Process Management

The fundamental task of any modern operating system is process management.
The operating system must allocate resources to processes, enable
processes to share and exchange information, protect the resources of each
process from other processes, and enable synchronization among processes.To meet
these requirements, the operating system must maintain a data structure for each
process that describes the state and resource ownership of that process and that enables
the operating system to exert process control.
On a multiprogramming uniprocessor, the execution of multiple processes can
be interleaved in time. On a multiprocessor, not only may process execution be interleaved,
but also multiple processes can execute simultaneously. Both interleaved
and simultaneous execution are types of concurrency and lead to a host of difficult
problems, both for the application programmer and the operating system.
In many contemporary operating systems, the difficulties of process management
are compounded by the introduction of the concept of thread. In a multithreaded
system, the process retains the attributes of resource ownership, while the
attribute of multiple, concurrent execution streams is a property of threads running
within a process.

Background

Before defining the term process, it is useful to summarize some of the concepts introduced
in Chapters 1 and 2:
1. A computer platform consists of a collection of hardware resources, such as
the processor, main memory, I/O modules, timers, disk drives, and so on.
2. Computer applications are developed to perform some task.Typically, they accept
input from the outside world, perform some processing, and generate output.
3. It is inefficient for applications to be written directly for a given hardware platform.
The principal reasons for this are as follows:

a. Numerous applications can be developed for the same platform.Thus, it makes
sense to develop common routines for accessing the computer’s resources.
b. The processor itself provides only limited support for multiprogramming.
Software is needed to manage the sharing of the processor and other
resources by multiple applications at the same time.
c. When multiple applications are active at the same time, it is necessary to
protect the data, I/O use, and other resource use of each application from
the others.
4. The OS was developed to provide a convenient, feature-rich, secure, and consistent
interface for applications to use.The OS is a layer of software between
the applications and the computer hardware (Figure 2.1) that supports applications
and utilities.
5. We can think of the OS as providing a uniform, abstract representation of
resources that can be requested and accessed by applications. Resources include
main memory, network interfaces, file systems, and so on. Once the OS
has created these resource abstractions for applications to use, it must also
manage their use. For example, an OS may permit resource sharing and
resource protection.
Now that we have the concepts of applications, system software, and resources,
we are in a position to discuss how the OS can, in an orderly fashion, manage the execution
of applications so that
• Resources are made available to multiple applications.
• The physical processor is switched among multiple applications so all will
appear to be progressing.
• The processor and I/O devices can be used efficiently.
The approach taken by all modern operating systems is to rely on a model in
which the execution of an application corresponds to the existence of one or more
processes.

Processes and Process Control Blocks
Recall from Chapter 2 that we suggested several definitions of the term process,
including
• A program in execution
• An instance of a program running on a computer
• The entity that can be assigned to and executed on a processor
• A unit of activity characterized by the execution of a sequence of instructions,
a current state, and an associated set of system resources
We can also think of a process as an entity that consists of a number of elements.
Two essential elements of a process are program code (which may be shared with
other processes that are executing the same program) and a set of data associated
with that code. Let us suppose that the processor begins to execute this program

code, and we refer to this executing entity as a process. At any given point in time,
while the program is executing, this process can be uniquely characterized by a
number of elements, including the following:
• Identifier: A unique identifier associated with this process, to distinguish it
from all other processes.
• State: If the process is currently executing, it is in the running state.
• Priority: Priority level relative to other processes.
• Program counter: The address of the next instruction in the program to be
executed.
• Memory pointers: Includes pointers to the program code and data associated
with this process, plus any memory blocks shared with other processes.
• Context data: These are data that are present in registers in the processor
while the process is executing.
• I/O status information: Includes outstanding I/O requests, I/O devices (e.g., tape
drives) assigned to this process, a list of files in use by the process, and so on.
• Accounting information: May include the amount of processor time and clock
time used, time limits, account numbers, and so on.
The information in the preceding list is stored in a data structure, typically
called a process control block (Figure 3.1), that is created and managed by the OS.
The significant point about the process control block is that it contains sufficient