OPERATING SYSTEMS. IIIT-Hyderabad

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1 OPERATING SYSTEMS IIIT-Hyderabad

2 OVERVIEW Introduction What is an OS/Kernel? Bootstrap program Interrupts and exceptions Volatile and Non volatile storage!!! Process Management What is a process/system calls? Creating a process (fork, exec) PCB / Process Scheduling Threads Interprocess communication Critical Section/ Locks/ Deadlock / Livelocks Memory Management Virtual Memory/ Physical Memory Paging /swapping Demand paging Thrashing

3 WHAT IS AN OPERATING SYSTEM? A program that helps us manage computer hardware Intermediary between the user and hardware Kernel: Central module of operating system that is first loaded in memory. It is responsible for : Disk management Task Management Memory Management

4 BOOTSTRAP PROGRAM Bootstrap program or Bootstrap loader loads kernel into main memory, and starts its execution Typically stored in ROM OS, when loaded, starts the first process init.

5 INTERRUPTS AND EXCEPTIONS Special kind of control transfer. Alter the normal program flow to handle external events or report errors The are not same!! Interrupts: external events by other processes Exceptions: internal events within a process

6 VOLATILE AND NON-VOLATILE STORAGE Volatile : Needs a power source, loses data otherwise (RAM) Non-volatile : data persists even without power (DISK) Flash: slower than RAM but needs no power to retain content

7 PROCESS MANAGEMENT

8 WHAT IS A PROCESS? A process is a program in execution. It needs certain resources like: CPU time, memory, I/O etc. System calls are an interface b/w program and OS

9 FORK AND EXEC Fork() creates a new process with a new address space copying the original address space Exec() is used when we need to do a task unrelated to the original task

10 EXAMPLE FORK() int main( void ) { int pid = fork(); if ( pid == 0 ) { printf( "This is being printed from the child process\n" ); } else {//pid > 0 printf( "This is being printed in the parent process:\n" " - the process identifier (pid) of the child is %d\n", pid ); } return 0; } **** If a parent is killed then child continues to function with init as the parent

11 EXAMPLE EXEC() int main( void ) { char *argv[3] = {"Command-line", ".", NULL}; int pid = fork(); if ( pid == 0 ) execvp( "find", argv ); /* Put the parent to sleep for 2 seconds--let the child finished executing */ wait( 2 ); printf( "Finished executing the parent process\n" " - the child won't get here--you will only see this once\n" ); return 0; }

12 PROCESS CONTROL BLOCK Each process in OS is represented by a PCB or task control block. It is a data structure in kernel that contains:

13 MULTITASKING Multitasking refers to term where multiple jobs are executed by the CPU simultaneously by switching between them.switches occur so frequently that the users may interact with each program while it is running. Logical extension of multiprogramming where more than one process lies in memory. Context Switch: Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process. This task is known as a context switch

14 PROCESS SCHEDULING Types of Schedulers Long term : loads ready processes into memory Short term : selects from ready processes for execution Medium term : removes processes from memory (such as processes waiting for I/O)

15 SCHEDULING ALGORITHMS First Come First Serve (FCFS) Jobs are executed on first come, first serve basis. Shortest-Job-First (SJF) needs prior knowledge of time required Priority assigns priority to processes depending on memory requirements etc Round Robin(RR) Each process is provided a fix time to execute called quantum. Multilevel Queue multiple queues for different type of processes like system, user etc. each with different scheduling algo

16 THREADS Lightweight processes (use less resources) Share address space (can write to same variables) Multithreading : Applications use thread to do multiples tasks at the same time. eg : word processor uses one thread to print a doc and other to take user inputs.

17 MULTIPROCESSING Utilization of multiple processors for simultaneous processing Two types Asymmetric: Each processor has exclusive work. e.g. : OS will be run by 1 processor only. So if this processor fails the whole computer crashes. Symmetric: Any processor can run any type of thread. Processors communicate using shared memory

18 INTER PROCESS COMMUNICATION ( IPC ) Needed when processes need to communicate. Two fundamental models: Shared Memory (explicitly share region to read and write) Message Passing (using message exchange, system calls)

19 MSG PASSING VS SHARED MEMORY Message passing : easier to implement for inter computer communication, good for small amount of data since it uses sys calls Shared memory : Faster since once shared memory established, normal routine calls (without kernel intervention)

20 CRITICAL SECTION In shared memory interprocess communication, read write conflicts can occur when modifying shared variables eg. incrementing a counter variable (not atomic) register = counter register = register+1 counter = register We need a mechanism for mutual exclusion

21 SOLUTION TO CRITICAL SECTION PROBLEM Hardware solutions : Atomic operations like TestAndSet(), Swap() Semaphores/mutex

22 SEMAPHORES VS MUTEX They are not same!! Binary semaphores can be called mutex Mutex is a locking mechanism and semaphore is a signalling mechanism

23 SEMAPHORES Used when a resource has many instances Two major functions associated: wait() and signal() wait() checks if any instance is available and decrements its count signal() is like release of resource (increments count) Used for synchronization problems

24 ..CONTD! wait(s) { while S <~0 ; // no-op!! S--; } Also called Spinlock(busy waiting) **For monitors refer notes!! signal(s) { S++ ; }

25 DEADLOCK Semaphores can be implemented with a waiting queue to avoid busy waiting changing its state to a waiting state and giving control back to scheduler to schedule another process This implementation results in a situation where 2 or more process are waiting indefinitely for an event that can be caused by another waiting process. This is called DEADLOCK.

26 NECESSARY CONDITIONS FOR DEADLOCK Mutual exclusion: at least one resource should be in non sharable mode. Hold and wait: A process must be holding one resource and waiting for another that is held by some other process. No preemption: Only voluntary release of resources after process has completed task. Circular wait: loop of waiting processes

27 PREVENTION OF DEADLOCK Make sure that at least one of the necessary conditions cannot hold Allow shared resources Allocate resources before it begins execution If resource cannot be allocated to a process then all resources of that process are preempted(released implicitly) Processes can request resources in increasing order of enumeration

28 LIVELOCK Similar to deadlock except that states of processes are continuously changing, with regards to each other but non progressing. eg: two people attempting to pass each other in a corridor, seeing that they block each other, either both move left or both move right.

29 MEMORY MANAGEMENT

30 VIRTUAL MEMORY An optimization technique and is implemented by the operating system in order to give an application program the impression that it has more memory than actually exists.

31 ADVANTAGES AND DISADVANTAGES OF VIRTUAL MEMORY Advantages: The first is to allow the use of programs that are too big to physically fit in memory. The other reason is to allow for multitasking multiple programs running at once Disadvantages: Slows performance If virtual memory is quite large as compared to real memory then, constant disk swapping key lead to thrashing

32 PAGING Memory management scheme that permits physical address space of a process to be non contiguous Page - virtual memory frames - physical memory MMU(memory management unit) contains mapping from pages to frames

33 PAGE FAULTS AND DEMAND PAGING When non memory resident pages are accessed then a page fault occurs In case of page faults swapping is done to bring page from disk to actual memory Demand paging: Lazy swapping. Swaps data only when needed. eg only error routines from a program can be swapped in instead of whole program.

34 THRASHING Accessing the hard disk is far slower than using system memory. Using too many programs at once in a system with an insufficient amount of RAM results in constant disk swapping Thrashing It is a state in which our CPU perform 'productive' work less and 'swapping' more!!

35 ALL THE BEST (Y)

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