Chapter 3 Processes Chapter 3 Processes n Process

Chapter 3: Processes

Chapter 3: Processes n Process Concept n Process Scheduling n Operations on Processes n Cooperating Processes n Interprocess Communication Operating System Concepts 3. 2 Silberschatz, Galvin and Gagne © 2005

Process Concept n n n A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task l CPU, memory, I/O, files l Initialization data Process termination requires reclaim of any reusable resources Single-threaded process has one program counter specifying location of next instruction to execute l Process executes instructions sequentially, one at a time, until completion Multi-threaded process has one program counter per thread Typically system has many processes, some user, some operating system running concurrently on one or more CPUs l Concurrency by multiplexing the CPUs among the processes / threads Operating System Concepts 3. 3 Silberschatz, Galvin and Gagne © 2005

Process Concept n An operating system executes a variety of programs: Batch system – jobs l Time-shared systems – user programs or tasks n Textbook uses the terms job and process almost interchangeably n Process – a program in execution; process execution must progress in sequential fashion n A process includes: l program counter l stack l l data section Operating System Concepts 3. 4 Silberschatz, Galvin and Gagne © 2005

Multiprogramming needed for efficiency Single user cannot keep CPU and I/O devices busy at all times l Multiprogramming organizes jobs (code and data) so CPU always has one to execute l A subset of total jobs in system is kept in memory l One job selected and run via job scheduling l When it has to wait (for I/O for example), OS switches to another job n Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing l Response time should be < 1 second l Each user has at least one program executing in memory process l If several jobs ready to run at the same time CPU scheduling l If processes don’t fit in memory, swapping moves them in and out to run l Virtual memory allows execution of processes not completely in memory l Operating System Concepts 3. 5 Silberschatz, Galvin and Gagne © 2005

Memory Layout for Multiprogrammed System Operating System Concepts 3. 6 Silberschatz, Galvin and Gagne © 2005

Process Management Activities The operating system is responsible for the following activities in connection with process management: n Creating and deleting both user and system processes n Suspending and resuming processes n Providing mechanisms for process synchronization n Providing mechanisms for process communication n Providing mechanisms for deadlock handling Operating System Concepts 3. 7 Silberschatz, Galvin and Gagne © 2005

Process State n As a process executes, it changes state l new: The process is being created l running: Instructions are being executed l waiting: The process is waiting for some event to occur l ready: The process is waiting to be assigned to a process l terminated: The process has finished execution Operating System Concepts 3. 8 Silberschatz, Galvin and Gagne © 2005

Diagram of Process State Operating System Concepts 3. 9 Silberschatz, Galvin and Gagne © 2005

Process Control Block (PCB) Each process has a process control block which includes: n Process state n Program counter (PC) n CPU registers n CPU scheduling information n Memory-management information n Accounting information n I/O status information Operating System Concepts 3. 10 Silberschatz, Galvin and Gagne © 2005

Process Control Block (PCB) Operating System Concepts 3. 11 Silberschatz, Galvin and Gagne © 2005

CPU Switch From Process to Process Operating System Concepts 3. 12 Silberschatz, Galvin and Gagne © 2005

Context Switch n 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 n Context-switch time is overhead; the system does no useful work while switching n Time dependent on hardware support Operating System Concepts 3. 13 Silberschatz, Galvin and Gagne © 2005

Process Creation n A parent process creates children processes by system calls, which, in turn create other processes, forming a tree of processes n Resource sharing l Parent and children share all resources l Children share subset of parent’s resources l Parent and child share no resources n Execution l Parent and children execute concurrently l Parent waits until children terminate Operating System Concepts 3. 14 Silberschatz, Galvin and Gagne © 2005

Process Creation (Cont. ) n Address space l Child duplicate of parent l Child has a program loaded into it n UNIX examples l fork system call creates new process l exec system call used after a fork to replace the process’ memory space with a new program Operating System Concepts 3. 15 Silberschatz, Galvin and Gagne © 2005

Process Creation Operating System Concepts 3. 16 Silberschatz, Galvin and Gagne © 2005

C Program Forking Separate Process int main() { Pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0); } } Operating System Concepts 3. 17 Silberschatz, Galvin and Gagne © 2005

Process Termination n Process executes last statement and asks the operating system to delete it (exit) l Output data from child to parent (via wait) l Process’ resources are deallocated by operating system n Parent may terminate execution of children processes (abort) l Child has exceeded allocated resources l Task assigned to child is no longer required l If parent is exiting 4 Some operating system do not allow child to continue if its parent terminates – Operating System Concepts All children terminated - cascading termination 3. 18 Silberschatz, Galvin and Gagne © 2005

Cooperating Processes n Independent process cannot affect or be affected by the execution of another process n Cooperating process can affect or be affected by the execution of another process n Advantages of process cooperation l Information sharing l Computation speed-up l Modularity l Convenience n Two interprocess communication models l Shared-memory model l Message-passing model Operating System Concepts 3. 19 Silberschatz, Galvin and Gagne © 2005

Two Interprocesss Communications Models a. message-passing model Operating System Concepts b. shared-memory model 3. 20 Silberschatz, Galvin and Gagne © 2005

Shared-Memory Model for IPC n Two or more processes share a common memory region. Processes can exchange information by reading and writing data to the shared region. n Advantages l Allow maximum spend l No assistance from the kernel is required n Disadvantages l Difficult to implement l No protection from one process accessing another process’s memory Operating System Concepts 3. 21 Silberschatz, Galvin and Gagne © 2005

Producer-Consumer Problem n Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process l unbounded-buffer places no practical limit on the size of the buffer l bounded-buffer assumes that there is a fixed buffer size Operating System Concepts 3. 22 Silberschatz, Galvin and Gagne © 2005

Bounded-Buffer – Shared-Memory Solution n Shared data #define BUFFER_SIZE 10 Typedef struct {. . . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; In points the next free position in the buffer l Out points the first full position in the buffer l The buffer is empty when in==out l The buffer is full when in+1 mod BUFFER_SIZE==out. l Operating System Concepts 3. 23 Silberschatz, Galvin and Gagne © 2005

Bounded-Buffer – Insert() Method while (true) { /* Produce an item */ while (((in = (in + 1) % BUFFER SIZE count) == out) ; /* do nothing -- no free buffers */ buffer[in] = item; in = (in + 1) % BUFFER SIZE; { Operating System Concepts 3. 24 Silberschatz, Galvin and Gagne © 2005

Bounded Buffer – Remove() Method while (true) { while (in == out) ; // do nothing -- nothing to consume // remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE; return item; { Operating System Concepts 3. 25 Silberschatz, Galvin and Gagne © 2005

Message-Passing Model for IPC n Communication takes place by message exchanged between the cooperating processes n It is easy to implement, but low speed, and needs kernel intervention. n IPC facility provides two operations: l send(message) – message size fixed or variable l receive(message) n If P and Q wish to communicate, they need to: l establish a communication link between them l exchange messages via send/receive n Implementation of communication link physical (e. g. , shared memory, hardware bus) l logical (e. g. , logical properties) l Operating System Concepts 3. 26 Silberschatz, Galvin and Gagne © 2005

Implementation Methods n Direct or indirect communication. n Synchronous or asynchronous communication n Automatic or explicit buffering Operating System Concepts 3. 27 Silberschatz, Galvin and Gagne © 2005

Direct Communication n Processes must name each other explicitly: l Send (P, message) – send a message to process P l Receive (Q, message) – receive a message from process Q n Properties of communication link l Links are established automatically l A link is associated with exactly one pair of communicating processes l Between each pair there exists exactly one link Operating System Concepts 3. 28 Silberschatz, Galvin and Gagne © 2005

Indirect Communication n Messages are directed and received from mailboxes (also referred to as ports) l Each mailbox has a unique id l Processes can communicate only if they share a mailbox n Properties of communication link l Link established only if processes share a common mailbox l A link may be associated with many processes l Each pair of processes may share several communication links, with each link corresponding to one mailbox Operating System Concepts 3. 29 Silberschatz, Galvin and Gagne © 2005

Indirect Communication n Mailboxes are owned by OS n Operations l create a new mailbox l send and receive messages through mailbox l destroy a mailbox n Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A Operating System Concepts 3. 30 Silberschatz, Galvin and Gagne © 2005

Indirect Communication n Mailbox sharing l P 1, P 2, and P 3 share mailbox A l P 1, sends; P 2 and P 3 receive l Who gets the message? n Solutions l Allow a link to be associated with at most two processes l Allow only one process at a time to execute a receive operation l Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was. Operating System Concepts 3. 31 Silberschatz, Galvin and Gagne © 2005

Synchronization n Message passing may be either blocking or non-blocking n Blocking is considered synchronous n l Blocking send has the sender block until the message is received l Blocking receive has the receiver block until a message is available Non-blocking is considered asynchronous l Non-blocking send has the sender send the message and continue l Non-blocking receive has the receiver receive a valid message or null Operating System Concepts 3. 32 Silberschatz, Galvin and Gagne © 2005

Buffering n 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 Operating System Concepts 3. 33 Silberschatz, Galvin and Gagne © 2005

End of Chapter 3