Process Concept
 An operating system executes a variety of programs
 Batch system – jobs
 Time-shared systems – user programs or tasks
 Process – a program in execution
 A process includes:
 Text segment
 Data segment
 Stack segment
 Program counter
Operating System Concepts
4.1
Silberschatz, Galvin and Gagne 2002
Process State
 As a process executes, it changes state
 new: The process is being created.
 running: Instructions are being executed on the CPU.
 waiting: The process is waiting for some event to occur.
 ready: The process is waiting for the CPU.
 terminated: The process has finished execution.
Operating System Concepts
4.2
Silberschatz, Galvin and Gagne 2002
Process Control Block (PCB)
Information associated with each process.
 Process state
 Program counter
 CPU register values
 CPU scheduling information
 Memory-management information
 Accounting information
 I/O status information
Operating System Concepts
4.3
Silberschatz, Galvin and Gagne 2002
Process Queues
 Job queue – set of all PCBs 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 a resource,
e.g., an I/O device.
 PCBs migrate between the various queues.
Operating System Concepts
4.4
Silberschatz, Galvin and Gagne 2002
Ready Queue And Various I/O Device Queues
Operating System Concepts
4.5
Silberschatz, Galvin and Gagne 2002
Context Switch
 When the CPU switches to another process, the system
must




Switch to kernel mode
Save the state of the old process
Deal with the interrupt
Load the saved state for the new process.
 Hardware (and possibly OS) saves the state of the old
process
 The scheduler selects the new process
 The dispatcher loads the new process
 Set the PC
 Switch to user mode
 Context-switch time is overhead; the system does no
useful work while switching
 Time dependent on hardware support.
Operating System Concepts
4.6
Silberschatz, Galvin and Gagne 2002
CPU Switch From Process to Process
Operating System Concepts
4.7
Silberschatz, Galvin and Gagne 2002
Process Creation
 Parent process create children processes, which, in turn
create other processes, forming a tree of processes.
 Address space (one of)
 Child duplicate of parent.
 Child has a program loaded into it.
 Execution (one of)
 Parent and child execute concurrently
 Parent waits until child terminates
 Resource sharing (one of)
 Parent and children share all resources.
 Children share subset of parent’s resources.
 Parent and child share no resources.
Operating System Concepts
4.8
Silberschatz, Galvin and Gagne 2002
Process Termination
 Process executes last statement and tells the OS
 Output status from child to parent
 Process’ resources are (eventually) deallocated by the
operating system
 Parent may terminate execution of children processes
 Child has exceeded allocated resources.
 Task assigned to child is no longer required.
 Parent is exiting
 With cascading termination, the operating system does
not allow child to continue if its parent terminates
Operating System Concepts
4.9
Silberschatz, Galvin and Gagne 2002
UNIX PCBs
Operating System Concepts
4.10
Silberschatz, Galvin and Gagne 2002
UNIX Shell
Operating System Concepts
4.11
Silberschatz, Galvin and Gagne 2002
Threads
 A thread (or lightweight process) is a basic unit of CPU
utilization; it consists of:
 Program counter
 Register set
 Stack space

A thread shares with its peer threads its:
 Code segment
 Data segment
 Operating-system resources
 A traditional or heavyweight process is equal to a task
with one thread.
Operating System Concepts
4.12
Silberschatz, Galvin and Gagne 2002
Single and Multithreaded Processes
Operating System Concepts
4.13
Silberschatz, Galvin and Gagne 2002
Thread Implementations
 User-level threads library
 POSIX Pthreads
 Mach C-threads
 Solaris threads
 Kernel Threads
 Windows 95/98/NT/2000
 Solaris
 Linux
Operating System Concepts
4.14
Silberschatz, Galvin and Gagne 2002
Many-to-One
 Many user-level threads mapped to single kernel thread.
 Used on systems that do not support kernel threads.
Operating System Concepts
4.15
Silberschatz, Galvin and Gagne 2002
One-to-One
 Each user-level thread maps to kernel thread.
 Examples
 Windows 95/98/NT/2000
 OS/2
 LinuxThreads
Operating System Concepts
4.16
Silberschatz, Galvin and Gagne 2002
Many-to-Many Model
 Allows many user level threads to be mapped to many
kernel threads.
 Allows the operating system to create a sufficient number
of kernel threads.
 Solaris 2
 Windows NT/2000 with the ThreadFiber package
Operating System Concepts
4.17
Silberschatz, Galvin and Gagne 2002
Pthreads
 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.
Operating System Concepts
4.18
Silberschatz, Galvin and Gagne 2002
Benefits
 Responsiveness
 In a multiple threaded task, while one thread is blocked and
waiting, a second thread in the same task can run.
 Kernel level threads provide a mechanism that allows
sequential processes to make blocking system calls while
also achieving parallelism.
 Resource Sharing
 Economy
 Utilization of MP Architectures
Operating System Concepts
4.19
Silberschatz, Galvin and Gagne 2002