Scheduling

Scheduling

1. Basic Concepts

   • CPU I/O Burst Cycle
     • The success of CPU scheduling depends on an observed property of processes: process execution consists of a cycle of CPU execution and I/O wait. Processes alternate between these two states.
   • CPU Scheduler
     • Whenever the CPU becomes idle, the operating system must select one of the processes in the ready queue to be executed. The selection process is carried out by the short-term scheduler, or CPU scheduler.  The scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process. 
   • Preemptive Scheduling
     • CPU scheduling decisions take place under the following circumstances: 1. When a process switches from the running state to the waiting state (for example, as the result of an I/O request) 2. When a process switches from the running state to the ready state (e.c. when an interrupt occurs) 3. When a process switches from the waiting state to the ready state (e.g. at completion of I/O) 4. When a process terminates
   • Dispatcher
     • Another component involved in the CPU-scheduling function is the dispatcher. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler.  This function involves the following: • Switching context • Switching to user mode • Jumping to the proper location in the user program to restart that program The dispatcher should be as fast as possible, since it is invoked during every process switch. The time it takes for the dispatcher to stop one process and start another is known as the dispatch latency.

2. Scheduling Criteria

3. Scheduling Algorithms

4. Thread Scheduling

5. Multiprocessor Scheduling

6. Realtime CPU Scheduling

7. OS Examples

8. Algorithm Evaluation