PROCESS MANAGEMENT SISTEM OPERASI

         PROCESS MANAGEMENT

A process can be thought of as a program in execution. A process will
need certain resources—such as CPU time, memory, files, and I/O devices
—to accomplish its task. These resources are allocated to the process
either when it is created or while it is executing.
A process is the unit of work in most systems. Systems consist of
a collection of processes: operating-system processes execute system
code, and user processes execute user code. All these processes may
execute concurrently.
Although traditionally a process contained only a single thread of
control as it ran, most modern operating systems now support processes
that have multiple threads.
The operating system is responsible for several important aspects of
process and thread management: the creation and deletion of both user
and system processes; the scheduling of processes; and the provision of
mechanisms for synchronization, communication, and deadlock handling
for processes.Processes
Early computers allowed only one program to be executed at a time. This
program had complete control of the system and had access to all the system’s
resources. In contrast, contemporary computer systems allow multiple programs
to be loaded into memory and executed concurrently. This evolution
required firmer control and more compartmentalization of the various programs;
and these needs resulted in the notion of a process, which is a program
in execution. A process is the unit of work in a modern time-sharing system.
The more complex the operating system is, the more it is expected to do on
behalf of its users. Although its main concern is the execution of user programs,
it also needs to take care of various system tasks that are better left outside the
kernel itself. A system therefore consists of a collection of processes: operatingsystem
processes executing system code and user processes executing user
code. Potentially, all these processes can execute concurrently, with the CPU (or
CPUs) multiplexed among them. By switching the CPU between processes, the
operating system can make the computer more productive. In this chapter, you
will read about what processes are and how they work.

3.1.1 The Process
Informally, as mentioned earlier, a process is a program in execution. A process
is more than the program code, which is sometimes known as the text section.
It also includes the current activity, as represented by the value of the program
counter and the contents of the processor’s registers. A process generally also
includes the process stack, which contains temporary data (such as function
parameters, return addresses, and local variables), and a data section, which
contains global variables.A process may also include a heap,which is memory
that is dynamically allocated during process run time. The structure of a process
in memory is shown in Figure 3.1.
We emphasize that a program by itself is not a process. A program is a
passive entity, such as a file containing a list of instructions stored on disk
(often called an executable file). In contrast, a process is an active entity,
with a program counter specifying the next instruction to execute and a set
of associated resources. A program becomes a process when an executable file
is loaded into memory. Two common techniques for loading executable filesare double-clicking an icon representing the executable file and entering the
name of the executable file on the command line (as in prog.exe or a.out).
Although two processes may be associated with the same program, they
are nevertheless considered two separate execution sequences. For instance,
several users may be running different copies of the mail program, or the same
user may invoke many copies of the web browser program. Each of these is a
separate process; and although the text sections are equivalent, the data, heap,
and stack sections vary. It is also common to have a process that spawns many
processes as it runs.We discuss such matters in Section 3.4.
Note that a process itself can be an execution environment for other
code. The Java programming environment provides a good example. In most
circumstances, an executable Java program is executed within the Java virtual
machine (JVM). The JVM executes as a process that interprets the loaded Java
code and takes actions (via native machine instructions) on behalf of that code.
For example, to run the compiled Java program Program.class, we would
enter
java Program
The command java runs the JVM as an ordinary process, which in turns
executes the Java program Program in the virtual machine. The concept is the
same as simulation, except that the code, instead of being written for a different
instruction set, is written in the Java language.

3.1.2 Process State
As a process executes, it changes state. The state of a process is defined in part
by the current activity of that process. A process may be in one of the following
states:
• New. The process is being created.
• Running. Instructions are being executed.
• Waiting. The process is waiting for some event to occur (such as an I/O
completion or reception of a signal).
• Ready. The process is waiting to be assigned to a processor.
• Terminated. The process has finished execution.
These names are arbitrary, and they vary across operating systems. The states
that they represent are found on all systems, however. Certain operating
systems also more finely delineate process states. 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, however. The state diagram corresponding
to these states is presented in Figure 3.2.

3.1.3 Process Control Block
Each process is represented in the operating system by a process control block
(PCB)—also called a task control block.APCBis shown in Figure 3.3. It contains
many pieces of information associated with a specific process, including these:• Process state. The state may be new, ready, running, waiting, halted, and
so on.
• Program counter. The counter indicates the address of the next instruction
to be executed for this process.
• CPU registers. The registers vary in number and type, depending on
the computer architecture. They include accumulators, index registers,
stack pointers, and general-purpose registers, plus any condition-code
information. Along with the program counter, this state information must
be saved when an interrupt occurs, to allow the process to be continued
correctly afterward (Figure 3.4).
• CPU-scheduling information. This information includes a process priority,
pointers to scheduling queues, and any other scheduling parameters.
(Chapter 6 describes process scheduling.)
• Memory-management information. This information may include such
items as the value of the base and limit registers and the page tables, or the
segment tables, depending on the memory system used by the operating
system (Chapter 8).

3.1.4 Threads
The process model discussed so far has implied that a process is a program that
performs a single thread of execution. For example, when a process is running
a word-processor program, a single thread of instructions is being executed.
This single thread of control allows the process to perform only one task at
a time. The user cannot simultaneously type in characters and run the spell
checker within the same process, for example. Most modern operating systems
have extended the process concept to allow a process to have multiple threads
of execution and thus to perform more than one task at a time. This feature
is especially beneficial on multicore systems, where multiple threads can run
in parallel. On a system that supports threads, the PCB is expanded to include
information for each thread. Other changes throughout the system are also
needed to support threads. Chapter 4 explores threads in detail.PROCESS REPRESENTATION IN LINUX
The process control block in the Linux operating system is represented by
the C structure task struct, which is found in the <linux/sched.h>
include file in the kernel source-code directory. This structure contains all the
necessary information for representing a process, including the state of the
process, scheduling and memory-management information, list of open files,
and pointers to the process’s parent and a list of its children and siblings. (A
process’s parent is the process that created it; its children are any processes
that it creates. Its siblings are children with the same parent process.) Some
of these fields include:
long state; /* state of the process */
struct sched entity se; /* 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 */
For example, the state of a process is represented by the field long state
in this structure.Within the Linux kernel, all active processes are represented
using a doubly linked list of task struct. The kernel maintains a pointer—
current—to the process currently executing on the system, as shown below:
struct task_struct

As an illustration of how the kernel might manipulate one of the fields in
the task struct for a specified process, let’s assume the system would like
to change the state of the process currently running to the value new state.
If current is a pointer to the process currently executing, its state is changed
with the following:
current->state = new state;