ch3_EN_BK_Process.pdf

Silberschatz, Galvin and Gagne ©2018
Operating System Concepts
Chapter 3: Processes

2
Operating System Concepts Silberschatz, Galvin and Gagne ©2018
Chapter 3: Processes
q Process Concept
q Process Scheduling
q Operations on Processes
q Inter-Process Communication (IPC)
q IPC in Shared-Memory Systems
q IPC in Message-Passing Systems
q Examples of IPC Systems
q Communication in Client-Server Systems

3
Objectives
q Identify the separate components of a process and illustrate how they
are represented and scheduled in an operating system.
q Describe how processes are created and terminated in an operating
system, including developing programs using the appropriate system
calls that perform these operations.
q Describe and contrast inter-process communication using shared
memory and message passing.
q Design programs that uses pipes and POSIX shared memory to
perform inter-process communication.
q Describe client-server communication using sockets and remote
procedure calls.
q Design kernel modules that interact with the Linux operating system.

4
Process Concept
q An operating system executes a variety of programs that run as
processes
q Process – a program in execution; process execution must progress
in sequential fashion
q Multiple parts
o The program code, also called text section
o Current activity including program counter, and processor registers
o Stack section containing temporary data
4 Function parameters, return addresses, local variables
o Data section containing global variables
o Heap section containing memory dynamically allocated during run time

5
Process Concept (Cont.)
q Program is passive entity stored on disk (e.g., executable file)
q Process is active entity
o Program becomes process when executable file loaded into memory
q Execution of program can be started via GUI mouse clicks, command
line (CLI) entry of its name, etc.
q One program can be several processes
o E.g., Consider multiple users executing the same program
#ps -aux

6
Process in Memory
(memory limits of a process)
#size <pid>

7
Memory Layout of a C Program

8
Process State
q As a process executes, it changes state
o New – The process is being created
o Running – Instructions are being executed
o Waiting – The process is waiting for some event to occur
o Ready – The process is waiting to be assigned to a processor
o Terminated – The process has finished execution

9
Diagram of Process State

10
Process Control Block (PCB)
q Process Control Block (PCB) – Information associated with
each process, also called Task Control Block (TCB), includes:
o Process state – running, waiting, etc.
o Process number – identity of the process
o Program counter – location of instruction to next execute
o CPU registers – contents of all process-centric registers
o CPU scheduling info – priorities, scheduling queue pointers
o Memory-management information – memory allocated to the
process
o Accounting information – CPU used, clock time elapsed
since start, time limits
o I/O status information – I/O devices allocated to process, list of
open files

11
Threads
q So far, process has a single thread of execution
q Consider having multiple program counters per process
o Multiple locations can execute at once
4 Multiple threads of control –> threads
q Must then have storage for thread details
q Multiple program counters in PCB
(Explore in detail in Chapter 4)

12
Process Representation in Linux
q Represented by the C structure task_struct
pid t_pid; /* process identifier */
long state; /* state of the process */
unsigned int time_slice /* scheduling information */
struct task_struct *parent; /* this process’s parent */
struct list_head children; /* this process’s children */
struct files_struct *files; /* list of open files */
struct mm_struct *mm; /* address space of this process */

13
Process Scheduling
q Maximize CPU use ⇢ quickly switch processes onto CPU core
q Process scheduler selects one process among available (ready)
processes for next execution on CPU core
q Maintains scheduling queues of processes
o Ready queue – set of all processes residing in main memory, ready and
waiting to execute
o Wait queues – set of processes waiting for an event (e.g., I/O)
q Processes migrate among the various queues

14
Ready and Wait Queues

15
Representation of Process Scheduling
new process

16
CPU Switch from Process to Process
q A context switch
occurs when the
CPU switches
from one process
to another.

17
Context Switch
q 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
via a context switch
q Context of a process represented in the PCB
q Context-switch time is overhead, the system does no useful work
while switching
o The more complex the OS and the PCB, the longer the context switch
q Time dependent on hardware support
o Some hardware provides multiple sets of registers per CPU, multiple
contexts loaded at once

18
Multitasking in Mobile Systems
q Some mobile systems (e.g., early version of iOS) allow only one
process to run, others suspended
q Due to screen real state, user interface limits iOS provides for a
o Single foreground process – controlled via user interface
o Multiple background processes – in memory, running, but not on the
display, and with limits
o Limits include single, short task, receiving notification of events, specific
long-running tasks like audio playback
q Android runs foreground and background, with fewer limits
o Background process uses a service to perform tasks
o Service can keep running even if background process is suspended
o Service has no user interface, small memory use

19
Operations on Processes
q System must provide mechanisms for:
o process creation
o process termination

20
Process Creation
q Parent processes create children processes, which, in turn create
other processes, forming a tree of processes
q Process identified and managed via a Process Identifier (PID)
q Resource sharing options
o Parent and children share all resources
o Children share subset of parent’s resources
o Parent and child share no resources

21
Process Creation (Cont.)
q Execution options
o Parent and children execute concurrently
o Parent waits until children terminate
q Address space
o Child duplicate of parent
o Child has a program loaded into it

22
A Tree of Processes in Linux
#pstree

23
Process Creation (Cont.)
q UNIX examples
o fork() system call creates new process
o exec() system call used after a fork() to replace the process’
memory space with a new program
o Parent process calls wait() waiting for the child to terminate

24
C Program Forking A Separate Process

25
Creating a Separate Process via Windows API

26
Process Termination
q Process executes last statement and then asks the operating system
to delete it using the exit() system call.
o Returns status data from child to parent (via wait())
o Process’resources are deallocated by operating system
q Parent may terminate the execution of children processes using the
abort() system call. Some reasons for doing so:
o Child has exceeded allocated resources
o Task assigned to child is no longer required
o The parent is exiting and the operating systems does not allow a child to
continue if its parent terminates

27
Process Termination (Cont.)
q Some operating systems do not allow child to exists if its parent has
terminated. If a process terminates, then all its children must also be
terminated.
o Cascading termination: All children, grandchildren, etc. are terminated
o The termination is initiated by the operating system
q The parent process may wait for termination of a child process by
using the wait() system call. The call returns status information
and the pid of the terminated process
pid = wait(&status);
q If no parent waiting (did not invoke wait()), process is a zombie
q If parent terminated without invoking wait(), process is an orphan

28
Importance Hierarchy of Android Process
q Mobile operating systems often have to terminate processes to
reclaim system resources such as memory. From most to least
important:
▲ Foreground process
▲ Visible process
▲ Service process
▲ Background process
▲ Empty process
q Android will begin terminating processes that are least important.

29
Multiprocess Architecture – Chrome Browser
q Many web browsers ran as a single process (some still do)
o If one web site causes trouble, entire browser can hang or crash
q Google Chrome Browser is multiprocess with 3 different types of
processes:
o Browser process manages user interface, disk and network I/O
o Renderer process renders web pages, deals with HTML, JavaScript. A
new renderer created for each website opened
4 Runs in sandbox restricting disk and network I/O, minimizing effect of security
exploits
o Plug-in process for each type of plug-in

30
Inter-Process Communication (IPC)
q Processes within a system may be independent or cooperating
o Independent process does not share data with any other processes
executing in the system
o Cooperating process can affect or be affected by other processes,
including sharing data
q Reasons for cooperating processes:
o Information sharing
o Computation speed-up
o Modularity
o Convenience
q Cooperating processes need Inter-Process communication (IPC)

31
Communication Models
q Two models of IPC
o Shared memory
o Message passing
Shared memory Message passing

32
Inter-Process Communication – Shared Memory
q An area of memory shared among the processes that wish to
communicate
q The communication is under the control of the users processes, not
the operating system.
q Major issues is to provide mechanism that will allow the user
processes to synchronize their actions when they access shared
memory.
(Synchronization is discussed in great details in Chapters 6 & 7)

33
Producer-Consumer Problem
q Producer-Consumer relationship
q Paradigm for cooperating processes, producer process produces
information that is consumed by a consumer process
o unbounded-buffer places no practical limit on the size of the buffer
o bounded-buffer assumes that there is a fixed buffer size

34
Bounded-Buffer – Shared-Memory Solution
q Shared data
#define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
q Solution is correct, but can only use BUFFER_SIZE-1 elements

35
Producer Process – Shared Memory
item next_produced;
while (true) {
/* produce an item in next produced */
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
}

36
Consumer Process – Shared Memory
item next_consumed;
while (true) {
while (in == out)
; /* do nothing */
next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
/* consume the item in next consumed */
}

37
Inter-Process Communication – Message
Passing
q Mechanism for processes to communicate and to synchronize their
actions
q Message system – processes communicate with each other without
resorting to shared variables
q IPC facility provides two operations:
o send(message)
o receive(message)
q The message size is either fixed or variable

38
Message Passing (Cont.)
q If processes P and Q wish to communicate, they need to:
o Establish a communication link between them
o Exchange messages via send/receive
q Implementation issues:
o How are links established?
o Can a link be associated with more than two processes?
o How many links can there be between every pair of communicating
processes?
o What is the capacity of a link?
o Is the size of a message that the link can accommodate fixed or variable?
o Is a link unidirectional or bi-directional?

39
Message Passing (Cont.)
q Implementation of communication link
o Physical
4 Shared memory
4 Hardware bus
4 Network equipment
o Logical
4 Direct or indirect
4 Synchronous or asynchronous
4 Automatic or explicit buffering
4 Network protocols

40
Direct Communication
q Processes must name each other explicitly:
o send (P, message) – send a message to process P
o receive(Q, message) – receive a message from process Q
q Properties of communication link
o Links are established automatically
o A link is associated with exactly one pair of communicating processes
o Between each pair there exists exactly one link
o The link may be unidirectional, but is usually bi-directional

41
Indirect Communication
q Messages are directed and received from mailboxes (also referred to
as ports)
o Each mailbox has a unique ID
o Processes can communicate only if they share a mailbox
q Properties of communication link
o Link established only if processes share a common mailbox
o A link may be associated with many processes
o Each pair of processes may share several communication links
o Link may be unidirectional or bi-directional

42
Indirect Communication (Cont.)
q Operations
o create a new mailbox (or port)
o send and receive messages through mailbox
o destroy a mailbox
q Primitives are defined as:
o send(A, message) – send a message to mailbox A
o receive(A, message) – receive a message from mailbox A

43
Indirect Communication (Cont.)
q Mailbox sharing
o Example
4 P1, P2, and P3 share mailbox A,
4 P1 sends; P2 and P3 receive.
4 Who gets the message?
o Solutions
4 Allow a link to be associated with at most two processes
4 Allow only one process at a time to execute a receive operation
4 Allow the system to select arbitrarily the receiver. Sender is notified who the
receiver was

44
Message Passing – Synchronization
q Message passing may be either blocking or non-blocking
q Blocking is considered synchronous
o Blocking send – the sender is blocked until the message is received
o Blocking receive – the receiver is blocked until a message is available
q Non-blocking is considered asynchronous
o Non-blocking send – the sender sends the message and continue
o Non-blocking receive – the receiver receives:
4 A valid message, or Null message
q Different combinations possible
o If both send and receive are blocking, we have a rendezvous

45
Producer – Message Passing
message next_produced;
while (true) {
/* produce an item in next_produced */
send(next_produced);
}

46
Consumer – Message Passing
message next_consumed;
while (true) {
receive(next_consumed)
/* consume the item in next_consumed */
}

47
Buffering
q Queue of messages attached to the link.
q Implemented in one of three ways
o Zero capacity – no messages are queued on a link
4 Sender must wait for receiver (rendezvous)
o Bounded capacity – finite length of n messages
4 Sender must wait if link full
o Unbounded capacity – infinite length
4 Sender never waits

48
Examples of IPC Systems - POSIX
q POSIX Shared Memory
o Process first creates shared memory segment
shm_fd = shm_open(name, O CREAT | O RDWR, 0666);
o Also used to open an existing segment
o Set the size of the object
ftruncate(shm_fd, 4096);
o Use mmap() to memory-map a file pointer to the shared memory object
o Reading and writing to shared memory is done by using the pointer
returned by mmap().

49
IPC POSIX Producer – Consumer

50
Examples of IPC Systems - Mach
q Mach communication is message based
o Even system calls are messages
o Each task gets two ports at creation – Task Self port and Notify port
o Messages are sent and received using the mach_msg() function
o Ports needed for communication, created via mach_port_allocate()
o Send and receive are flexible, for example four options if mailbox full:
4 Wait indefinitely
4 Wait at most n milliseconds
4 Return immediately
4 Temporarily cache a message

51
Mach Messages
#include<mach/mach.h>
struct message {
mach_msg_header_t header;
int data;
};
mach port t client;
mach port t server;

52
Mach Message Passing - Client

53
Mach Message Passing - Server

54
Examples of IPC Systems – Windows
q Message-passing centric via advanced Local Procedure Call (LPC)
facility
o Only works between processes on the same system
o Uses ports (like mailboxes) to establish and maintain communication
channels
o Communication works as follows:
4 The client opens a handle to the subsystem’s connection port object
4 The client sends a connection request
4 The server creates two private communication ports and returns the handle to
one of them to the client
4 The client and server use the corresponding port handle to send messages or
callbacks and to listen for replies

55
Local Procedure Calls in Windows

56
Pipes
q Acts as a conduit allowing two processes to communicate
q Issues:
o Is communication unidirectional or bidirectional?
o In the case of two-way communication, is it half or full-duplex?
o Must there exist a relationship (e.g., parent-child) between the
communicating processes?
o Can the pipes be used over a network?
q Ordinary pipes – cannot be accessed from outside the process that
created it. Typically, a parent process creates a pipe and uses it to
communicate with a child process that it created.
q Named pipes – can be accessed without a parent-child relationship.

57
Ordinary Pipes
q Ordinary Pipes allow communication in standard producer-consumer
style
o Producer writes to one end (the write-end of the pipe)
o Consumer reads from the other end (the read-end of the pipe)
q Ordinary pipes are therefore unidirectional
q Require parent-child relationship between communicating processes

58
Named Pipes
q Named pipes are more powerful than ordinary pipes
q Communication is bidirectional
q No parent-child relationship is necessary between the communicating
processes
q Several processes can use the named pipe for communication
q Provided on both UNIX and Windows systems

59
Communications in Client-Server Systems
q Sockets
o A socket is defined as an endpoint for communication
o It is a concatenation of IP address and port – a number included at start
of message packet to differentiate network services on a host
4 E.g., The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
o Communication consists between a pair of sockets
o All ports below 1024 are well known, used for standard services
o Special IP address 127.0.0.1 (loopback) to refer to system on which
process is running
q Remote Procedure Calls (RPC)

60
Socket Communication

61
Sockets in Java – Server
q Three types of sockets
o Connection-oriented
(TCP)
o Connectionless
(UDP)
o MulticastSocket
class– data can be
sent to multiple
recipients
q Consider this “Date”
server in Java:

62
Sockets in Java – Client
q The equivalent
“Date” client

63
Remote Procedure Calls
q Remote Procedure Call (RPC) abstracts procedure calls between
processes on networked systems
o Again uses ports for service differentiation
q Stubs – proxies for the actual procedure on the server and client
sides
o The client-side stub locates the server and marshals the parameters
o The server-side stub receives this message, unpacks the marshalled
parameters, and performs the procedure on the server
q On Windows, stub code compile from specification written in
Microsoft Interface Definition Language (MIDL)

64
Remote Procedure Calls (Cont.)
q Data representation handled via External Data Representation
(XDR) format to account for different architectures
o E.g., Big-endian (Motorola) and little-endian (Intel x86)
q Remote communication has more failure scenarios than local
o Messages can be delivered exactly once rather than at most once
q OS typically provides a rendezvous (or matchmaker) service to
connect client and server

65
Execution of RPC

66
Summary
q A process is a program in execution, and the status of the current
activity of a process is represented by the program counter, as well
as other registers.
q The layout of a process in memory is represented by four different
sections: (1) text, (2) data, (3) heap, and (4) stack.
q As a process executes, it changes state. There are four general
states of a process: (1) ready, (2) running, (3) waiting, and (4)
terminated.
q A process control block (PCB) is the kernel data structure that
represents a process in an operating system.
q The role of the process scheduler is to select an available process
to run on a CPU.

67
Summary (Cont.)
q An operating system performs a context switch when it switches
from running one process to running another.
q The fork() and CreateProcess() system calls are used to
create processes on UNIX and Windows systems, respectively.
q When shared memory is used for communication between
processes, two (or more) processes share the same region of
memory. POSIX provides an API for shared memory.
q Two processes may communicate by exchanging messages with one
another using message passing. The Mach operating system uses
message passing as its primary form of inter-process communication.
Windows provides a form of message passing as well.

68
Summary (Cont.)
q A pipe provides a conduit for two processes to communicate. There
are two forms of pipes, ordinary and named. Ordinary pipes are
designed for communication between processes that have a parent-
child relationship. Named pipes are more general and allow several
processes to communicate.
q UNIX systems provide ordinary pipes through the pipe() system
call. Ordinary pipes have a read end and a write end. A parent
process can, for example, send data to the pipe using its write end,
and the child process can read it from its read end. Named pipes in
UNIX are termed FIFOs.

69
Summary (Cont.)
q Windows systems also provide two forms of pipes—anonymous and
named pipes. Anonymous pipes are similar to UNIX ordinary pipes.
They are unidirectional and employ parent-child relationships
between the communicating processes. Named pipes offer a richer
form of inter-process communication than the UNIX counterpart,
FIFOs.
q Two common forms of client-server communication are sockets
and remote procedure calls (RPCs). Sockets allow two processes on
different machines to communicate over a network. RPCs abstract
the concept of function (procedure) calls in such a way that a function
can be invoked on another process that may reside on a separate
computer.
q The Android operating system uses RPCs as a form of inter-process
communication using its binder framework.

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