcher (handles the scheduler)gives control to the thread selected by the short term scheduler, a dispatch will run a selected thread until it is distracted by one of these condition: is preemptedby a higher- priority thread, it terminates, its time quantum end, or blocking system call (such as I/O) . In Windows scheduling (dispatcher), when higher priority thread is ready to run and need access,it preemptthe running lower priority. Priorities are divided into two classes:the variable class and the real-time class. The dispatcheruses a queue for each scheduling priority and order the set of queues from the highest to lowest until it finds a thread that is ready to run, if it doesn’tfind a ready thread, it executes an idle thread. The priority for each thread is based on both the priority class and its relative priority.
- 4. Solaris scheduler Solaris uses six classes priority-based thread scheduling in which each class is divided into different priorities and algorithms. The default is time sharing; which alters priorities and assigns different lengths time-slices dynamically by multilevel feedbackqueue (inverse relationship; the higher the priority, the smaller the time- slice and vice versa). The interactive processes -higherpriority typically- gives a good response time. It uses a scheduling policy as time-sharing but gives windowed-applications a higher priority. The Solaris kernel is fully promotable, this means that all threads to allow a higher priority thread to run. Threads in real-time class are given the highest priority, guaranteed response time. In system class, kernel processesare assigned to this class, and priorities are static. In fixed and fair priority, no dynamic adjustment of priorities. Solaris uses a dispatch table; this is used to assign a new priority to a process intwo cases:time quantum expired without blocking, process blockedbefore time quantum expired.