OS Unit - 2 part 1.pptx

CSE3007

OPERATING SYSTEM

UNIT - II

Department of computer Science and Engineering SCHOOL OF ENGINEERING

Course Content

Unit I Introduction 8 lectures

Introduction to Operating System Concepts (including Multitasking, multiprogramming, multi user, Multithreading etc)., Classification of Operating systems- Batch, Time sharing, Real Time System, Distributed OS, Network OS, Real Time OS; Various Operating system services, architecture, System programs and calls

Unit II Process Management 8 lectures

Process concept, process scheduling, operation on processes; CPU scheduling, scheduling criteria, scheduling algorithms -First Come First Serve (FCFS), Shortest-Job-First (SJF), Priority Scheduling, Round Robin(RR), Multilevel Queue Scheduling

Unit III Memory Management 7 lectures

Logical & Physical Address Space, swapping, contiguous memory allocation, non-contiguous memory allocation paging and segmentation techniques, segmentation with paging; virtual memory management - Demand Paging & Page-Replacement Algorithms; Demand Segmentation.

Unit IV File System 8 lectures

Different types of files and their access methods, directory structures, various allocation methods, disk scheduling and management and its associated algorithms, Introduction to distributed file system.

Critical Section Problems, semaphores; methods for handling deadlocks-deadlock prevention, avoidance & detection; deadlock recovery.

Unit V I/O Systems 8 lectures

I/O Hardware, Application I/O Interface, Kernel, Transforming I/O requests, Performance Issues.

PROCESS

 _{A process is a program in execution.}

 _{A process is more than the program code, which is sometimes}

known as the text section.

 _{It also includes the current activity, as represented by the value}

PROCESS STATE

• _{As a process executes, it changes state. The state of a process}

is defined in part by the current activity of that process.

– _{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}

processor

CONTEXT SWITCHING

 _{When CPU switches to another process, the system must save}

the state of the old process and load the saved state for the new process via a context switch

 _{Context of a process represented in the PCB}

 _{Context-switch time is overhead; the system does no useful}

work while switching

 _{The more complex the OS and the PCB}  _{the longer the} context switch

 _{Time dependent on hardware support}

 _{Some hardware provides multiple sets of registers per CPU}

PROCESS SCHEDULING

The objective of multiprogramming is to have some process running at all times, so as to maximize CPU utilization.

PROCESS SCHEDULING – Scheduling Queues

 _{Maintains scheduling queues of processes}

 _{Job queue – set of all processes in the system}

 _{Ready queue – set of all processes residing in main memory,}

ready and waiting to execute

 _{Device queues – set of processes waiting for an I/O device}

Processes migrate among the various queues

Representation of Process Scheduling

Operations on Processes

 _{The processes in most systems can execute concurrently, and}

they may be created and deleted dynamically. Thus, these systems must provide a mechanism for process creation and termination.

 _{Two main operations on processes are:}

1. Process Creation

Operations on Processes – Process Creation

 _{A process may create several new processes, via a create –}

process system call, during the course of 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}

processes, forming a tree of processes.

 _{Generally, process identified and managed via a process}

Operations on Processes – Process Creation

• _{Resource sharing options}

• _{Parent and children share all resources}

• _{Children share subset of parent’s resources – this}

restriction avoids system overloading

•_{Execution options}

Process Creation

fork() creates a duplicate of the  current process

exec() replaces the program in the  current process with another 

Operations on Processes – Process Deletion

• Some operating systems do not allow child to exists if its parent has terminated.

• If a process terminates, then all its children must also be terminated.

• cascading termination. All children, grandchildren, etc. are terminated.

The termination is initiated by the operating system.

The parent process may wait for termination of a child process by using the

wait()system call. The call returns status information and the pid of the terminated

process

Operations on Processes – Process Deletion

If no parent waiting (did not invoke wait()) process is a zombie

A process which has finished the execution but still has entry in the process table to

report to its parent process is known as a zombie process. A child process always

first becomes a zombie before being removed from the process table. The parent

process reads the exit status of the child process which reaps off the child process

entry from the process table.

If parent terminated without invoking wait , process is an orphan

A process whose parent process no more exists i.e. either finished or terminated

Schedulers

A  process migrates among the various scheduling queues 

throughout its lifetime. The operating system must select, for 

scheduling purposes, processes from these queues in some 

fashion. The selection process is carried out by the appropriate 

Schedulers

 _{Long-term scheduler (or job scheduler) – selects from job}

queue which processes should be brought into the ready queue  _{Long-term scheduler is invoked infrequently (seconds,}

minutes)  (may be slow)

 _{The long-term scheduler controls the degree of}

multiprogramming

 _{Short-term scheduler (or CPU scheduler) – selects from the}

ready queue which processes should be executed next and allocates CPU

 _{Sometimes the only scheduler in a system}

Schedulers

 _{Processes can be described as either:}

 _{I/O-bound process – spends more time doing I/O than} computations, many short CPU bursts

 _{CPU-bound process – spends more time doing computations;} few very long CPU bursts

Long-term scheduler strives for good process mix otherwise the 

Addition of Medium Term Scheduling

 _{Medium-term scheduler can be added if degree of multiple}

programming needs to decrease

Dispatcher

 _{The dispatcher module gives control of the CPU to the process}

selected by the short-term scheduler; this involves:  _{switching context}

 _{switching to user mode}

 _{jumping to the proper location in the user program to restart} that program

 _{The dispatcher needs to run as fast as possible, since it is}

invoked during process context switch

 _{The time it takes for the dispatcher to stop one process and}

Although the SJF algorithm is optimal, in that it gives the minimal average waiting time for a given set of processes, it cannot be implemented at the level of short term CPU scheduling. There is no way to know the length of the next CPU burst.

Priority Scheduling

• _{A priority number (integer) is associated with each process} • _{The CPU is allocated to the process with the highest priority.} • _{Equal priority processes are scheduled in FCFS order.}

• _{Priority are generally some fixed range of numbers such as from}

0 to 7, or from 0 to 4095. However, there is no general agreement on whether 0 is the highest or lowest priority.

• _{Some systems use low numbers to represent low priority;}

others use low numbers to represent high priority,

Priority scheduling

• _{Priority scheduling can be either pre-emptive or non-preemptive} – _{A pre-emptive approach will preempt the CPU if the priority of}

the newly-arrived process is higher than the priority of the currently running process

– _{A non-preemptive approach will simply put the new process}

(with the highest priority) at the head of the ready queue

• _{SJF is a priority scheduling algorithm where priority is the}

predicted next CPU burst time

• _{The main problem with priority scheduling is starvation, that is,}

low priority processes may never execute

• _{A solution is aging; as time progresses, the priority of a process in}

Priority Scheduling

•

_{For example, if priorities range from 127 (low)}

to 0 (high)., we could decrement the priority

Priority Scheduling

• _{This method is quite same as the SJF but the difference is that}

instead of choosing the next process to work on by the shortest

burst time, CPU chooses the next process by the shortest priority

value. Here, all the processes are given a priority value. The process

with the shortest (The most shortest is 1) priority will be worked on

first and so on. Now consider a CPU and also consider a list in which