Process Scheduling

1. Threads

1.1. The process model discussed so far has implied that a process is a program that performs a single thread of execution.

1.2. Most modern operating systems have extended the process concept to allow a process to have multiple threads of execution and thus to perform more than one task at a time.

1.3. On a system that supports threads, the PCB is expanded to include information for each thread.

2. The state of a process is defined in part by the current activity of that process

2.1. New - The process is being created.

2.2. Running - instructions are being executed

2.3. Waiting - The process is waiting for some event to occur

2.4. Ready - The process is waiting to be assigned to a processor

2.5. Terminated - The process has finished execution

3. The objective of time sharing is to switch the CPU among processes so frequently that users can interact with each program while it is running. The process scheduler play its role to selects an available process for program execution on the CPU.

4. Schedulers

4.1. Batch system – jobs

4.2. Time-shared systems – user programs or tasks

4.3. On a single-user system, a user may be able to run several programs at one time:

4.4. On an embedded device that does not support multitasking, the operating system may need to support its own internal programmed activities