Processes 2 Introduction to Operating Systems: Module 4

Process states terminated ready waiting running terminate block dispatch pre-empt wakeup inactive create activate suspend

Process states u Inactive (New or Suspended)  Process has been initiated; the OS has not yet allocated it any memory resources  Process has been active, OS needs to reduce the degree of multiprogramming to improve system performance u Ready  Process has some code and data in memory, is eligible to get CPU u Running  The CPU is currently executing code for this process  In single processor system, only one process is running at a time

Process states u Waiting (Blocked)  Process is in a queue waiting for some device or system resource u Terminated  Program is done; process resources need to be recovered (accounting)

Operations on processes: creation u When a process is created, the operating system updates its data structures to include the new process u Allocation of memory resources does not occur until activation, though the process may be stored on disk u UNIX examples  fork system call creates new process  execve system call used after a fork to replace the process’ memory space with a new program

Operations on processes: creation u Reasons for process creation  User logs on  User starts a program  OS creates process to provide a service (e.G., Printer daemon to manage printer)  Program starts another process (e.G., Netscape calls xv to display a picture)

Operations on processes: termination u When a process is terminated, the operating system must free its resources and remove it from OS data structures u Reasons for process termination  Normal completion  Arithmetic error, or data misuse (e.G., Wrong type)  Invalid instruction execution  Insufficient memory available, or memory bounds violation  Resource protection error  I/O failure

Operations on processes u Preempt a process (running  ready)  Process’ time quantum expires  Higher priority process arrives at the ready queue u Block a process (running  waiting)  begin waiting for I/O, resource u Wakeup a process (waiting  ready)  when notified I/O complete or resource is available

Operations on processes u Dispatch a process (ready  running)  Called on process at head of CPU queue after preempt or block u Change the priority of a process  in UNIX, the nice command u Suspend a process (ready  inactive)  Used to reduce the system load during a peak loading situation  Process state is stored on disk u Activate a process (inactive  ready)  Brings a new inactive process into memory  Resumes the execution of a suspended process

Swapping u What if memory cannot hold all processes?  Don't allow the creation of more processes  What about existing processes?  Dynamically allocated memory u Some processes need to have their address space written to disk so that others can continue  Such processes are said to be swapped to disk  Modern OSs don't swap out entire processes, they can selective "swap" parts of the address space We'll learn more of this when we study memory management

Process control block (PCB) u Representation of a process u Used to save and restore process context u Exact contents are system dependent  May reside in linked lists u When is the PCB read from memory? Written to memory? process state next previous process id program counter registers memory structure open file table etc