Download Module 4: Processes

Silberschatz, Galvin and Gagne ©2006
Operating System Principles
Chapter 3:
Processes - Concept
Mi-Jung Choi
[email protected]
Dept. of Computer and Science
Ch3. Process - Concept
Chapter 3: Processes
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Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems
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Process Concept
 An operating system executes a variety of programs:
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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:
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program counter
stack
data section
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Process in Memory
 Current activity of a process is represented by
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the value of program counter
The contents of the processor’s registers
 Stack
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contains temporary data
Function parameters, return address, local
variables
 Data section
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contains global variables
 Heap
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is a memory that is dynamically allocated
during process run time
 Program is a passive entity.
 Process is an active entity.
 Two processes may be associated with the
same program.
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Process State
 As a process executes, it changes state
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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 (CPU)
terminated: The process has finished execution
 The state of a process is defined in part by the current activity of that
process.
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Diagram of Process State
 It is important to realize that only one process can be running on any
processor at any instant.
 Many processes may be ready and waiting states.
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Process Control Block (PCB)
 Each process is represented in the operating system by a
process control block (PCB).
 Information associated with each process
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Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information
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Process Control Block (PCB)
 Process state
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New, ready, running, waiting, terminated
 Program counter
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The address of the next instruction to be executed
for this process
 CPU registers
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Accumulators, index registers, stack pointers, and
general-purpose registers, etc.
 CPU scheduling information
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Process priority, pointers to scheduling queue, etc.
 Memory-management information
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Value of base and limit registers, the page tables
or segment tables, etc.
 Accounting information
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The amount of CPU and real time used, time limits,
account number, job and process number
 I/O status information
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List of I/O devices allocated, open files, etc.
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CPU Switch From Process to Process
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Process Scheduling
 The objective of multiprogramming is to have some
process running at all times, to maximize CPU utilization.
 The objective of time sharing is to switch the CPU among
processes so frequently that users can interact with each
program while it is running.
 To meet these objectives, the process scheduler selects
an available process for program execution on the CPU
 For a single-processor system, there will never be more
than one running process. The rest will have to wait until
the CPU is free and can be rescheduled.
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Process Scheduling Queues
 Processes migrate among the various queues for
process scheduling.
 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

Each device has it’s own device queue.
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Ready Queue And Various I/O Device Queues
 Queues are generally
implemented as
linked lists.
 Queue header
contains pointers to
the first and last
PCBs in the list
 Each PCB includes a
pointer to the next
PCB.
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Representation of Process Scheduling
 Two types of queues: one ready queue, a set of device queues
 Two types of resources: CPU, I/O
 Arrow indicates the flow of processes in the system
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Process Scheduling
 A new process is initially put in the ready queue and waits
there until it is selected for execution. (dispatched)
 Once the process is allocated the CPU and is executing
until one of the following events occur:
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The process issues an I/O request
The process creates a new child process
The process waits for an interrupt (timer event)
The time slice assigned is expired.
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Schedulers
 is a component of OS the role of which is to select a
process for execution among a set of processes in
various queues
 In a batch and multiprogramming system
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More processes are submitted than can be executed immediately.
These processes are spooled to a disk, where they are kept for
later execution
Long-term scheduler (or job scheduler) – selects which
processes should be brought from disk into the ready queue
Short-term scheduler (or CPU scheduler) – selects which
process should be executed next from the ready queue and
allocates CPU
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Schedulers
 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
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The number of processes in memory
The degree of multiprogramming must be stable
 The rate of process creation == the rate of process termination
The long term scheduler may need to be invoked only when a
process leaves the system.
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Schedulers
 It is important that the long-term scheduler make a careful
selection.
 Processes can be described as either:
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I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts (ex, find)
CPU-bound process – spends more time doing computations; few
very long CPU bursts (ex. Sort)
 It is important that the long-term scheduler selects a good
process mix of I/O-bound and CPU-bound processes.
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If all processes are I/O bound, the ready queue will almost always
be empty
If all processes are CPU bound, the I/O waiting queue will almost
always be empty
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Schedulers
 In time-sharing system such as UNIX and Windows XP
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No long-term scheduler is used.
simply put every new process in memory for the short-term
scheduler.
The stability of these systems depends on a physical limitation or
on the self-adjusting nature of human users.
 In time-sharing system, a medium-term scheduler is used.
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It sometimes can be advantageous to remove processes from
memory and thus reduce the degree of multiprogramming.
Later, the process can be reintroduced into memory, and its
execution can be continued.
We call it swapping
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Addition of Medium Term Scheduling
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Context Switch
 is a task of switching CPU to another process
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which requires performing a state save of the current process and
a state restore of a different process
 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
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When an interrupt occurs, the OS saves the context of current
running process in its PCB.
 Context-switch time is overhead; the system does no
useful work while switching
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Time dependent on memory speed, number of registers, etc.
Typical speeds are a few millisecond.
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Operations of Processes
The Processes in most system can execute concurrently, they may be created and
deleted dynamically.
Process Creation
Process Termination
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Process Creation
 Parent process create children processes, which, in turn
create other processes, forming a tree of processes
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via create-process system call
 Three possibilities in terms of resource sharing
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Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources, child receives resources from
OS directly.
 Two possibilities in terms of execution
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Parent and children execute concurrently
Parent waits until children terminate
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A tree of processes on a typical Solaris
 A process is identified by a
unique process identifier
(PID)
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An integer number
 At top of the tree
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Sched process
pid =0
Process scheduling
 Pageout process (2)
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Memory management
 Fsflush process (3)
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File system management
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Process Creation
 Two possibilities in terms of address space
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Child is a duplicate of parent;
 child has same program and data as the parent
Child has a program loaded into it
 UNIX examples
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fork system call creates new process
exec system call used after a fork to replace the process’ memory
space with a new program
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fork() & exec()
 A new process is created by the fork() system call
 The new process consists of a copy of the address space
of the original process
 Both processes (parent & child) continue execution at the
instruction after the fork().
 The return code for the fork() is zero for the child process,
whereas the (nonzero) pid of the child process is returned
to the parent
 The exec() loads a binary file into memory, destroys the
memory image of the program, and starts its execution.
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C Program Forking Separate Process
int main()
{
pid_t pid;
pid = fork();
if (pid < 0)
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0)
{
execlp("/bin/ls", "ls", NULL);
}
else
{
/* fork another process */
/* error occurred */
/* child process */
/* parent process */
/* parent will wait for the child to complete */
wait (NULL);
printf ("Child Complete");
exit(0);
}
}
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C Program Forking Separate Process
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Process Creation
 Windows example
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CreateProcess() function is used, which is similar to fork
creates a new child process
requires loading a specified program into the address space of the
child process at process creation
expects no fewer than ten parameters.
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C Program Forking Separate Process
void _tmain( int argc, TCHAR *argv[] )
{
STARTUPINFO si;
PROCESS_INFORMATION pi;
ZeroMemory( &si, sizeof(si) );
si.cb = sizeof(si);
ZeroMemory( &pi, sizeof(pi) );
// Start the child process.
if( !CreateProcess(
NULL,
“c:\\Windows\\system32\mspaint.exe”,
NULL,
NULL,
FALSE,
0,
NULL,
NULL,
&si,
&pi )
)
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// No module name (use command line)
// program name
// Process handle not inheritable
// Thread handle not inheritable
// Set handle inheritance to FALSE
// No creation flags
// Use parent's environment block
// Use parent's starting directory
// Pointer to STARTUPINFO structure
// Pointer to PROCESS_INFORMATION structure
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C Program Forking Separate Process
{
printf( "CreateProcess failed (%d)\n", GetLastError() );
return;
}
// Wait until child process exits.
WaitForSingleObject( pi.hProcess, INFINITE );
// Close process and thread handles.
CloseHandle( pi.hProcess );
CloseHandle( pi.hThread );
}
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Process Termination
 Process executes last statement and asks the operating
system to delete it (exit)
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Status value returned from child to parent (via wait)
Process’ resources are deallocated by operating system
 Memory, open files, I/O buffers
 Parent may terminate execution of children processes
(abort) with various reason
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Child has exceeded allocated resources
Task assigned to child is no longer required
 If parent is exiting
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Some operating system do not allow child to continue if its
parent terminates
– All children terminated - cascading termination
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User
Process 1
User
Process 2
Operating System
Hardware
Cooperating Processes
The Processes executing concurrently in the operating system may be either
independent processes or cooperating processes
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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
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Information sharing
Computation speed-up; parallelism may speed up the computation
Modularity; dividing the system function into separate process
Convenience; concurrently running environment.
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User
Process 1
User
Process 2
Operating System
Hardware
InterProcess Communication
Cooperating processes require an InterProcess Communication (IPC) mechanism that
will allow them to exchange data and information
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Interprocess Communication (IPC)
 is a mechanism for processes to communicate and to synchronize their
actions
 Two fundamental models
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Shared memory
Message passing
 In the shared-memory model, a region of memory that is shared by
cooperating process is established.
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IPC is performed by reading and writing data to the shared region
 In the message-passing model, IPC takes place by means of messages
exchanged between the cooperating processes.
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IPC is performed by sending and receiving data
 Many system implements both.
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Communications Models
(a) message-passing mechanism
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(b) shared-memory mechanism
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Interprocess Communication (IPC)
 Message-passing
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is useful for exchanging smaller amounts of data, due to no
conflicts
is easier to implement than shared memory
 Shared-memory
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allows maximum speed and convenience of communication
It can be done at memory speeds
is faster than message passing
 Message-passing systems are typically implemented using system
calls and thus require more time consuming task of kernel intervention
 Shared-memory systems requires system calls for only establishing
shred-memory region.
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Shared-Memory System
 Creation of shared-memory
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A process creates a shared-memory segment in the address space
of the process.
Other processes attach it to their address space
 Termination of shared-memory
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requires the agreement from all processes to remove it.
 Exchange of information
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By reading and writing data in the shared areas
 The form of the data and the location are determined by these process,
not OS
 Processes are responsible for synchronized access.
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Producer-Consumer Problem
 Paradigm for cooperating processes,
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producer process produces information that is consumed by a
consumer process
 Example
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compiler produces assembly code, consumed by assembler
Assembler produces object modules, consumed by loader
 Producer-consumer problem can be solved by sharedmemory
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Producer and consumer processes runs concurrently
They communicate to each other via shared-memory (buffer)
Producer produces an item and store it to the buffer
Consumer consumes an item from the buffer
The producer and consumer must be synchronized, so that the
consumer consumes an item only when available.
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Producer-Consumer Problem
 Two types of buffer can be used.
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unbounded-buffer places no practical limit on the size of the
buffer
bounded-buffer assumes that there is a fixed buffer size
 In unbounded-buffer
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The consumer may have to wait for new items
The producer can always produce new items
 In bounded-buffer
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The consumer must wait if the buffer is empty
The producer must wait if the buffer is full
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Bounded-Buffer – Shared-Memory Solution
 Shared data among producer and consumer
#define BUFFER_SIZE 10
typedef struct {
int
content;
} item;
item buffer[BUFFER_SIZE];
int in
= 0;
int out
= 0;
// initial state
// empty
 Shared buffer is implemented as a circular array with two
logical pointer: in and out.
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can store no more than BUFFER_SIZE-1 elements
Empty when in == out
Full
when (in+1)%BUFFER_SIZE == out
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Bounded-Buffer – Insert() Method
item nextProduced;
while (true)
{
/* Produce an item in nextProduced */
while ( ( (in + 1) % BUFFER_SIZE ) == out );
/* do nothing -- no free buffers */
/* insert an item into the buffer */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE ;
}
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Bounded Buffer – Remove() Method
item nextConsumed;
while (true)
{
while (in == out);
// do nothing -- nothing to consume
// remove an item from the buffer
nextConsumed = buffer[out];
out = (out + 1) % BUFFER SIZE;
return nextConsumed;
}
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Message-Passing System
 OS provides the means for cooperating processes to
communicate with each other via message-passing
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processes may resides on different computers connected by a
network
processes communicate with each other without resorting to
shared variables
 Two operations:
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send(message) – message size fixed or variable
receive(message)
Those are system calls initiated by cooperating processes to
communicate each other.
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Message-Passing System
 If processes P and Q wish to communicate, they need to:
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establish a communication link between them
exchange messages via send/receive operations
 Implementation of communication link

It can be done in a variety of ways
 Three logical Issues in the implementation
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Direct or Indirect communication
Synchronous or asynchronous communication
Autonomic or Explicit buffering
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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?
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Direct Communication
 Processes must name each other explicitly:
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
send (P, message) – send a message to process P
receive (Q, message) – receive a message from process Q
 Properties of communication link
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

Links are established automatically, processes need to know only
each other’s identity
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
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Direct Communication
 Two addressing mode
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Symmetry in addressing
 Both sender and receiver must name the other to communicate
Asymmetry in addressing
 Only the sender names the recipient, the recipient is not
required to name the sender.
send (P, message) – send a message to process P
receive (id, message) – receive a message from any process, id is
set to the name of the sender with which communication has taken
place
 Disadvantages in direct communication (symmetry & Asymmetry)


Limited modularity of the resulting process definition
Changing the id of a process may necessitate examining all other
process definitions.
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Indirect Communication
 Messages are directed and received from mailboxes (also
referred to as ports)



Mailbox is an object into which messages can be placed and from
which messages can be removed.
Each mailbox has a unique id
Processes can communicate only if they share a mailbox
 Properties of indirect 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
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Indirect Communication
 Operations



create a new mailbox
send and receive messages through mailbox
destroy a mailbox
 The owner of the mailbox


User process
 The owner only receives, others only send messages.
Operating system
 A mailbox is independent and is not attached to any process.
 Primitives are defined as:


send (A, message) – send a message to mailbox A
receive (A, message) – receive a message from mailbox A
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Indirect Communication
 Mailbox sharing example



P1, P2, and P3 share mailbox A
P1, sends; P2 and P3 execute a receive() from A
Who gets the message?
 Answer depends on which one below you chose.



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.
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Synchronization
 Message passing may be either blocking or non-blocking
 Blocking is considered synchronous


Blocking send has the sender blocked until the message is
received
Blocking receive has the receiver blocked 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
 When both send() and receive() are blocking, we have a
rendezvous between sender and receiver
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Synchronization
 For the producer and consumer problem.


Blocking method is desirable
Use blocking send() and blocking receive() primitives.
 The producer


merely invokes the blocking send() call and
waits until the message is delivered to either the receiver or the
mailbox.
 When the consumer invokes receive(),

it blocks until a message is available
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Buffering
 is queue of messages attached to the communication link;

Messages resides in a temporary queue during communication
 Three ways of queue implemented

Zero capacity - 0 messages in the queue
 Maximum length of queue is 0
 Sender must wait for receiver receives message (rendezvous)

Bounded capacity – finite length of n messages
 Sender must wait if link full

Unbounded capacity – infinite length
 Sender never waits
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Client-Server Communication
 Sockets
 Remote Procedure Calls
 Remote Method Invocation (Java)
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Sockets
 A socket is defined as an endpoint for communication
 Concatenation of IP address, protocol, and port
 The socket 161.25.19.8:TCP:1625 refers to protocol TCP,
port 1625 on host 161.25.19.8
 Communication consists between a pair of sockets
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Socket Communication
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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, performs the procedure on the
server, and return the results.
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Ch3. Process - Concept
Execution of RPC
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Ch3. Process - Concept
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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Ch3. Process - Concept
Remote Procedure Call
Marshalling Parameters
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Ch3. Process - Concept
Summary
 A process is a program in execution.
 As a process executes, the process may be in one of the 5 states:
new, ready, running, waiting, terminated
 Each process is represented in the OS by its PCB
 Ready queue contains all the processes that are ready to execute and
are waiting for the CPU, I/O queue exists for each I/O device.
 Long-term scheduler selects processes from disk and put them into
ready queue.
 Short-term scheduler selects a process from ready queue and assigne
CPU
 OS provides a mechanism for parent process to create a child process
 Cooperating processes require an IPC mechanism to communicate
with each other.
 Two types of IPC: shared-memory and message-passing
 Communication in client-server system may use sockets, RPC, RMI.
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Silberschatz, Galvin and Gagne ©2006
Operating System Principles
End of Chapter 3
Mi-Jung Choi
[email protected]
Dept. of Computer and Science