1. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 0 Lesson 9 Process Management

2. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 1 LESSON OVERVIEW  The process model  Implementation of processes  Interprocess communication (IPC)  Process scheduling  Deadline scheduling

3. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 2 The Process Model  Virtual time  Virtual memory  Process hierarchies  Process states and transitions

4. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 3 Implementation Of Process  Process table  Process table entry (PTE)  Level principle  Data Structure  Interrupt vector

5. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 4 Race Conditions It is occurs when two processes can interact and the outcome depends on the order in which the processes execute.

6. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 5 Requirements For Critical Sections Implementation  No two processes may be simultaneously inside their critical section.  No assumption may be made about the speeds or the number of CPUs.  No process outside its critical section may block other processes.  No process should have to wait forever to enter its critical section.

7. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 6 Objectives Of A Good Scheduling Policy  Degrade gracefully under load.  Fair across projects  Low response time  High throughput  Fairness  Efficiency  Repeatability  Low turnaround time

8. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 7 Preemption  It means the operating system moves a process from running to ready without the process requesting it.  Without it, the system implements ``run to completion (or yield or block)''.  It needs a clock interrupt (or equivalent).  It is needed to guarantee fairness.  Found in all modern general-purpose operating systems.

9. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 8 Deadline Scheduling This is used for real time systems. The objective of the scheduler is to find a schedule for all the tasks (there are a fixed set of tasks) so that each meets its deadline. The run time of each task is known in advance.

10. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 9 First Come First Served (FCFS) If the OS doesn't schedule, it still needs to store the PTEs somewhere. If it is a queue you get FCFS. If it is a stack (strange), you get LCFS (Last Come First Served).  Only FCFS is considered.  The simplest scheduling policy.  Non-preemptive.

11. Copyright © Genetic Computer School 2008 Computer Systems Architecture SA 9- 10 Round Robbin (RR)  An important preemptive policy.  Essentially the preemptive version of FCFS.  The key parameter is the quantum size q.