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Chapter 3: Process Description and Control

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  1. Chapter 3: Process Description and Control CS 472 Operating Systems Indiana University – Purdue University Fort Wayne

  2. Typical processes • Batch job • Perhaps a background application • Interactive process • Operating system function • Monitor performance • I/O manager • Spawned subprocesses • Children of a parent process

  3. Recall . . . • A process consists of • Activity • Data • Execution context • The execution context includes all information the operating system needs to manage the process

  4. The execution context includes . . . • Identifier • Processor state (hardware PCB) • Registers • Program counter • Stack pointer • PSW • Execution state (running, ready, blocked) • Priority and privilege • Memory pointers • I/O status information • Accounting information

  5. Process Control Block (PCB) • Data structure containing the execution context • Contains sufficient information to allow the process to be interrupted and later resumed • Created and managed by the operating system • Allows support for multiple processes

  6. Recall . . . • Each process has an execution state • Running • Executing on the processor • Ready • Ready to run • Waiting in a queue for time on the processor • Blocked • Waiting for an event to occur • Cannot run until the event happens • See next slide for a chronological scenario

  7. Process switching I/O request timer interrupts Process A Process B Process C Dispatcher time Interleaved execution of processes on a single processor

  8. Seven-state process model • We have seen that a process may be • Running • Ready • Blocked • A typical state transition model includes four additional states • New • Exit • Ready/Suspended • Blocked/Suspended

  9. Seven-state process model Note: all states but New have an implicit transition to Exit

  10. Additonal states • New • The process control block of the new process has been created • The process has not yet been admitted to the pool of executable processes • Perhaps sufficient resources do not yet exist • Exit • The process has been released from the pool of executable processes • Process information is available for performance analysis, accounting, or other clean up

  11. Additonal states • Ready / Suspended • A ready process is completely swapped out to secondary memory • The process is inactive while in this state • Blocked / Suspended • A blocked process is completely swapped out to secondary memory waiting for an event • Event completion moves the process to the Ready/Suspended state

  12. Reasons to suspend a process • To free memory for the remaining processes to use • This might be done to reduce the virtual memory page fault rate • A utility process may not be needed for a while • Perhaps only executed periodically • User request or parent process request • Lack of use • Deadlock among processes • Etc.

  13. Reasons to terminate a process

  14. Reasons to terminate a process

  15. Implementation of Running, Ready, and Blocked states

  16. Implementation • In a large operating system, there may be hundreds of processes in the blocked state • It is more efficient to have one blocked queue for each event • Queue holds all processes associated with that event • Avoids the need to search for processes to release whenever an event occurs • Occurrence of the event releases all processes in the associated queue • If processes have priorities, there would be one ready queue for each priority level

  17. Implementation • OS control system structures are needed to hold information about the current status of each process and resource • Tables are constructed for each entity the operating system manages

  18. Memory tables keep track of . . . • Main memory allocated to the OS and the various processes • Secondary memory allocated to each process • Any allocated shared memory • This includes protection attributes for access to shared memory regions • Information needed to manage virtual memory • In paged virtual memory, this includes the page tables of the various processes

  19. I/O tables • Availability of each I/O device • Is the device assigned to a process? • If so, which process is the owner? • Status of I/O operations • What is the location in main memory being used as the source or destination of the I/O transfer? • Which process is associated with the I/O operation?

  20. File tables • File tables provide information about . . . • Existence of files • Location on secondary memory • Current status • Closed or open • If open, by which process? • Attributes • Sometimes this information is maintained by a file management system

  21. Process table • Maintains the process image data structure for each process • The process image includes . . . • User data • User activity (program) • System stack (more user data) • Local variables • Parameters • Process control block (PCB)

  22. Operating system control tables

  23. Process control block • The process control block was discussed earlier in simplified form • In more detail, the elements of the PCB are . . . • Process identifiers • Processor state information (hardware PCB) • User-visible registers • Control and status registers • Stack pointers • Process control information • Scheduling and state information • Related processes • Interprocess communication information • Process privileges • Memory management information • Resource ownership and utilization

  24. Process identifiers • Identifiers that may be stored with the process control block include • Internal identifier of this process • Identifier of the process that created this process (parent process) • User identifier

  25. Processor state info (hardware PCB) • User-visible registers • Control and status registers • Program counter (PC) • Process status word (PSW) containing . . . • Condition codes • Result of the most recent arithmetic or logical operation • For example: sign, zero, carry, equal, overflow • Status information such as . . . • Trap enable bits for various traps (overflow, underflow, trace) • Interrupt priority level • Execution mode (user, kernel) • Interrupt enable / disable bit • Stack pointer (SP) • Possibly one SP for each execution mode

  26. Process control information • Scheduling and state information • Process execution state • Running, ready, blocked, ready/suspended, etc. • Priority relative to other processes • Scheduling-related information • The details are specific to each scheduling algorithm used • For example, the length of time that the process has been waiting • Events • Identification of the events that must occur before the process can be resumed

  27. Process control information • Related processes • A process may be linked to other processes in a queue, ring, etc. • A process may exhibit a parent-child (creator-created) relationship with another process • The process control block may contain pointers to other processes that support these relationships • Interprocess communication (IPC) information • Various flags, signals, and messages may be associated with communication between two separate processes • Some or all of this information may be maintained in the process control block

  28. Process control information • Process privileges • Memory that may be accessed • The types of instructions that may be executed • Allowed system utilities and services • Memory management information • References to segment and/or page tables that describe the virtual memory assigned to this process • Resource ownership and utilization • Open files • Devices currently controlled • History of utilization of the processor or other resources • This may be needed by the scheduler

  29. Execution modes • Systems typically have two execution modes • User mode • Less-privileged mode • User programs typically execute in this mode • Kernel mode • More-privileged mode • Kernel of the operating system executes in this mode • Also called system mode or control mode

  30. Execution modes • A user-mode process may run in kernel mode when necessary • The way this is possible is summarized below • The user-mode process executes a change-mode instruction • This a special supervisor call instruction • The change-mode instruction has an integer operand indicating a specific OS supervisor routine to be activated • The supervisor routine runs in kernel mode • For example, it may control a device • When done, the supervisor routine executes a special return instruction that changes the processor mode back to user mode • In this way, a user-mode process may temporarily gain necessary privilege in a controlled manner

  31. Mode switch • An interrupt or supervisor call typically results in a mode switch to the interrupt handler or supervisor routine • Also called a context switch • A mode switch is typically hardware implemented • Efficient to perform • Easy to restore the old mode (context) • The running process does not undergo a change of state from Running to Ready as a result of a mode switch

  32. Mode switch • What is involved • The hardware context (hardware PCB) of the interrupted process is saved • The hardware PCB is loaded with a new context associated with the appropriate interrupt handler or supervisor call

  33. Typical causes for a mode switch • Clock interrupt • The process has executed for the maximum allowable time slice • I/O interrupt • Memory fault (or page fault) • There is a reference to a page in virtual memory that has not been loaded into main memory • Trap • Error or exception occurred • This may cause process to be moved to the Exit state • Supervisor call • For example, to open a file

  34. Process switch • In contrast to a mode switch, during a process switch the running process does undergo a change of state • Examples • Running to Ready • Running to Blocked • A process switch is performed by operating system software

  35. Process switch • What is involved • A mode switch is performed to an interrupt handler or supervisor routine • The handler/routine updates software PCB of running process • Running process is moved to appropriate queue for its new state • Ready, Blocked, Ready/Suspended, etc. • A new process is selected to run next • The software PCB for the new process is updated • The new process is removed from its queue (typically Ready) • Memory-management data structures are updated if needed • A mode switch is performed from interrupt handler or supervisor routine to the new process

  36. Relationship of the OS to processes • Is the OS kernel a process? • Options • Non-process kernel • The OS kernel operates outside of every process • Operating system code is executed as a separate entity that operates in kernel mode • Execution within user processes • Except for process switching activity, the OS kernel executes in the context of the current user process • The OS code is in an shared area of virtual memory accessible to all processes • Supervisor call overhead is just two mode switches • Not two process switches • Process-based operating system • Except for process switching activity, the OS is a collection of processes • Cleaner architecture in a multiprocessor or multicomputer system

  37. UNIX SVR4 process management • Most of the operating system executes within the environment of a user process

  38. UNIX process states

  39. UNIX process states • The states “Preempted” and “Ready to run in memory” are really the same (same queue) • Reason for dotted line • Preemption can only occur when a process is about to move from kernel mode to user mode • Perhaps a higher priority process has become ready • While a process is running in kernel mode, it may not be preempted • This makes UNIX in this form unsuitable for real-time processing

  40. UNIX process states