Week03-Ch3-Process Concept.pptx.gfhgvjhg

Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10th Edition
Chapter 3: Processes

3.2 Silberschatz, Galvin and Gagne ©2018
Objectives
 To introduce the notion of a process -- a program in
execution, which forms the basis of all computation
 To describe the various features of processes, including
scheduling, creation and termination, and communication
 To explore interprocess communication using shared memory
and message passing
 To describe communication in client-server systems

Process Concept
 An operating system executes a variety of
programs that run as processes.
 Process: a program in execution.
 Process consists of multiple parts:
• Text section: contains program code
• Current activity including program counter,
processor registers
• Stack containing temporary data such as
local variables
• Data section containing global variables
• Heap containing memory dynamically
allocated during run time

Process Concept (Cont.)
 Program is passive entity stored on hard disk (executable
file); process is active entity
• Program becomes process when an executable file is
loaded into memory
 Execution of program starts via mouse clicks, command line
entry of its name, etc.
 One program can result in several processes
• Consider multiple users executing the same program

Process State
 As a process executes, it changes state
• New: The process is being created
• Ready: The process is waiting to be assigned to a processor
• Running: Instructions are being executed
• Waiting: The process is waiting for some event to occur
• Terminated: The process has finished execution

Process Scheduling
 Process scheduler selects among available processes for next
execution on CPU
 Goal -- Maximize CPU use, quickly switch processes onto CPU
 Maintains scheduling queues of processes
• Ready queue: set of all processes residing in main memory,
ready and waiting to execute
• Wait queues: set of processes waiting for an event (I/O operation
such as reading data from hard disk)
• Processes migrate among the various queues

Schedulers
 Long-term scheduler (or job scheduler): selects which processes
should be brought into the ready queue from the job queue.
 The long-term scheduler controls the degree of multiprogramming,
(how many processes could be brought into the ready queue)
 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
 Long-term scheduler strives for a good combination of I/O bound and
CPU bound processes.
 Short-term scheduler (or CPU scheduler): selects which process
should be executed next and allocated to CPU

Medium-Term Scheduling
 Medium-term scheduler: performs swapping. It can be added
to reduce the degree of multiple programming.
 Swap out: medium-term scheduler removes a process from
memory, and then stores it on hard disk, as suspended process
in virtual memory.
 Swap in: when the process is brought back into memory from
hard disk (Virtual Memory).

CPU Switch From Process to Process
A context switch occurs when the CPU switches from one process
to another.
 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 of a process is represented in
the Process Control Block (PCB)
 Context-switch time is pure overhead;
the system does no useful work while
switching.

Process Creation
 Parent process create children processes, which, in turn
create other processes, forming a tree of processes
 Generally, process identified and managed via a process
identifier (pid)
 Resource sharing options
• Parent and children share all resources
• Children share subset of parent’s resources
• Parent and child share no resources
 Execution options
o Parent and children execute
concurrently
o Parent waits until children terminate

Process Creation (Cont.)
 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
• Parent process calls wait()waiting for the child to terminate

Process Termination
 Process executes last statement and then asks the operating
system to delete it using the exit() system call.
• Returns status data from child to parent (via wait())
• Process’resources are deallocated by operating system
 Parent may terminate the execution of children processes using
the abort() system call. Some reasons for termination:
• Child has exceeded allocated resources
• Task assigned to child is no longer required

Process Termination (Cont.)
 For some operating systems, if a process terminates, then all its
children must also be terminated.
 The parent process may wait for termination of a child process
by using the wait()system call.
 If no parent is waiting, process is a zombie
 If parent is terminated, process is an orphan

Interprocess Communication
 Processes within a system may be independent or cooperating
 Reasons for cooperating processes:
• Information sharing
• Computation speedup
• Modularity
 Cooperating processes need
interprocess communication (IPC)
 Two models of IPC
• Shared memory
• Message passing
(a) Shared memory. (b) Message passing.

Examples of IPC Systems – Windows
 It follows message-passing model via local procedure call:
• Only works between processes on the same system
• Uses ports (like mailboxes) to establish and maintain communication
channels
• Communication works as follows:
 The client opens a handle (reference) to the subsystem’s connection
port object.
 The client sends a connection request.
 The server creates two private communication ports and returns the
handle to one of them to the client.
 The client and server then exchange messages.

Sockets
 A socket is a concatenation of IP address and port number.
 It is included at start of message packet to differentiate network
services on a host
 The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
 Communication occurs between a pair of sockets
 Special IP address 127.0.0.1 (loopback) to refer to system on which
process is running

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 pass the parameters to
the remote procedure
 The server-side stub receives this
parameters, and performs the
procedure on the server.
 After server executes the
requested procedure, the output is
sent back to client.
 The client receives the output, and
continue the rest of the program.

Pipes
 Acts as a conduit (channel) allowing two processes to communicate
 Ordinary pipes –a parent process creates a pipe and uses it to communicate
with its child process.
 Producer writes to one end of pipe
 Consumer reads from the other end of pipe
 Ordinary pipes are unidirectional
 Require parent-child relationship between communicating processes
 Named pipes – can be accessed without a parent-child relationship.
 more powerful than ordinary pipes
 Communication is bidirectional
 Several processes can use the named pipe for communication

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