Operating System: Processes and Threads

AMBO UNIVERSITY WOLISO CAMPUS School of Technology and Informatics Department of Information Systems Course Title: Operating System Course Code: INSY2051; ECTS: 5 1 Chapter...

AMBO UNIVERSITY WOLISO CAMPUS School of Technology and Informatics Department of Information Systems Course Title: Operating System Course Code: INSY2051; ECTS: 5 1 Chapter Two Processes and Threads INSY2051 : Operating System BSc(IS) 2rd Year, Second Semester, 2017 E.C JAFAR B. By Jafar 4/12/2025  Outlines 2  Processes  Threads  Scheduling  Interposes communication By Jafar 4/12/2025  Process concept 3  Early systems  One program at a time was executed and a single program has a complete control.  Modern OS allow multiple programs to be loaded in to memory and to be executed at the same time.  This requires firm control over execution of programs.  The notion of process emerged to control the execution of programs. By Jafar 4/12/2025  Processes 4  What is a process?  A process is basically a program in execution.  It is a program under execution or the current execution of instructions  The entity that can be assigned to and executed on a processor  An activity of some kind which has a program, input, output, and a state.  a program in execution; process execution must progress in sequential fashion  Conceptually, each process has its own virtual CPU.  In reality, of course, the real CPU switches back and forth from process to process.  Provide the illusion of parallelism, which is some times called pseudo parallelism. By Jafar 4/12/2025  Process concept(con’t…) 5  When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap, text and data.  The following image shows a simplified layout of a process inside main memory By Jafar 4/12/2025  Cont’d 6  Stack: The process Stack contains the temporary data such as method/function parameters, return address and local variables.  Heap: This is dynamically allocated memory to a process during its run time.  Text: This includes the current activity represented by the value of Program Counter and the contents of the processor's registers.  Data: This section contains the global and static variables. By Jafar 4/12/2025  Program Vs Process 7 Program o It is sequence of instructions defined to perform some task o It is a passive entity o A program is a static set of instructions o Not executed by itself  Process o It is a program in execution o It is an instance of a program running on a computer o It is an active entity o A processor performs the actions defined by a process o A process is a dynamic execution of a program o Need CPU to Executed By Jafar 4/12/2025  Process Creation 8  A process may create several new process via a create-process system call , during the course of action.  The creation process is called a parent process and a new processes are the children of that process  Each of these new process may in turn create other processes forming a tree of processes  Process identified and managed via a process identifier Pid. By Jafar 4/12/2025  Process Creation… 9  Address space  Child duplicate of parent ( When a process creates new process, a duplicate of that process is created.  This new process is called the child and the process that created it is called the parent.  The child process then replaces the copy for the code the parent process created with the code the child process is supposed to execute )  Child has a program loaded into new program  UNIX examples  Fork system call creates new process  Exec system call used after a fork to replace the process' memory space with a new program By Jafar 4/12/2025  Process Creation.. 10 By Jafar 4/12/2025  Process Creation 11  There are four principal events that cause processes to be created: 1. System initialization:  When an operating system is booted, typically several processes are created.  These processes can be:  Foreground processes : processes that interact with (human) users and perform work for them.  Background processes: processes which are not associated with particular users, but instead have some specific function. E.g.: process to Accept mail. Driver update, Network configration By Jafar 4/12/2025  Process Creation.. 12 2. Execution of a process creation system call by a running process  Running process will issue system calls to create one or more new processes to help it do its job.  Creating new processes is particularly useful when the work to be done can easily be formulated in terms of several related, but otherwise independent interacting processes.  A process Fetching large amount of data and execute it will create two different process one for fetching data and another to execute it. By Jafar 4/12/2025  Process Creation (….) 13 3. A user request to create a new process.  In interactive systems, users can start a program by typing a command or(double) clicking an icon.  Taking either of these actions starts a new process and runs the selected program in it. 4. Initiation of a batch job.  users can submit batch jobs to the system (possibly remotely).  When the operating system decides that it has the resources to run another job, it creates a new process and runs the next job from the input queue in it. By Jafar 4/12/2025  Process Termination 14 o After a process has been created, it starts running and does whatever its job is. o However, nothing lasts forever, not even processes. o Sooner or later the new process will terminate, usually due to one of the following conditions:  Conditions which terminate processes  Normal exit (voluntary)  Error exit (voluntary)  Fatal error (involuntary)  Killed by another process (involuntary) By Jafar 4/12/2025  Process Termination…. 15 1.Normal exit (voluntary) o Most processes terminate because they have done their work.  Example ,When a compiler has compiled the program, it executes a system call to tell the operating system that it is finished.  This call is exit in UNIX and Exit Process in Windows o Screen-oriented programs also support voluntary termination.  Example Word processors, Internet browsers and similar programs always have an icon or menu item that the user can click to tell the process to remove any temporary files it has open and then terminate. 2. Error exit (voluntary) o A process is terminate if it is discovers a fatal error.  For example, if a user types the command cc foo.c to compile the program foo.c and no such file exists, the compiler simply exits. By Jafar 4/12/2025  Process Termination…. 16 3. Fatal error (involuntary) o The another reason for termination is an error caused by the process, often due to a program bug.  Examples include executing an illegal instruction, referencing non-existent memory, or dividing by zero. 4. Killed by another process (involuntary) o The fourth reason a process might terminate is that the process executes a system call telling the operating system to kill some other process. o In UNIX this call is kill. The corresponding Win32 function is Terminate Process. By Jafar 4/12/2025  Process States 17  The process state define the current activity of the process.  As a process executes, it changes state  The state in which may in is differ from one system to another.  Below we see three states a process may be in:  Running : Instructions of program are being executed.  Ready: The process is waiting to be assigned to a processor.  Blocked :The process is waiting for some event to occur By Jafar 4/12/2025  Process States… 18 Four transitions are possible among these three states.  Transition 1:Process blocks for an event to occur.  Transition 2:Scheduler picks another process to have CPU time.  Transition 3:Scheduler picks first process get the CPU to run again  Transition 4:event becomes occurred to awakened for blocked process. By Jafar 4/12/2025  Five State Process Module and Transaction 19  Five state 1. New : a process is being Created 2. Ready: process is waiting to run(runnable), temporarily stopped to let another process run. 3. Running : process is actually using CPU. 4. Blocked/Waiting : unable to run until some external events happen. 5. Exit/Terminated :The process has finished the executions. By Jafar 4/12/2025  Process control Block(PCB) 20  A process control Block(PCB) is a data structure maintain by the OS for every process.  PCB is used for storing the collection of the information about the process.  The PCB is identified by an integer process id(PID).  A PCB keeps all information needed to keep track a process.  The PCB maintained for the process through is lifetime and its delete once the process terminate.  The Architecture of PCB is completely dependent on the OS and may contain different information in different operating System.  PCB lies in kernel memory space. By Jafar 4/12/2025  Process control Block(PCB) contains 21 The process identifier: It is a unique key that is used to uniquely identify processes. Process state: the current state of the process.i.e where it is ready, running or waiting Program counter: a pointer to the address of the next instruction that requires to be executed for the process. Priority: priority of the process CPU Registers: Various CPU register where process need to be stored for the execution for running state. I/O Information: This include a list of I/O device allocated to the process. Accounting Information: This include the Amount of CPU for process , the time limits. PCB By Jafar 4/12/2025  CPU Switch From Process to Process 22 Context Switching Context switch means stopping one process and restarting another process. When an event occur , the OS save the state of an Active Process and restore the state of the new process. Context Switching is purely overload b/c system does not perform any useful work while context switch. Steps performed by OS during Context Switching Sequence of Actions: 1. OS takes control (through Interrupt) 2. Save Context of Running Process in the process of PCB 3. Reload Context of new process from new process PCB. 4. Return Control to the new process By Jafar 4/12/2025  Implementation of Processes (1) 23 o To implement the process model, the operating system maintains a table (an array of structures), called the process table, with one entry per process. these entries process control blocks(PCB) also called task control block.  PCB Contains information associated with each process. 1. Process state:- can be ready, running, waiting, and etc. 2. Program counter:- indicates the address of the next instruction to be executed. 3. CPU registers:- includes general-purpose registers, stack Pointers, index registers and accumulators. By Jafar 4/12/2025  Implementation of Processes…. 24 4. Memory-management information:- includes the value of base and limit register. The information is useful for reallocating the memory when the process terminates. 5. CPU scheduling information:- includes the CPU scheduling information for each and every process(Eg. process priorities, pointers to scheduling queues, etc. 6. Accounting information:- includes the amount of CPU and real time used, time limits, job or process numbers, account numbers etc. 7. I/O status information:- includes list of opened files 8. Event information:- for a process in the blocked (wait) state this field contains information concerning the event for which the process is waiting. By Jafar 4/12/2025  Thread 25  Thread concept  Thread usage  Thread library  Thread implementation By Jafar 4/12/2025  Thread concept 26  Process model is based on two independent concepts: resource grouping and execution.  One way of looking at a process is that it is a way to group related resources together.  A process has an address space containing program text and data, as well as other resources. These resource may include open files, child processes, pending alarms, signal handlers, accounting information, and more.  By putting them together in the form of a process, they can be managed more easily.  Thread is a light weight process created by a process  It is a single sequence stream within a process By Jafar 4/12/2025  Thread concept(con’t..) 27  The other concept a process has is a thread of execution, usually shortened to just thread.  The thread has a program counter that keeps track of which instruction to execute next.  It has registers, which hold its current working variables.  It has a stack, which contains the execution history, with one frame for each procedure called but not yet returned from.  Processes are used to group resources together; threads are the entities scheduled for execution on the CPU.  The term multithreading is also used to describe the situation of allowing multiple threads in the same process. By Jafar 4/13/2025  Thread concept(con’t..) 28 o A thread consists of:  thread id  program counter  register set  stack o Threads belonging to the same process share:  its code  its data section  other OS resources By Jafar 4/12/2025  Processes and Threads 29 Similarities Differences Both share CPU and only one Unlike processes, threads thread/process is active (running) are not independent of one at a time. another. Like processes, threads within a Unlike processes, all threads process execute sequentially. can access every address in Like processes, thread can create the task. children. Unlike processes, thread are Like process, if one thread is blocked, another thread can run. design to assist one other. Note that process might or might not assist one anther b/c processes may be  Pr. Vs Th originated from different user By Jafar 4/13/2025  Thread usage 30 o Several reasons for having multiple threads: o Many applications need multiple activities are going on at once.  Decomposing such an application into multiple sequential threads that run in quasi-parallel, the programming model becomes simpler. o They are lighter weight than processes, they are easier (i.e., faster) to create and destroy than processes. o Having multiple threads within an application provide higher performance argument.  If there is substantial computing and also substantial I/0, having threads allows these activities to overlap, thus speeding up the application. o Threads are useful on systems with multiple CPUs By Jafar 4/12/2025  Thread library 31 o Thread libraries provide programmers an API to create and manage threads There are three basic libraries used: POSIX pthreads They may be provided as either a user or kernel library, as an extension to the POSIX standard  Systems like Solaris, Linux and Mac OS X implement pthreads specifications WIN32 threads These are provided as a kernel-level library on Windows systems. Java threads Since Java generally runs on a Java Virtual Machine, the implementation of threads is based upon whatever OS and hardware the JVM is running on, i.e. either Pthreads or Win32 threads depending on the system. By Jafar 4/12/2025  Thread implementation 32 o There are two main ways to implement a threads package: in user space and in the kernel. Implementing Threads in User Space  All code and data structure are reside in user space.  Invoking a function in the library results in a local procedure call in user space not system call.  the kernel is not aware of the existence of threads. Advantage:  To do thread switching, it calls a run-time system procedure, which is least an order of magnitude-may be more-faster than trapping to the kernel  They allow each process to have its own customized scheduling algorithm. Disadvantage:  Problem of how blocking system calls are implemented  Problem of page faults  No other thread in that process will ever run unless the first thread voluntarily gives up the CPU. By Jafar 4/12/2025  Thread implementation…. 33 Implementing Threads in kernel Space o All code and data structure are reside in kernels pace. o Invoking a function in the library results system call. o The kernel is aware of the existence of threads. Advantage:  All calls that might block a thread are implemented as system calls  If one thread in a process causes a page fault, the kernel can easily check to see if the process has any other runnable threads, and if so, run one of them while waiting for the required page to be brought in from the disk. o kernel threads solve some While problems, they do not solve all problem o what happens when a multithreaded process forks?  In many cases, the best choice depends on what the process is planning to do next. By Jafar 4/12/2025  Thread implementation…. 34 Implementing Threads in kernel Space(con’t..) o When a signal comes in, which thread should handle it?  Possibly threads could register their interest in certain signals but there may be two or more threads register for the same signal. Hybrid Implementations o Use kernel-level threads and then multiplexes user-level threads onto some or all of the kernel threads. By Jafar 4/12/2025  Benefits 35  Responsiveness – may allow continued execution if part of process is blocked, especially important for user interfaces  Resource Sharing – threads share resources of process, easier than shared memory or message passing  Economy – cheaper than process creation, thread switching has lower overhead than context switching  Scalability – multithreading can take advantage of multiprocessor architectures By Jafar 4/12/2025  Multicore Programming 36  A recent trend in computer architecture is to produce chips with multiple cores, or CPUs on a single chip.  Multi-core programming provides a mechanism for more efficient use of multiple computing cores and improved concurrency.  Multicore or multiprocessor systems putting pressure on programmers, challenges include:  Dividing tasks  Balance  Data splitting  Data dependency  Testing and debugging  Parallelism implies a system can perform more than one task simultaneously  Concurrency supports more than one task making progress  Single processor / core, scheduler providing concurrency By Jafar 4/12/2025  Concurrency vs. Parallelism 37 Concurrent execution on single-core system: Parallelism on a multi-core system: By Jafar 4/12/2025  Amdahl’s Law 38 Identifies performance gains from adding additional cores to an application that has both serial and parallel components S is serial portion N processing cores That is, if application is 75% parallel / 25% serial, moving from 1 to 2 cores results in speedup of 1.6 times As N approaches infinity, speedup approaches 1 / S Serial portion of an application has disproportionate effect on performance gained by adding additional cores By Jafar 4/12/2025  Multicore Programming (Cont.) 39  Generally, there are two types of parallelism  Data parallelism – divides the data up amongst multiple cores ( threads ), and performs the same task on each subset of the data.  Task parallelism – divides the different tasks to be performed among the different cores and performs them simultaneously.(each perform unique operation)  In most cases applications use a hybrid of the two strategies By Jafar 4/12/2025  Multithreading Models 40  Reading assignment  Amdahl’s Law(serial and parallel components)  Multithreading Models  common ways of establishing Multithreading Models such a relationship are:  Many-to-One  One-to-One  Many-to-Many  Implicit Threading  Thread Pools  Thread Cancellation By Jafar 4/14/2025  Ended! 41 Chapter Two Processes and Threads Course Code: INSY2051 :Operating System BSc(IS) 2nd Year, Second Semester, 2017 E.C By Jafar 4/12/2025