1. Chapter 3: Processes

2. 3.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication

3. 3.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Concept 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 CPU, memory, I/O, files Initialization data Process termination requires reclaim of any reusable resources Single-threaded process has one program counter specifying location of next instruction to execute 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 Concurrency by multiplexing the CPUs among the processes / threads

4. 3.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

5. 3.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multiprogramming Multiprogramming needed for efficiency Single user cannot keep CPU and I/O devices busy at all times Multiprogramming organizes jobs (code and data) so CPU always has one to execute A subset of total jobs in system is kept in memory One job selected and run via job scheduling When it has to wait (for I/O for example), OS switches to another job 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 Response time should be < 1 second Each user has at least one program executing in memory  process If several jobs ready to run at the same time  CPU scheduling If processes don’t fit in memory, swapping moves them in and out to run Virtual memory allows execution of processes not completely in memory

6. 3.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Memory Layout for Multiprogrammed System

7. 3.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Management Activities The operating system is responsible for the following activities in connection with process management: Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Providing mechanisms for deadlock handling

8. 3.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process State As a process executes, it changes state 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 process terminated: The process has finished execution

9. 3.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Diagram of Process State

10. 3.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Control Block (PCB) Each process has a process control block which includes: Process state Program counter (PC) CPU registers CPU scheduling information Memory-management information Accounting information I/O status information

11. 3.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Control Block (PCB)

12. 3.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts CPU Switch From Process to Process

13. 3.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 Time dependent on hardware support

14. 3.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Creation A parent process creates children processes by system calls, which, in turn create other processes, forming a tree of processes Resource sharing Parent and children share all resources Children share subset of parent’s resources Parent and child share no resources Execution Parent and children execute concurrently Parent waits until children terminate

15. 3.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

16. 3.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Creation

17. 3.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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); }

18. 3.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Termination Process executes last statement and asks the operating system to delete it (exit) 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

19. 3.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 Advantages of process cooperation Information sharing Computation speed-up Modularity Convenience Two interprocess communication models Shared-memory model Message-passing model

20. 3.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Two Interprocesss Communications Models a. message-passing model b.shared-memory model

21. 3.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Shared-Memory Model for IPC Two or more processes share a common memory region. Processes can exchange information by reading and writing data to the shared region. Advantages Allow maximum spend No assistance from the kernel is required Disadvantages Difficult to implement No protection from one process accessing another process’s memory

22. 3.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 bounded-buffer assumes that there is a fixed buffer size

23. 3.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Bounded-Buffer – Shared-Memory Solution 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 Out points the first full position in the buffer The buffer is empty when in==out The buffer is full when in+1 mod BUFFER_SIZE==out.

24. 3.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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; {

25. 3.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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; {

26. 3.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Message-Passing Model for IPC Communication takes place by message exchanged between the cooperating processes It is easy to implement, but low speed, and needs kernel intervention. IPC facility provides two operations: send(message) – message size fixed or variable receive(message) 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)

27. 3.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Implementation Methods Direct or indirect communication. Synchronous or asynchronous communication Automatic or explicit buffering

28. 3.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

29. 3.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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, with each link corresponding to one mailbox

30. 3.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Indirect Communication Mailboxes are owned by OS 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

31. 3.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Indirect Communication Mailbox sharing P 1, P 2, and P 3 share mailbox A P 1, sends; P 2 and P 3 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.

32. 3.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 has the sender send the message and continue Non-blocking receive has the receiver receive a valid message or null

33. 3.33 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

34. End of Chapter 3