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Chapter 4: 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

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.

Process Control Block (PCB) Information associated with each process. Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information

CPU Switch From Process to Process

Process Scheduling Queues 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. Process migration between the various queues.

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue. Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

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.

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.

Process Creation Parent process create children processes, 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.

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.

Processes Tree on a UNIX System

Process Termination Process executes last statement and asks the operating system to decide 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. Parent is exiting. Operating system does not allow child to continue if its parent terminates. 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 Advantages of 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. bounded-buffer assumes that there is a fixed buffer size.

Bounded-Buffer – Shared-Memory Solution Shared data #define BUFFER_SIZE 10 Typedef struct { . . . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; Solution is correct, but can only use BUFFER_SIZE-1 elements

Bounded-Buffer – Producer Process item nextProduced; while (1) { while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; }

Bounded-Buffer – Consumer Process item nextConsumed; while (1) { while (in == out) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; }

Interprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. 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)

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.

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 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.

Synchronization Message passing may be either blocking or non-blocking. Blocking is considered synchronous Non-blocking is considered asynchronous send and receive primitives may be either blocking or non-blocking.

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.

Client-Server Communication Sockets Remote Procedure Calls Remote Method Invocation (Java)

Sockets A socket is defined as an endpoint for communication . Concatenation of IP address and port The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Communication consists between a pair of sockets.

Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems. Stubs – client-side proxy for the actual procedure on the server. The client-side stub locates the server and marshalls the parameters. The server-side stub receives this message, unpacks the marshalled parameters, and peforms the procedure on the server.

Remote Method Invocation Remote Method Invocation (RMI) is a Java mechanism similar to RPCs. RMI allows a Java program on one machine to invoke a method on a remote object.

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