6-Process Structures - Process creation, management in Unix-25-07-2024.pdf

Operating System Principles Module – 2 Lecture 2 - Process  Module 2 Process  A process is a program in execution  It is the foundation for all the computations that happen on...

Operating System Principles Module – 2 Lecture 2 - Process  Module 2 Process  A process is a program in execution  It is the foundation for all the computations that happen on your computer  A process is unit of work that OS manages.  The status of the current activity of a process is represented by the value of the program counter and the contents of the 30-07-2024 processor’s registers. 2  Module 2 Process  A program is a passive entity, such as a file containing a list of instructions stored on disk  A process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.  A program becomes a process when an executable file is 30-07-2024 loaded into memory. 3  Module 2 Process  Two common techniques for loading executable files are double-clicking an icon representing the executable file and entering the name of the executable file on the command line.  Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. 30-07-2024 4  Module 2 Process – Memory Layout  A process in an operating system has its own virtual address space, separate from other processes. This virtual space is typically divided into distinct regions:  Text  Data  Heap  Stack 30-07-2024 5  Module 2 Process – Memory Layout 30-07-2024 Program’s machine code instructions executed by CPU 6  Module 2 Process – Memory Layout Global Data – Initialized and Uninitialized (BSS segment) 30-07-2024 Program’s machine code instructions executed by CPU 7  Module 2 Process – Memory Layout Dynamic Memory allocation during program runtime – Data Structures & objects Global Data – Initialized and Uninitialized (BSS segment) 30-07-2024 Program’s machine code instructions executed by CPU 8  Module 2 Process – Memory Layout Dynamically allocated. function calls, storing local variables, function arguments Dynamic Memory allocation during program runtime – Data Structures & objects Global Data – Initialized and Uninitialized (BSS segment) 30-07-2024 Program’s machine code instructions executed by CPU 9  Module 2 Process – Memory Layout Dynamically allocated. function calls, storing local Grows toward lower memory addresses Starts from the top of the virtual address space When a variables, function arguments function returns, its stack frame is deallocated, and the stack pointer moves back up Grows toward higher memory addresses When `free()` or `delete` is called, the memory is deallocated Dynamic Memory allocation during program runtime – Data Structures & objects Global Data – Initialized and Uninitialized (BSS segment) 30-07-2024 Program’s machine code instructions executed by CPU 10  Module 2 Process – Memory Layout - Example #include #include int global_var = 10; // Global variable void my_function() { // Function with local variables int local_var = 20; char *ptr = malloc(10); printf("Inside function:\n"); printf("Global variable: %d\n", global_var); printf("Local variable: %d\n", local_var); printf("Malloced memory: %s\n", ptr); free(ptr); } int main() { my_function(); 30-07-2024 printf("Outside function:\n"); printf("Global variable: %d\n", global_var); return 0; } 11  Module 2 Process – Memory Layout - Example #include #include int global_var = 10; void my_function() { int local_var = 20; char *ptr = malloc(10); printf("Inside function:\n"); printf("Global variable: %d\n", global_var); printf("Local variable: %d\n", local_var); Program converted to Machine Code and this printf("Malloced memory: %s\n", ptr); machine code is stored in text section free(ptr); } int main() { my_function(); printf("Outside function:\n"); 30-07-2024 printf("Global variable: %d\n", global_var); return 0; } 12  Module 2 Process – Memory Layout - Example #include #include int global_var = 10; Global variable which is initialized and will be stored in Data section and void my_function() { no uninitialized variable available so (block starting symbol) BSS section int local_var = 20; will be empty. char *ptr = malloc(10); printf("Inside function:\n"); printf("Global variable: %d\n", global_var); printf("Local variable: %d\n", local_var); printf("Malloced memory: %s\n", ptr); free(ptr); } int main() { my_function(); printf("Outside function:\n"); 30-07-2024 printf("Global variable: %d\n", global_var); return 0; } 13  Module 2 Process – Memory Layout - Example #include #include int global_var = 10; void my_function() { My_function () and its local variables local_var int local_var = 20; will be stored in Stack. char *ptr = malloc(10); In case of char *ptr variable only the variable and printf("Inside function:\n"); return address will be stored in the stack. printf("Global variable: %d\n", global_var); (local variables, Function parameters, return printf("Local variable: %d\n", local_var); address) printf("Malloced memory: %s\n", ptr); free(ptr); } int main() { Int main() will be stored in Stack. my_function(); printf("Outside function:\n"); 30-07-2024 printf("Global variable: %d\n", global_var); return 0; } 14  Module 2 Process – Memory Layout - Example #include #include int global_var = 10; void my_function() { The program is asking for 10 bytes of memory int local_var = 20; while in executing status so heap segment will char *ptr = malloc(10); provide memory in runtime and all the intermediary printf("Inside function:\n"); data will be stored in heap segment printf("Global variable: %d\n", global_var); printf("Local variable: %d\n", local_var); printf("Malloced memory: %s\n", ptr); free(ptr); } int main() { my_function(); printf("Outside function:\n"); 30-07-2024 printf("Global variable: %d\n", global_var); return 0; } 15  Module 2 Process – Information Stored in  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 30-07-2024  Heap containing memory dynamically allocated during run time 16  Module 2 Process – Memory Layout  Text Segment (Code): This region stores the program's machine code instructions that the CPU executes. It's often read-only and can be shared among processes using the same program or libraries.  Data Segment: This area holds the process's statically allocated data, like global variables defined in the program code. 30-07-2024 17  Module 2 Process – Memory Layout  Heap: This is a dynamically allocated memory region. Processes can request memory from the heap during runtime to store data structures or objects they create. The heap grows and shrinks as needed.  Stack: The stack is another dynamically allocated region that grows in the opposite direction of the heap. It's used for function calls, storing local variables, function arguments, and the return address for each function. 30-07-2024 18  Module 2 Process – Memory Layout  Each time a function is called, an activation record containing function parameters, local variables, and the return address is pushed onto the stack; when control is returned from the function, the activation record is popped from the stack.  Although the stack and heap sections grow toward one another, the operating system must ensure they do not overlap one another 30-07-2024 19  Module 2 Process – Memory Layout – Example 2 30-07-2024 20  Module 2 Process – Memory Layout – Example 2 30-07-2024 21  Module 2 Process – Memory Layout  It describes the organization of the memory for a process.  The operating system's memory management unit (MMU) translates virtual addresses used by the process into physical memory addresses. 30-07-2024 22  Module 2 Process – States  As a process executes, it changes state. The state of a process is defined in part by the current activity of that process.  A process may be in one of the following states:  New. The process is being created.  Running. Instructions are being executed.  Waiting. The process is waiting for some event to occur (such as an I/O completion or reception of a signal).  Ready. The process is waiting to be assigned to a processor. 30-07-2024  Terminated. The process has finished execution. 23  Module 2 Process – States 30-07-2024 24  Module 2 Process – Process Control Block (PCB)  A Data structure containing all the information about a process  The PCB contains pointers to the process’s memory layout.  Each process is represented in the operating system by a process control block (PCB)—also called a task control block.  The PCB acts as the handle for the OS to manage the 30-07-2024 process’s memory layout, scheduling and other resources. 25  Module 2 Process – Process Control Block (PCB) Process ID Process State Process Priority Program Counter CPU registers Heap & Stack Memory Limits I/O Permissions & Status CPU Scheduling Accounting Information List of Open files ……….. 30-07-2024 Pointers 26  Module 2 Process – Process Control Block (PCB)  Process state. The state may be new, ready, running, waiting, halted, and so on.  Program counter. The counter indicates the address of the next instruction to be executed for that process.  CPU registers. The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information. Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterward 30-07-2024 when it is rescheduled to run. 27  Module 2 Process – Process Control Block (PCB)  CPU-scheduling information. This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters.  Memory-management information. This information may include such items as the value of the base and limit registers and the page tables, or the segment tables, depending on the memory system used by the operating system.  Accounting information. This information includes the amount of CPU used, time limits, account numbers, job or process numbers, and so on.  I/O status information. This information includes the list of I/O devices allocated to the process, a list of open files, and so on. 30-07-2024 28  Module 2 Process – Process Table  A data structure maintained by the operating system to store information about all active processes.  It's a table or array of process control blocks (PCBs).  Each entry in the process table represents a process and contains a pointer to its corresponding PCB. 30-07-2024 29  Module 2 Process – Creation  The act of creating a new process in an operating system.  A process can create several new processes through creating process system calls during the process execution.  The creating process is called a parent process, and the new processes are called the children of that process.  Each of these new processes may in turn create other 30-07-2024 processes, forming a tree of processes. 30  Module 2 Process – Creation  Identify processes according to a unique process identifier (or pid), which is typically an integer number.  The pid provides a unique value for each process in the system, and it can be used as an index to access various attributes of a process within the kernel. 30-07-2024 31  Module 2 Process – Creation 30-07-2024 Figure: A tree of processes on a typical Linux system. 32  Module 2 Process – Creation  systemd (pid of 1) is the first daemon to start during booting and the last daemon to terminate during shutdown.  It creates processes which provide additional services such as a web or print server, an ssh server, etc.  The logind process is responsible for managing clients that directly log onto the system and the sshd process is responsible for managing clients that connect to the system by using ssh 30-07-2024 33  Module 2 Process – Parent Process and Child Process  How Resource Allocation, Resource Sharing, Address Space and Execution happens when child processes are created? 30-07-2024 34  Module 2 Process – Parent Process and Child Process Resource Allocation  When a child process is created, it needs resources like CPU time, memory, files, and I/O devices to execute. 1. The child process can obtain resources directly from the OS 2. The child process can inherit a subset of resources from the parent process (The parent may have to partition its resources among its children) 30-07-2024 35  Module 2 Process – Parent Process and Child Process Process Execution  When a new process (child) is created, there are two possibilities for execution: 1. Concurrent Execution: The parent process continues executing simultaneously with its child processes. 2. Sequential Execution: The parent process waits until one or all of its child processes have finished executing before continuing. 30-07-2024 36  Module 2 Process – Parent Process and Child Process Address Layout When a new (child) process is created, there are two possibilities for its address space: 1. Duplicate Address Space: The child process has the same program and data as the parent process 2. New Address Space: The child process has a new program loaded into it, with its own separate address space. 30-07-2024 37  Module 2 Process – Parent Process and Child Process - Example  Consider Unix System Calls fork(), Wait(), exec() and exit()  fork() - Creates a new process, called the child process, which is a copy of the current process, called the parent process. The child process starts executing from the point where `fork()` was called.  wait() - Suspends the current/parent process until one of its child processes terminates. The `wstatus` parameter is used to store the exit status of the terminated child process. 30-07-2024 38  Module 2 Process – Parent Process and Child Process - Example  Consider Unix System Calls fork(), Wait(), exec() and exit()  exec() - Replaces the current/child process image with a new program loaded from the specified file.  exit() - Terminates the current process and returns the specified status code to the parent process. The parent process can use this status code to determine the reason for the child process's 30-07-2024 termination. 39  Module 2 Creating a separate process using the UNIX fork() system call. 30-07-2024 40  Module 2 Process – Parent Process and Child Process - Example  Consider Unix System Calls fork(), Wait(), exec() and exit() Time Parent-Child Process Execution Parent (pid > 0) Parent Process Parent pid = fork() wait() Resumes Copy of Parent parent process Child Process Child exit() 30-07-2024 created or exec() Process executes 41  Module 2 Process Termination  A process terminates when it finishes executing its final statement and asks the operating system to delete it by using the exit() system call.  All the resources of the process are deallocated and reclaimed by the operating system. A parent process can terminate another process using TerminateProcess().  A user or a misbehaving application could arbitrarily kill another user’s processes. 30-07-2024 42  Module 2 Process Termination  A parent may terminate the execution of one of its children for a variety of reasons:  The child has exceeded its usage of the allocated resources.  The task assigned to the child is no longer required.  The parent is exiting  Cascading Termination - if a parent process terminates (either normally or abnormally), then all its children must 30-07-2024 also be terminated. 43  Module 2 Process Termination  A parent process may wait for the termination of a child process by using the wait() system call.  The wait() system call is passed a parameter that allows the parent to obtain the exit status of the child. pid = wait(&status);  When a process terminates, its resources are deallocated by the operating system. However, its entry in the process table must 30-07-2024 remain there until the parent calls wait() 44  Module 2 Process – Parent Process and Child Process - Example Circumstance 1 – Zombie Process Time Parent Process Parent pid = fork() exit() executes Copy of Parent parent process Child Process Child exit() 30-07-2024 created or exec() Process executes 46  Module 2 Process – Parent Process and Child Process - Example Parent is not waiting to Circumstance 1 – Zombie Process acknowledge child Time process termination Parent Process Parent pid = fork() wait() executes Copy of Parent parent process Child Process becomes Child Process Child Zombie exit() 30-07-2024 created or exec() Process executes 47  Module 2 Process – Parent Process and Child Process - Example Circumstance 1 – Zombie Process  A zombie process is a process that has finished executing, but its parent process has not yet acknowledged its termination.  The zombie process still exists in the process table, but it is no longer running.  The parent process is responsible for waiting for the zombie process to finish and retrieving its exit status using the `wait()` system call.  If the parent process does not wait for the zombie process, it will remain in the process table until the parent process terminates or the system reboots. 30-07-2024 48  Module 2 Process – Parent Process and Child Process - Example Circumstance 2 – Orphan Process Time Parent Process Parent pid = fork() exit() executes Copy of Parent parent process Child Process Child Process exit() 30-07-2024 created or exec() executes 49  Module 2 Process – Parent Process and Child Process - Example Circumstance 2 – Orphan Process Time Parent exited before child process Parent Process termination Parent pid = fork() exit() executes Copy of Parent Child Process parent process becomes Orphan Child Process Child Process exit() 30-07-2024 created or exec() executes 50  Module 2 Process – Parent Process and Child Process - Example Circumstance 2 – Orphan Process  An orphan process is a process whose parent process has terminated, but the child process is still running.  The orphan process is adopted by the `init` process (PID 1), which becomes its new parent.  The `UNIX systems addressed this scenario by assigning the 30-07-2024 init process as the new parent to orphan processes. 51  Module 2 Process – Parent Process and Child Process - Example Circumstance 2 – Orphan Process  The init process periodically invokes wait(), thereby allowing the exit status of any orphaned process to be collected and releasing the orphan’s process identifier and process-table entry. 30-07-2024 52  Module 2 Android Process Hierarchy  Because of resource constraints such as limited memory, mobile operating systems may have to terminate existing processes.  Rather than terminating an arbitrary process, Android has identified an importance hierarchy of processes, and therefore it terminates processes in order of increasing importance. 30-07-2024 53  Module 2 Android Process Hierarchy  Foreground process—The current process visible on the screen  Visible process— A process performing an activity whose status is displayed on the foreground process.  Service process—A process that is performing an activity that is apparent to the user.  Background process—A process that may be performing an activity but is not apparent to the user.  Empty process—A process that holds no active components associated 30-07-2024 with any application 54