1. Ceng 334 - Operating Systems 2.1-1 Chapter 2.1 : Processes Process concept Process scheduling Interprocess communication Deadlocks Threads

2. Ceng 334 - Operating Systems 2.1-2 What is a Process? A process (task) is the activity resulting from the execution of a program with its data on a sequential processor A process is an abstract form of a program

3. Ceng 334 - Operating Systems 2.1-3 Sequential Execution Programs are run one after the other (run to completion) Program 2Program 1Program 3 Start Terminate

4. Ceng 334 - Operating Systems 2.1-4 Concurrent Execution All programs execute concurrently (share the time of the CPU) Program 2Program 1Program 3 Start Terminate Start Terminate

5. Ceng 334 - Operating Systems 2.1-5 How to Implement Concurrency? Separate a program into code, data and environment (Process table) segments Process table contains all the necessary data to stop and restart a process at any time Start Terminate Start Terminate Code 1 Data 1 PCB 1 Code 2 Data 2 PCB 2 Code 3 Data 3 PCB 3

6. Ceng 334 - Operating Systems 2.1-6 Process Table (PCB - Process Control Block) Contents of registers Program counter PSW (Program Status Word) Process state Pointers to code and data Process identifier (PID) Process priority File descriptors Pointer to the parent of the process Pointers to all children of the process The processor it is running on

7. Ceng 334 - Operating Systems 2.1-7 Switching Processes Save the “real world” of the currently active process in process table Restore the “real world” from the process table of the selected process (the one to be executed next)

8. Ceng 334 - Operating Systems 2.1-8 System Process Tables Process tables Code Data Stack Process

9. Ceng 334 - Operating Systems 2.1-9 Shared Code The shared code must be re-entrant (ie., it must not modify itself) Shared Code Data 1 PCB 1 Data 2 PCB 2

10. Ceng 334 - Operating Systems 2.1-10 Process States Ready Blocked Running Terminate Create SUSPEND CONTINUE SCHEDULE PREEMPT

11. Ceng 334 - Operating Systems 2.1-11 Process States Create –A process is created by an existing process (parent/child); a process is created by a service, or as a new batch job, or an interactive logon Terminate –Normal completion (exit); External completion (forced completion: by operator, by parent); Internal completion (error, time overrun) Schedule –A process is selected out of the ready queue (is scheduled) and dispatched to the running state Preempt –The running process is preempted because some other process of higher priority has become ready (or it yields)

12. Ceng 334 - Operating Systems 2.1-12 Process States Suspend –A process starts a time consuming IO operation, or is swapped out, or makes a spontaneous request to sleep, or is blocked in a synchronization operation. When a process is suspended, it is on some queue, namely the queue of all the processes waiting for the same resource. Processes during their life move from queue to queue, with stays in the running state Continue –The inverse of Suspension: the resource requested becomes available (timer, IO completes, unblocking)

13. Ceng 334 - Operating Systems 2.1-13 How to Create New Processes? Fork & Exec fork –create a new process that is a copy of current one exec –change the program that a process is executing Some systems combine the two into one

14. Ceng 334 - Operating Systems 2.1-14 void main() { int child; if ( (child = fork())==-1 ) return( -1 ); else if ( child==0 ) { printf( "This is the child executing\n"); exit(1); } printf( "This is the parent\n" ); }

15. Ceng 334 - Operating Systems 2.1-15 Parent Child Relationship root is the parent p1, p2 and p3 are children of root process p4 and p5 are children of p3 root p5 p3 p4 p2 p1