Unit1_Process_Management (1).ppt

Chapter 3: Processes  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems  Process Concept An operating system executes a variety of...

Chapter 3: Processes  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems  Process Concept An operating system executes a variety of programs:  Batch system – jobs  Time-shared systems – user programs or tasks Textbook uses the terms job and process almost interchangeably Process – a program in execution; process execution must progress in sequential fashion A process includes:  program counter  stack  data section 3.01  Process State As a process executes, it changes state  new: The process is being created(dead state)  running: Instructions are being executed  waiting: The process is waiting for some event to occur(process is running but waiting for the resources)  ready: The process is waiting to be assigned to a processor  terminated: The process has finished execution Diagram of Process State  Process Control Block (PCB) - Data structure that physically represents a process in the memory. Each process is represented by PCB. -Suppose certain processes are running in a system,let us say one of the process is taken from system and put in the ready or waiting state.Now when the process has to resume,there must me some information that is required by computer to locate that process where it was interrupted.  PCB Information associated with each process Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information Process Control Block (PCB) The Process Control Block is: a) Process type variable b) Data Structure c) A secondary storage section d) A Block in memory  Process Scheduling Queues Job queue – set of all processes in the system Ready queue – set of all processes ready and waiting to execute (in main memory) Device queues – set of processes waiting for an I/O device Process migration between the various queues Ready Queue And Various I/O Device Queues Representation of Process Scheduling Which of the following do not belong to queues for processes ? a) Job Queue b) PCB queue c) Device Queue d) Ready Queue When the process issues an I/O request : a) It is placed in an I/O queue b) It is placed in a waiting queue c) It is placed in the ready queue d) It is placed in the Job queue  Schedulers selects one of the available processes for execution. Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue from job queue. Short-term scheduler (or CPU scheduler) – selects which process should be executed next. Medium term scheduler (MTS)-Medium term scheduler is used to swap out processes from the main memory Addition of Medium Term Scheduling  Schedulers (Cont.) Short-term scheduler is invoked very frequently (milliseconds)  (must be fast) Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow) The long-term scheduler controls the degree of multiprogramming 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 What is a long-term scheduler ? a)) It selects which process has to be brought into the ready queue b) It selects which process has to be executed next and allocates CPU c) It selects which process to remove from memory by swapping d) None of the mentioned What is a medium-term scheduler ? a) It selects which process has to be brought into the ready queue b) It selects which process has to be executed next and allocates CPU c) It selects which process to remove from memory by swapping d) None of the mentioned  Context Switch 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 Context-switch time is overhead; the system does no useful work while switching Dependent on hardware support CPU Switch From Process to Process  Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes Each child has one parent but parent has many children. Resource sharing  Parent and children share all resources  Children share subset of parent’s resources  Parent and child share different address space.(any change in one will not reflect in other.) Execution  Parent and children execute concurrently  Parent waits until children terminate  Process Creation (Cont.) Address space  Child duplicate of parent  Child has a program loaded into it 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 3.11  Process Termination When Process executes it’s last statement and asks the operating system to terminate it (exit) It will be removed from any system list or table with it’s PCB.  Output data from child to parent (via wait)  Process’ resources are deallocated by operating system Parent may terminate execution of children processes (abort)  Child has exceeded allocated resources  Task assigned to child is no longer required  If parent is exiting  Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination  Cooperating Processes Independent process cannot affect or be affected by the execution of another process Cooperating process can affect or be affected by the execution of another process Reasons for providing an environment that allows process cooperation  Information sharing  Computation speed-up  Modularity  Convenience  Producer-Consumer Problem Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process  unbounded-buffer places no practical limit on the size of the buffer. Consumer waits  bounded-buffer assumes that there is a fixed buffer size. Producer waits. 3.13  Interprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions Message system – processes communicate with each other by passing of messages. IPC facility provides two operations:  send(message)  receive(message) Messages can be of 2 types: Fixed length, variable length. If P and Q wish to communicate, they need to:  establish a communication link between them  exchange messages via send/receive Implementation of communication link  physical (e.g., shared memory, hardware bus)  logical (e.g., logical properties)  Several methods for logically implementing a link and the send/receive operations. For example: - Direct or indirect communication - Symmetric or asymmetric communication  Implementation Questions How are links established? Can a link be associated with more than two processes? How many links can there be between every pair of communicating processes? What is the capacity of a link? Is the size of a message that the link can accommodate fixed or variable? Is a link unidirectional or bi-directional?  Direct Communication Processes must name each other explicitly:  send (P, message) – send a message to process P  receive(Q, message) – receive a message from process Q Properties of communication link  Links are established automatically  A link is associated with exactly one pair of communicating processes  Between each pair there exists exactly one link  The link may be unidirectional, but is usually bi-directional  A variant of this scheme employs asymmetry in addressing. Only the sender names the recipient; the recipient is not required to name the sender. receive (id, message) - Receive a message from any process; the variable id is set to the name of the process with which communication has taken place.  Indirect Communication Messages are directed and received from mailboxes (also referred to as ports)  Each mailbox has a unique id  Processes can communicate only if they share a mailbox Properties of communication link  Link established only if processes share a common mailbox  A link may be associated with many processes  Each pair of processes may share several communication links  Link may be unidirectional or bi-directional  Indirect Communication Operations  create a new mailbox  send and receive messages through mailbox  destroy a mailbox Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A  Indirect Communication Mailbox sharing  P1, P2, and P3 share mailbox A  P1, sends; P2 and P3 receive  Who gets the message? Solutions  Allow a link to be associated with at most two processes  Allow only one process at a time to execute a receive operation  Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.  Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous  Blocking send has the sender block until the message is received  Blocking receive has the receiver block until a message is available Non-blocking is considered asynchronous  Non-blocking send as the sender sends the message and continue  Non-blocking receive has the receiver receive a valid message or null  Buffering Queue of messages attached to the link; implemented in one of three ways 1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous) 2. Bounded capacity – finite length of n messages Sender must wait if link full 3. Unbounded capacity – infinite length Sender never waits