Topic 3 - Process Management

Process

A sequential execution of program

Process

A name given to a program instance (computer program or application) that has been loaded into memory and managed by the operating system

Process

Sometimes called a job or task

Thread

sequence of instructions within a process

- The executable program itself

- The address space of the process

- The execution context of the process

Components of a Process

Stack

Local variables and functions are allocated at compile-time and placed here

stack, heap

As memory is used by a process, the _____ and the _____ go toward each other

Shared Libraries

f a process makes use of ___________, these are mapped in the memory between the stack and heap

Heap

Data structures explicitly allocated at run-time is placed here

Global Variables

have their own segment in memory that allows for reading and writing

Text

Program code (converted to machine instructions) and other read-only data are placed here

Process Scheduling

Deciding which process/thread should occupy each resource (CPU, disk, etc.) at each moment

-new

-running

-waiting

-ready

-terminated

Process 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.

Process Control Block (PCB)

OS maintains a data structure for each process ____________ that contains the information needed to manage a particular process.

- Process ID number (PID)

- Address space of the process

- Execution context of the process

- List of open files being used by the process

- accounting information

Information stored in PCB includes:

Process Context

Minimal set of state information that must be stored to allow a process to be stopped and re-started later

information stored in the CPU

Context Switching

The Operating System switches the CPU from one process to another

Scheduler

The part of the OS that makes the choice is called the

scheduling algorithm

what algorithm does the scheduler use?

single-processor system

only one process can run at a time; any others must wait until the CPU is free and can be rescheduled

multiprogramming systems

whenever two or more processes are simultaneously in the ready state, a choice has to be made which process to run next.

CPU-I/O Burst Cycle

Eventually, the final CPU burst ends with a system request to terminate execution

- job queue

- ready queue

- device queue

3 Process Scheduling Queues

Job Queues

set of all processes in the system.

Ready Queue

set of all processes residing in main memory, ready and waiting to execute.

Device Queue

set of processes waiting for an I/O device. Each device has its own device queue.

Ready Queue

is stored as linked list

Ready Queue Header

will contain pointers to the first and last PCBs in the list

Long-term scheduler (Job scheduler)

selects which processes should be brought into the ready queue (i.e, selects processes from pool (disk) and loads them into memory for execution).

Short-term scheduler (CPU scheduler)

selects which process should be executed next and allocates CPU (i.e, selects from among the processes that are Ready to execute, and allocates the CPU to one of them)

Short-term scheduler

is invoked very frequently (milliseconds) (must be fast).

Long-term scheduler

s invoked very infrequently (seconds, minutes) (may be slow)

Long-term scheduler

controls the degree of multiprogramming (the number of processes in memory).