Processes

Processes

1. What's a 'process'?

   • A program IN execution
     • A program that has its own address space includes:
       • Text Section
         • Stores the code that the processor executes
       • Data section
         • Stores global variables
       • Stack
         • Stores local variables + function parameters
       • Heap
         • Memory dynamically allocated during runtime
     • Processors are separated from one another
   • Unit of work in a modern computer system
   • NOT a program
     • Program = passive entity
     • Process = active entity
   • Can be Independent or Cooperating
     • e..g for info sharing, computation speedup, convience
       • via shared memory or message passing

2. Process Concept

   • A key OS abstraction that users see
     • The environment you interact with is built out of processes
       • e.g. Shell
       • e.g. executing a program you've compiled
       • e.g. web browser
       • Organising system activities around processes is useful for separating activities
         • Resource Sharing
           • Key Issue: Ensure all processes get fair share of CPU
             • Pre-Emptive Multitasking
               • OS runs one process for a while, takes CPU from that process and lets another process run
                 • Must save and restore process state
                   • Process State
                     • new
                       • process is being created
                     • running
                       • instructions being executed
                     • waiting
                       • process waiting for some event to occur (I/O)
                     • ready
                       • process waiting to be assigned processor by scheduler dispatcher
                     • terminated
                       • process finished execution
                     • Process Control Block (PCB)
                       • Holds current state of a process
                         • Process state
                         • Program counter
                         • CPU registers
                         • CPU scheduling information
                         • Memory management info
                         • Accounting info
                         • I/O status
                     • Context Switching
                       • Saving + Loading states
                       • Context switch time = overhead
                         • Solution: Threads
                 • Scheduling Queues
                   • Job Queue
                     • All processes in system
                   • Ready Queue
                     • All processes residing in memory 'ready'
                   • Device Queues
                     • Processes waiting for an I/O device

3. Operations on Processes

   • Process Creation
     • Parent process creates child processes
       • Produces tree of processes
       • Resource Sharing Possibilities
         • Share all resources
         • Subset of resources
         • No resources shared
       • Execution Possibilites
         • P/C execute concurrently
         • P waits for C termination
       • Address Space Possibilites
         • C duplicate of P
         • C has mew program loaded
   • Process Termination
     • Process executes last statement
       • Asks OS to delete it (exit system call)
         • Output data from child to parent (wait system call)
         • Process resources de-allocated by OS
     • Parent may terminate execution of child process (abort system call)
       • Cascading Termination = All children terminated

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