operating system
CMET 331 Operating Systems
3. Processes
• Process Concept • Process Scheduling • Operations on Processes • Cooperating Processes • Interprocess Communication
Outline
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 vs. Program • Process – dynamic, Program – static • Process – temporary, Program – permanent
Process in Memory
A Heap Memory is a area in main memory that is used to assign dynamically allocated memory to a application.
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
processor —terminated: The process has finished execution
Diagram of Process State
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
Process Control Block (PCB)
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 • Processes migrate among the various queues
Schedulers • The OS must select, for scheduling purposes,
processes for these queues in some fashion. The selection process is carried out by the appropriate scheduler.
• 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 (the number of processes in memory)
• 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
A tree of processes on a typical Solaris
Process Creation - 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
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
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
• Interprocess Communication (IPC)
Interprocess Communication Model
(a) Message passing (b) Memory sharing
Three Issues of IPC • how one process can pass information to
another • making sure two or more processes do not get
into each other's way when engaging in critical activities (suppose two processes each try to grab the last 1 MB of memory)
• proper sequencing when dependencies are present: if process A produces data and process B prints it, B has to wait until A has produced some data before starting to print.
The Producer-Consumer Problem • Paradigm for cooperating processes –
Producer process produces information that is consumed by a Consumer process. —Example 1: a print program produces
characters that are consumed by a printer.
—Example 2: an assembler produces object modules that are consumed by a loader.
The Producer-Consumer Problem • We need a buffer to
hold items that are produced and later consumed: —unbounded-buffer
places no practical limit on the size of the buffer.
—bounded-buffer assumes that there is a fixed buffer size.
The Producer-Consumer Problem • Producer-Consumer Problem also known as the
bounded buffer problem —Two processes share a common, fixed-size buffer. —one of them, the producer, puts information into the
buffer —the other one, the consumer, takes it out —How to handle when producer wants to put a new
item in the buffer, but it is already full? • Solution is for the producer to go to sleep, to be awakened
when the consumer has removed one or more items; Similarly, if the consumer wants to remove an item from the buffer and sees that the buffer is empty, it goes to sleep until the producer puts something in the buffer and wakes it up
Idea for Producer-Consumer Solution • The bounded buffer is implemented as a circular array
with 2 logical pointers: in and out. • The variable in points to the next free position in the
buffer. • The variable out points to the first full position in the
buffer.
• Shared data #define BUFFER_SIZE 10 typedef struct {
. . . } item;
item buffer[BUFFER_SIZE]; int in = 0; int out = 0;
• The shared buffer is implemented as a circular array with pointers; in and out — in points to next free position in the buffer — out points to the first full position in the buffer — buffer is empty when in==out — buffer is full when ((in+1)%BUFFER_SIZE)==out
• Solution is correct, but can only use BUFFER_SIZE-1 elements
Bounded-Buffer – Shared-Memory Solution
Bounded-Buffer – Insert() Method
while (true) { /* Produce an item */ while (( (in + 1)% BUFFER_SIZE)
== out); /* do nothing -- no free buffers */
buffer[in] = item; in = (in + 1) % BUFFER SIZE;
}
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;
Message Passing • 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 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
—P1, P2 and P3 share mailbox A —P1 sends; P2 and P3 receive —Who gets the message?
• Answers depend on which of the following methods we chosen: —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 delivered
—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
End of Chapter 3