Applied OS Process Management PDF

Process Management Process A process is basically a program in execution. The execution of a process must progress in a sequential fashion. A process is defined as an entity which represents the basic unit of work to be implemented in the system. To put it in simple terms, we write our computer progr...

Process Management Process A process is basically a program in execution. The execution of a process must progress in a sequential fashion. A process is defined as an entity which represents the basic unit of work to be implemented in the system. To put it in simple terms, we write our computer programs in a text file and when we execute this program, it becomes a process which performs all the tasks mentioned in the program. 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 − S.N. Component & Description 1 Stack The process Stack contains the temporary data such as method/function parameters, return address and local variables. 2 Heap 3 This is dynamically allocated memory to a process during its run time. Program A program is a piece of code which may be a single line or millions of lines. A computer program is usually written by a computer programmer in a programming language. For example, here is a simple program written in C programming language − #include int main() { printf("Hello, World! \n"); return 0; } A computer program is a collection of instructions that performs a specific task when executed by a computer. When we compare a program with a process, we can conclude that a process is a dynamic instance of a computer program. A part of a computer program that performs a well-defined task is known as an algorithm. A collection of computer programs, libraries and related data are referred to as a software. Process Life Cycle When a process executes, it passes through different states. These stages may differ in different operating systems, and the names of these states are also not standardized. In general, a process can have one of the following five states at a time. S.N. State & Description 1 Start This is the initial state when a process is first started/created. 2 Ready The process is waiting to be assigned to a processor. Ready processes are waiting to have the processor allocated to them by the operating system so that they can run. Process may come into this state after Start state or while running it by but interrupted by the scheduler to assign CPU to some other process. 3 Running Once the process has been assigned to a processor by the OS scheduler, the process state is set to running and the processor executes its instructions. 4  Waiting Process moves into the waiting state if it needs to wait for a resource, such as waiting for user input, or waiting for a file to become available. 5 Terminated or Exit Once the process finishes its execution, or it is terminated by the operating system, it is moved to the terminated state where it waits to be removed from main memory. Process Control Block (PCB) A Process Control Block is a data structure maintained by the Operating System for every process. The PCB is identified by an integer process ID (PID). A PCB keeps all the information needed to keep track of a process as listed below in the table − S.N. Information & Description 1 Process State The current state of the process i.e., whether it is ready, running, waiting, or whatever. 2 Process privileges This is required to allow/disallow access to system resources. 3 Process ID Unique identification for each of the process in the operating system. 4 Pointer A pointer to parent process. 5 Program Counter  Program Counter is a pointer to the address of the next instruction to be executed for this process. 6 CPU registers Various CPU registers where process need to be stored for execution for running state. 7 CPU Scheduling Information Process priority and other scheduling information which is required to schedule the process. 8 Memory management information This includes the information of page table, memory limits, Segment table depending on memory used by the operating system. 9 Accounting information This includes the amount of CPU used for process execution, time limits, execution ID etc. 10 IO status information This includes a list of I/O devices allocated to the process. The architecture of a PCB is completely dependent on Operating System and may contain different information in different operating systems. Here is a simplified diagram of a PCB − The PCB is maintained for a process throughout its lifetime, and is deleted once the process terminates. Process Scheduling Definition The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy. Process scheduling is an essential part of a Multiprogramming operating systems. Such operating systems allow more than one process to be loaded into the executable memory at a time and the loaded process shares the CPU using time multiplexing. Categories of Scheduling There are two categories of scheduling: 1. Non-preemptive: Here the resource can’t be taken from a process until the process completes execution. The switching of resources occurs when the  running process terminates and moves to a waiting state. 2. Preemptive: Here the OS allocates the resources to a process for a fixed amount of time. During resource allocation, the process switches from running state to ready state or from waiting state to ready state. This switching occurs as the CPU may give priority to other processes and replace the process with higher priority with the running process. Process Scheduling Queues The OS maintains all Process Control Blocks (PCBs) in Process Scheduling Queues. The OS maintains a separate queue for each of the process states and PCBs of all processes in the same execution state are placed in the same queue. When the state of a process is changed, its PCB is unlinked from its current queue and moved to its new state queue. The Operating System maintains the following important process scheduling queues − Job queue − This queue keeps all the processes in the system. Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue. Device queues − The processes which are blocked due to unavailability of an I/ O device constitute this queue. The OS can use different policies to manage each queue (FIFO, Round Robin, Priority, etc.). The OS scheduler determines how to move processes between the ready and run queues which can only have one entry per processor core on the system; in the above diagram, it has been merged with the CPU. Two-State Process Model Two-state process model refers to running and non-running states which are described below − S.N. State & Description 1 Running When a new process is created, it enters into the system as in the running state. 2 Not Running Processes that are not running are kept in queue, waiting for their turn to execute. Each entry in the queue is a pointer to a particular process. Queue is implemented by using linked list. Use of dispatcher is as follows. When a process is interrupted, that process is transferred in the waiting queue. If the process has completed or aborted, the process is discarded. In either case, the dispatcher then selects a process from the queue to execute. Schedulers Schedulers are special system software which handle process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. Schedulers are of three types − Long-Term Scheduler Short-Term Scheduler Medium-Term Scheduler Long Term Scheduler It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling. The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system. On some systems, the long-term scheduler may not be available or minimal. Time- sharing operating systems have no long term scheduler. When a process changes the state from new to ready, then there is use of long-term scheduler. Short Term Scheduler It is also called as CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them. Short-term schedulers, also known as dispatchers, make the decision of which process to execute next. Short-term schedulers are faster than long-term schedulers. Medium Term Scheduler Medium-term scheduling is a part of swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium-term scheduler is in- charge of handling the swapped out-processes. A running process may become suspended if it makes an I/O request. A suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other processes, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix. Comparison among Scheduler S.N. Long-Term Scheduler Short-Term Scheduler Medium-Term Scheduler 1 It is a job scheduler It is a CPU scheduler It is a process swapping scheduler. 2 Speed is lesser than short Speed is fastest among Speed is in between both term scheduler other two short and long term scheduler. 3 It controls the degree of It provides lesser control It reduces the degree of multiprogramming over degree of multiprogramming. multiprogramming 4 It is almost absent or It is also minimal in time It is a part of Time sharing minimal in time sharing sharing system systems. system 5 It selects processes from It selects those processes It can re-introduce the process pool and loads them into which are ready to execute into memory and execution memory for execution can be continued. Context Switching A context switching is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. Using this technique, a context switcher enables multiple processes to share a single CPU. Context switching is an essential part of a multitasking operating system features. When the scheduler switches the CPU from executing one process to execute another, the state from the current running process is stored into the process control block. After this, the state for the process to run next is loaded from its own PCB and used to set the PC, registers, etc. At that point, the second process can start executing. Context switches are computationally intensive since register and memory state must be saved and restored. To avoid the amount of context switching time, some hardware systems employ two or more sets of processor registers. When the process is switched, the following information is stored for later use. Program Counter Scheduling information Base and limit register value Currently used register Changed State I/O State information Accounting information  Scheduling algorithms A Process Scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms. There are six popular process scheduling algorithms which we are going to discuss in this chapter − First-Come, First-Served (FCFS) Scheduling Shortest-Job-Next (SJN) Scheduling Priority Scheduling Shortest Remaining Time Round Robin(RR) Scheduling Multiple-Level Queues Scheduling These algorithms are either non-preemptive or preemptive. Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state. First Come First Serve (FCFS) Jobs are executed on first come, first serve basis. It is a non-preemptive, pre-emptive scheduling algorithm. Easy to understand and implement. Its implementation is based on FIFO queue. Poor in performance as average wait time is high. Wait time of each process is as follows − Process Wait Time : Service Time - Arrival Time P0 0-0=0 P1 5-1=4 P2 8-2=6 P3 16 - 3 = 13 Average Wait Time: (0+4+6+13) / 4 = 5.75 Shortest Job Next (SJN) This is also known as shortest job first, or SJF This is a non-preemptive, pre-emptive scheduling algorithm. Best approach to minimize waiting time. Easy to implement in Batch systems where required CPU time is known in advance. Impossible to implement in interactive systems where required CPU time is not known. The processer should know in advance how much time process will take. Given: Table of processes, and their Arrival time, Execution time Process Arrival Time Execution Time Service Time P0 0 5 0 P1 1 3 5 P2 2 8 14 P3 3 6 8 Waiting time of each process is as follows − Process Waiting Time P0 0-0=0 P1 5-1=4 P2 14 - 2 = 12 P3 8-3=5 Average Wait Time: (0 + 4 + 12 + 5)/4 = 21 / 4 = 5.25 Priority Based Scheduling Priority scheduling is a non-preemptive algorithm and one of the most common scheduling algorithms in batch systems. Each process is assigned a priority. Process with highest priority is to be executed first and so on. Processes with same priority are executed on first come first served basis. Priority can be decided based on memory requirements, time requirements or any other resource requirement. Given: Table of processes, and their Arrival time, Execution time, and priority. Here we are considering 1 is the lowest priority. Process Arrival Time Execution Time Priority Service Time P0 0 5 1 0 P1 1 3 2 11 P2 2 8 1 14 P3 3 6 3 5 Waiting time of each process is as follows − Process Waiting Time P0 0-0=0 P1 11 - 1 = 10 P2 14 - 2 = 12 P3 5-3=2 Average Wait Time: (0 + 10 + 12 + 2)/4 = 24 / 4 = 6 Shortest Remaining Time Shortest remaining time (SRT) is the preemptive version of the SJN algorithm. The processor is allocated to the job closest to completion but it can be preempted by a newer ready job with shorter time to completion. Impossible to implement in interactive systems where required CPU time is not known. It is often used in batch environments where short jobs need to give preference. Round Robin Scheduling Round Robin is the preemptive process scheduling algorithm. Each process is provided a fix time to execute, it is called a quantum. Once a process is executed for a given time period, it is preempted and other process executes for a given time period. Context switching is used to save states of preempted processes. Wait time of each process is as follows − Process Wait Time : Service Time - Arrival Time P0 (0 - 0) + (12 - 3) = 9 P1 (3 - 1) = 2 P2 (6 - 2) + (14 - 9) + (20 - 17) = 12  P3 (9 - 3) + (17 - 12) = 11 Average Wait Time: (9+2+12+11) / 4 = 8.5 Multiple-Level Queues Scheduling Multiple-level queues are not an independent scheduling algorithm. They make use of other existing algorithms to group and schedule jobs with common characteristics. Multiple queues are maintained for processes with common characteristics. Each queue can have its own scheduling algorithms. Priorities are assigned to each queue. For example, CPU-bound jobs can be scheduled in one queue and all I/O-bound jobs in another queue. The Process Scheduler then alternately selects jobs from each queue and assigns them to the CPU based on the algorithm assigned to the queue. Process Deadlocks A deadlock happens in operating system when two or more processes need some resource to complete their execution that is held by the other process. In the above diagram, the process 1 has resource 1 and needs to acquire resource 2. Similarly process 2 has resource 2 and needs to acquire resource 1. Process 1 and process 2 are in deadlock as each of them needs the other’s resource to complete their execution but neither of them is willing to relinquish their resources. Coffman Conditions A deadlock occurs if the four Coffman conditions hold true. But these conditions are not mutually exclusive. The Coffman conditions are given as follows − Mutual Exclusion There should be a resource that can only be held by one process at a time. In the diagram below, there is a single instance of Resource 1 and it is held by Process 1 only. Hold and Wait A process can hold multiple resources and still request more resources from other processes which are holding them. In the diagram given below, Process 2 holds Resource 2 and Resource 3 and is requesting the Resource 1 which is held by Process 1. No Preemption A resource cannot be preempted from a process by force. A process can only release a resource voluntarily. In the diagram below, Process 2 cannot preempt Resource 1 from Process 1. It will only be released when Process 1 relinquishes it voluntarily after its execution is complete. Circular Wait A process is waiting for the resource held by the second process, which is waiting for the resource held by the third process and so on, till the last process is waiting for a resource held by the first process. This forms a circular chain. For example: Process 1 is allocated Resource2 and it is requesting Resource 1. Similarly, Process 2 is allocated Resource 1 and it is requesting Resource 2. This forms a circular wait loop. Deadlock Detection A deadlock can be detected by a resource scheduler as it keeps track of all the resources that are allocated to different processes. After a deadlock is detected, it can be resolved using the following methods − All the processes that are involved in the deadlock are terminated. This is not a good approach as all the progress made by the processes is destroyed. Resources can be preempted from some processes and given to others till the deadlock is resolved. Deadlock Prevention It is very important to prevent a deadlock before it can occur. So, the system checks each transaction before it is executed to make sure it does not lead to deadlock. If there is even a slight chance that a transaction may lead to deadlock in the future, it is never allowed to execute. Deadlock Avoidance It is better to avoid a deadlock rather than take measures after the deadlock has occurred. The wait for graph can be used for deadlock avoidance. This is however only useful for smaller databases as it can get quite complex in larger databases. Difference between Deadlock and Starvation Deadlock: Deadlock occurs when each process holds a resource and wait for other resource held by any other process. Necessary conditions for deadlock to occur are Mutual Exclusion, Hold and Wait, No Preemption and Circular Wait. In this no process holding one resource and waiting for another get executed. For example, in the below diagram, Process 1 is holding Resource 1 and waiting for resource 2 which is acquired by process 2, and process 2 is waiting for resource 1. Hence both process 1 and process 2 are in deadlock. Starvation: Starvation is the problem that occurs when high priority processes keep executing and low priority processes get blocked for indefinite time. In heavily loaded computer system, a steady stream of higher-priority processes can prevent a low- priority process from ever getting the CPU. In starvation resources are continuously utilized by high priority processes. Problem of starvation can be resolved using Aging. In Aging priority of long waiting processes is gradually increased. Difference between Deadlock and Starvation: S. NO Deadlock Starvation All processes keep waiting for each High priority processes keep other to complete and none get executing and low priority processes 1. executed are blocked Resources are blocked by the Resources are continuously utilized 2. processes by high priority processes Necessary conditions Mutual Exclusion, Hold and Wait, No Priorities are assigned to the 3. preemption, Circular Wait processes 4. Also known as Circular wait Also known as lived lock It can be prevented by avoiding the 5. necessary conditions for deadlock It can be prevented by Aging