Process threads operating system.

3.1 Silberschatz, Galvin and GagneOperating System Concepts – 8th
Edition
Process
An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
Process – a program in execution; process execution must
progress in sequential fashion
A process includes:
program counter
stack
data section
Topics:
Operations in Process
Scheduling
Interprocess Communication

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Process in Memory

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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

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Process States and Transition

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Process Control Block (PCB)

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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 via a context switch.
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work
while switching
Time dependent on hardware support

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CPU Switch From Process to Process

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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)
Options in Resource sharing
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Options Execution
Parent and children execute concurrently
Parent waits until children terminate

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Process Creation (Cont.)
Options n 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

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Unix Fork/Exec/Exit/Wait Example
int pid = fork();
Create a new process that is a
clone of its parent.
exec*(“program” [, argvp, envp]);
Overlay the calling process
virtual memory with a new
program, and transfer control
to it.
exit(status);
Exit with status, destroying the
process.
int pid = wait*(&status);
Wait for exit (or other status
change) of a child.
fork parent fork child
wait exit
exec
initialize
child
context

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Example: Process Creation in Unix
int pid;
int status = 0;
if (pid = fork()) {
/* parent */
…..
pid = wait(&status);
} else {
/* child */
…..
exit(status);
}
Parent uses wait to sleep
until the child exits; wait
returns child pid and
status.
Wait variants allow wait
on a specific child, or
notification of stops and
other signals.
The fork syscall
returns twice: it
returns a zero to the
child and the child
process ID (pid) to the
parent.

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C Program Forking Separate Process
int main()
{
int pid;
/* fork another process */
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL);
}
else { /* parent process */
/* parent will wait for the child to
complete */
wait (NULL);
printf ("Child Complete");
exit(0);
}
}

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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

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Communications Models: Shared
memory or Message Passing

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Synchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send has the sender block until the message is
received
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

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Motivation for multi-threaded servers

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Single and Multithreaded Processes

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Benefits
Responsiveness
Resource Sharing
Economy
Scalability

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Kernel Threads
Recognized and supported by the OS Kernel
OS explicitly performs scheduling and context switching of kernel threads
Examples
Windows XP/2000
Solaris
Linux
Tru64 UNIX
Mac OS X

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User Threads
Thread management done by user-level threads library
OS kernel does not know/recognize there are multiple threads running
in a user program.
The user program (library) is responsible for scheduling and context
switching of its threads.
Three primary thread libraries:
POSIX Pthreads
Win32 threads
Java threads

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User- vs. Kernel-level Threads
From W. Stallings, Operating Systems, 6th
Edition

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Pthreads
May be provided either as user-level or kernel-level
A POSIX standard (IEEE 1003.1c) API for thread creation and
synchronization
API specifies behavior of the thread library, implementation is up to
development of the library
Common in UNIX operating systems (Solaris, Linux, Mac OS X)

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Java Threads
Java threads are managed by the JVM
Typically implemented using the threads model provided by
underlying OS
Java threads may be created by:
Extending Thread class
Implementing the Runnable interface

Process threads operating system.

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