Processes

Processes

1. Definition

   • Program in execution
     • Process execution must progress in a sequential function

2. Changing States

   • New
     • Create new process - gets various resources
   • Ready
     • Has all resources (apart from CPU)
   • Waiting
     • Process can give up processesor
   • Terminated
     • OS claims all resources back

3. Process Control Block

   • Process information stored in kernel data structure
     • Includes information like state, program counters, CPU registers, scheduling info

4. Process creation

   • Tree structure
     • Parent process creates children processes which, in turn, create other processes
     • Identified / managed by PiD
   • Resource Sharing
     • Parent and children share all resources
     • Children share subset of parent's resources
     • Parent and child share no resources
   • Execution
     • Parent and children execute concurrently
     • Parent waits until children terminate

5. Process termination

   • Parent may terminate execution of child processes (abort)
     • Child may have exceeded allocated resources
     • Task assigned to child is no longer required
     • If parent exits, OS may cascade to children
   • Process executes last statement and asks the OS to exit
     • Output data from child to parent (via wait)
     • Process resources are deallocated by OS

6. Context switch

   • When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process
   • Context of process represented in PCB
   • Context-switch time is overhead, system does no useful work while switching - need to minimise
     • Time dependant on hardware support

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