OS - Chapter # 3 for the development of os
- 2. Chapter 3 - Outline Process Concept Program vs Process Process Control Block Process Table Context Switch Process states Process states transitions Process life cycle Five state process model Unix process state transition diagram
- 3. Process Concept A process is basically a program in execution or that which a processor executes A process is unit of work in a system A process is an activity performed by a system It may be program’s data, program counter, pointer or register used in a program A process uses resources like CPU, memory, files and I/O devices Operating system processes execute system code and user processes executes user code
- 4. Program vs Process A program is a passive entity A process is a dynamic entity represented by the values stored in registers, memory, program counters etc. A program does not compete for the computing resources whereas a process does One program can have several processes Consider multiple users executing the same program
- 5. Process Concept Multiple parts The program code, also called text section Current activity including program counter, processor registers Stack containing temporary data Function parameters, return addresses, local variables Data section containing global variables Heap containing memory dynamically allocated during run time
- 6. Process Concept () Process idea was first used in Multics All programming OS, such as Windows NT, or MVS (multiple virtual storage) were built around the concept of process Major requirements that the OS must meet can all be expressed with reference to process OS must interleave the execution of a number of processes to maximize processor use OS must allocate resources to process in confirming to a specify policy e.g. certain functions or applications are of higher priority at the same time avoiding deadlock OS may support inter-process communication
- 7. Process Control Block (PCB) When a new user program is initiated OS creates a data structure, called Process Control Block (PCB), which gives substance to the process and serves to control it Each process is represented by its own process control block PCB is a data block or record containing information associated with a specified process Allows the OS to locate all key information about a process When the OS switches the attention of the CPU among various active processes It uses the same area in PCB to hold the information That information used to restart process when the process next gets CPU Process control block is also called task control block
- 8. Process Control Block (PCB) PCB contains following information about process Current state of the process Unique identification of process A pointer to process parent Pointer to process’s child processes The process’s priority Pointer to locate the process memory Pointer to allocate resources The register save area – CPU register The processor it is running on Accounting information I/O state information
- 9. Process Control Block (PCB) Pointer Process state Process number Program counter Registers Memory limits List of open files . . .
- 10. Process Table OS maintains the process table with one entry per process This entry contains information about process’s state its program counter stack pointer memory allocation the status of its open file its accounting and scheduling information its priority
- 11. The Shell It is a command interpreter It is a process which starts by typing a command at prompt It provides an interface to the OS for the user A facility for online users rather than batch Command can be entered directly at an online terminal through shell program or script a text file having series of commands forms a script DOS shell is alternative as screen oriented interface
- 12. The Shell In Unix when any user login in, a shell is started up It starts up as soon as user types some thing at command prompt, e.g. date The shell creates a child process and run the date program When child is running shell waits for its termination When child finishes, the shell types $ prompt again and wait for next command
- 13. The Shell Shell Process Process executing grep File studentInfo $ grep First studentInfo First
- 14. Operations on Process () A system managing processes may perform following operations Create a process Destroy a process Suspend a process Resume a process Block a process Wakeup a process Dispatch a process Change process’s priority Enable a process to communicate with other processes For long term suspension, process’s resources should be free Changing the priority of process normally involved modifying the priority value in PCB
- 15. Process Creation & Termination A process may create other processes When a process creates a new process The creator process is called the parent process Created process is called the child process Act of creating a process is called spawning a process A child process may share the resources of its parent process
- 16. Process Creation & Termination A parent may wait for its child process to terminate before resuming or may run concurrently with the child process A child process may run the same program as that of its parent or may run a separate program In DOS, parent and child process do not run in parallel and parent process will suspend itself till child has finished
- 17. Process Creation & Termination () When a process is created the following operations are performed Process naming (process id) Creates the process control block Insert it in process table Determine the process initial priority Allocate the process’s initial resources Allocate the address space to be used by the process When a process is destroyed following operations are performed Its resources are returned to the system Its process control block is erased It is purged from any system list or table
- 18. Reasons for Process Creation 1. New batch job In a batch environment, a process is created in response to the submission of a job 2. Interactive log on A user at a terminal logs on to the system 3. Created by OS to provide a service OS can create a process to perform a function on behalf of a user program, without user having to wait, e.g. printing 4. Spawned by existing process A user program can create a number of processes Typically related processes need to communicate and cooperate each other Achieving this cooperation is a difficult task for the programmer of OS
- 19. Reasons for Process Termination There must be some means for a process to indicate its completion A batch job should include a halt instruction, which generates an interrupts to alert the OS that a process has completed For an interactive application, the action of the user will indicate whether the process is completed
- 20. Reasons for Process Termination Following are the possible reasons for process termination; 1. Normal termination The process executes an OS service call to indicate that it has completed running 2. Time limit exceeds The process has run longer than the specified total time limit which includes total time elapsed amount of time spent executing in case of interactive process, he amount of time since the user last provided any input 3. Memory unavailable The process requires more memory than the system can provide
- 21. Reasons for Process Termination 4. Bounds violation The process tries to access the memory locations that is not allowed to access 5. Protection error The process attempt to use a file that is not allowed to use or, It tries to use it in improper fashion, such as writing to a read only file 6. Arithmetic error The process tries a prohibited computation such as division by zero 7. Time overrun The process has waited longer than specified maximum for a certain events to occur
- 22. Reasons for Process Termination 8. I/O failure An error occur in input or output, such as inability to find a file failure to read a write after a specified maximum no of tries (when, e.g. a defective area is encountered on a diskette) or invalid operation (such as reading from the line printer) 9. Invalid instruction The process attempts to execute a non-existent instruction (often a result of branching into a data area and attempting to execute data) 10. Privileged instruction The process attempts to use an instruction reserved for the operating system
- 23. Reasons for Process Termination () 11. Data misuse A piece of data is of the wrong type or is not initialized 12. OS or operator intervention For some reason, the operator or the OS has terminated the process (e.g. if a deadlock exists) 13. Parent termination When a parent terminates, the OS should be design to automatically terminate all the offspring of that parent 14. Parent request A parent process typically has the authority to terminate any of its offspring
- 24. Process States A process may have the following three states Running Instructions are being executed Waiting/Blocked The process is waiting for some event to occur (may be I/O completion) Ready The process is waiting to be assigned to processor At any instant a process is in one and only one of three states Process entering the system must go initially into the ready state Process can only enter the running state via the ready state Many OS are constructed using only these three states However, there is good justification for adding additional states to the model
- 26. Process State Transitions Transition-1: Occurs when a process cannot continue may be due to I/O request Transition-2: Request when scheduler decides that the running process has run long enough , and it’s the time to give CPU to other processes in ready state Transition-3: Occurs when all the other process have had their CPU time and its time for the first process to run again Transition-4: Occurs when external event for which process was waiting happens. If no other process is running at that time, transition-3 will be triggered and the process will start execution
- 27. Process State Transitions Ready Running Blocked 2 1 3 4 The four transitions can be indicated as follows Block running block Timerunout running ready Dispatch ready running Wakeup block ready
- 28. Context Switch Context switching is the procedure of switching of process from processor to memory and vice versa in a multiprogramming environment When CPU switches to another process The system must save the state of the old process Load the saved state of the new process Context switch involves storing and loading the values of each of the process’s control block, and process registers, such as; The program counter Index register General register Stack pointer etc
- 29. Context Switch Context switch is pure overhead The system does no useful work while switching Context switch time depends on hardware support e.g, memory speed, the number of registers etc. Its range is from 1 to 100 microsecond This speed vary from system to system
- 30. Context Switch
- 31. Process Life Cycle As a process executes, it changes state new: The process is being created running: Instructions are being executed waiting: The process is waiting for some event to occur ready: The process is waiting to be assigned to a process terminated: The process has finished execution
- 33. Process Life Cycle Assume that process is running with a Unix operating system with no user process Various events in life of processes within a multiprogramming system are given below; User using the shell command interpreter, types in a program name, say solvit The shell finds this program and program code will be loaded A system call will be used to generate a process corresponding to the execution of the solvit program The OS creates process control block in the memory The process solvit will now begin to run – the process is said to be in running state After a while, solvit needs to read some data from a disk file and issue an appropriate system call The process is now said to be in the blocked state
- 34. Process Life Cycle In the mean time, another user wants to run a program called myprog A new process is created for myprog and since solvit is currently idle, execution begins; myprog is now running The I/O delay which is blocking solvit now ends and solvit wants to restart; solvit is now said to be in ready state The OS scheduler now decides that myprog has enough processor time (Timeout) and moves it into the ready queue Solvit is restarted and enters the running state once more Switching between active processes, waiting for I/O transfer etc. will continue for the life of the processes Eventually, solvit completes its tasks and terminates; it leaves the running state and disappears from the system Similarly myprog terminates
- 35. Five State Process Model Blocked Suspended Admit I/O Completion Suspended I/O Request Release Time-out Dispatch Suspended Resume Admit Resume Suspended Running Blocked Ready Ready Suspended
- 36. Five State Process Model There are two queues; a ready queue and a blocked queue OS chooses a process the ready queue to run In the absence of any priority scheme, this can be a simple FIFO A running process is removed from execution, it is either Terminated Time quantum is over Performed I/O After I/O completion processes in blocked queue are moved to ready queue When an event occurs, the OS must scan the entire blocked queue In a large OS, there could be hundreds or even thousands of processes in that queue It would be more efficient to have a number of queues, one for each event When an event occurs, the entire list of processes in the appropriate queue can be moved to the ready state
- 37. Five State Process Model Consider a system that does not employ virtual memory Each process to be executed must be loaded fully into memory In multiple blocked queuing model, all the processes in all the queues must be resident in memory Memory holds multiple processes Solutions Expand the memory it has own flaw Cost associated with expansion Swapping Moving part or all of a process (blocked) from memory to disk (suspended queue) The space that is freed up in memory can be then be used to bring another process When a process is suspended, it becomes dormant and merely waits until it is resumed by the system or user
- 38. Five State Process Model A process can be suspended for the following reasons; Process being swapped put of the memory by the memory management system in order to free memory for higher priority processes A user may wish to suspend execution of a program for purpose of debugging a program Process investigates previous effects of the process Processes designed to run periodically to monitor system usage The OS may suspend a background or utility process or a process that is suspected of causing a problem The process can be suspended while in running, ready or blocked state
- 39. Unix Process State Transition 2 8 1 7 5 9 4 6 Kernel running Zombie Sleep in memory User running Ready to run in memory Exit Preempted Sleep swapped Ready to run swapped Created System call interrupt Return Not enough memory Preempt wakeup wakeup Swap out Swap out Enough memory Fork Return to user Swap in Sleep 3
- 40. Unix Process State Transition Nine process states are recognized by the Unix System V operating system Unix employ two running states to indicate weather the process is executing in user mode or kernel mode Unix process states are; User running Executing in user mode Kernel running Executing in kernel mode Ready to run (in memory) Ready to run as soon as kernel schedules it Sleeping in memory Unable to run until an event occurs Ready to run (swapped) The swapper must swap the process into memory before the kernel can schedules it to execute
- 41. Unix Process State Transition Sleeping (swapped) The process is waiting for an event, swapped to a disk Preempted Process is returning from kernel to user mode, but the kernel preempts it and does a context switch to schedule another process Created Process is newly created and not yet ready to run Zombie Process no longer exist, but it leaves a record for its parent process A distinction is made between two states; ready to run, in memory and preempted These are essentially the same states as indicated by the dotted line Preemption can occur when a process is about to move from kernel mode to user mode