Lecture 3: Processes
Operating System
Fall 2006
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Major Requirements of an
Operating System
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Interleave the execution of several processes
to maximize processor utilization while
providing reasonable response time
Allocate resources to processes in
conformance with a specific policy while at
the same time avoiding deadlock
Support interprocess communication and user
creation of processes, both of which may aid
in the structuring of applications
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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Processes - Definition
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Also called a job
Execution of an individual program
Process components:
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An executable program – text section
The associated data needed by the program
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Stack – temporary data (e.g. function parameters, return
addresses, and local variables)
Data section – global variables
Heap – memory which is dynamically allocated during process
run time
The execution context of the program
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All information the operating system needs to manage the
process
Including the value of program counter and the contents of the
processor’s registers
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Process in memory
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Process Trace
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Processes can be traced
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For a program to be executed, a process is
created for that program.
We can characterize the behavior of an individual
process by listing the sequence of instructions that
execute for that process.
Such a listing is called a trace of the process.
We can characterizing behavior of the processor
by showing how the traces of the various
processes are interleaved.
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Example for processes tracing
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Process A OS Process B OS Process C OS Process A OS Process C
time
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Example for processes tracing
(cont.)
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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Process State
- Two-State Process Model
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Process may be in one of two states
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Running
Not-running
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Not-Running Process in a
Queue
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Process State
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Not-running
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Waiting (also called blocked)
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ready to execute
waiting for I/O
Dispatcher cannot just select the
process that has been the longest in the
queue because it may be blocked
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Process State
- A Five-State Model
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New: The process is being created
Running: Instructions are being executed
Waiting (blocked): 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 State
- A Five-State Model
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Queueing-Diagram Representation
of Five-State Model
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Queueing-Diagram Representation
of Five-State Model
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Suspended Process – The
Need for Swapping
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The three principal states just described
(Ready, Running, Waiting/Blocked)
provide a systematic way of modeling
the behavior of processes and guide the
implementation of the OS.
However, there is good justification for
adding other states to the model – the
need for swapping
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Suspended Process – The
Need for Swapping
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Processor is faster than I/O so all processes
could be waiting for I/O
Swap these processes to disk to free up more
memory
Waiting(Blocked) state becomes suspend
state when swapped to disk
Two new states
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Waiting(Blocked), suspend
Ready, suspend
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One Suspend State
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Two Suspend States
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Reasons for Process Suspension
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Contents
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Process Definition
Process Trace
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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Process Scheduling Queues
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Job queue – set of all processes 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 an I/O device
Processes migrate among the various queues
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Representation of Process Scheduling
(Five-State Model)
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Schedulers
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Long-term scheduler (or job scheduler) –
selects which processes should be brought
into the ready queue
Short-term scheduler (or CPU scheduler)
– selects which process should be executed
next and allocates CPU
Medium-term scheduler – corresponds to
suspended state (swapping out of the
memory)
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Addition of Medium Term Scheduling
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Schedulers (Cont.)
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Short-term scheduler is invoked very frequently
(milliseconds)  (must be fast)
Long-term scheduler is invoked very infrequently
(seconds, minutes)  (may be slow)
The long-term scheduler controls the degree of
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Processes can be described as either:
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multiprogramming
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I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts
CPU-bound process – spends more time doing
computations; few very long CPU bursts
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Context Switch
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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
Context-switch time is overhead; the system
does no useful work while switching
Time dependent on hardware support
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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What information does the OS need to
control processes and manage resources
for them?
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Memory Tables
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Used to keep track of both main(real) and
secondary memory.
Must include the following information:
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Allocation of main memory to processes.
Allocation of secondary memory to processes.
Protection attributes of blocks of main or virtual
memory, such as which processes may access
certain shared memory regions.
Information needed to manage virtual memory.
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I/O Tables
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Used by the OS to manage the I/O
devices.
May include the following information:
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I/O device is available or assigned
Status of I/O operation
Location in main memory being used as
the source or destination of the I/O
transfer
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File Tables
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Provide information about the existence
of files:
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Existence of files
Location on secondary memory
Current Status
Attributes
Sometimes this information is maintained
by a file-management system
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Process Tables
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Used to manage processes
Include the following information:
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Where process is located
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Depend on the memory management scheme being
used.
In the simplest case, the process image is maintained as
a contiguous block of memory. This block is maintained
in secondary memory, usually disk.
Attributes necessary for its management
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Process ID
Process state
Location in memory
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Typically Elements of a Process Image
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User Data
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User Program
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The program to be executed
System Stack
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The modifiable part of the user space.
May include program data, a user stack area, and programs
that may be modified.
Each process has one or more system stacks associated with
it.
A stack is used to store parameters and calling addresses for
procedure and system calls.
Process Control Block
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Data needed by the OS to control the process.
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Process Control Block
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Process Identification
Processor State Information
Process Control Information
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Process Control Block
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Process Identification
Processor State Information
Process Control Information
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Process identification
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Numeric identifiers that may be stored
with the process control block include
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Identifier of this process
Identifier of the process that created this
process (parent process)
User identifier
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Process Control Block
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Process Identification
Processor State Information
Process Control Information
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Processor State Information
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User-Visible Registers
Control and Status Registers
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PC
PSW
Stack Pointers
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Each process has one or more system
stacks associated with it.
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Process Control Block
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Process Identification
Processor State Information
Process Control Information
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Process Control Information
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Scheduling and State Information:
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Process State (e.g. running, ready, waiting,
etc.)
Priority
Scheduling-related information
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Depend on the scheduling algorithm used.
e.g. amount of time it has already run, how
long it has waited
Event
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Identity of event the process is awaiting
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Process Control Information (cont.)
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Data Structuring
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Interprocess Communication
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A process may be linked to other processes, e.g to
its parent
Various flags, signals, and messages may be
associated with communication between two
independent processes.
Process Privilege
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Processes are granted privileges in terms of the
memory that may be accessed and the types of
instructions that may be executed.
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Process Control Information (cont.)
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Memory Management
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Including pointers to segment and/or page tables
that describe the virtual memory assigned to this
process.
Resource Ownership and Utilization
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Resources controlled by the process may be
indicated, such as opened files.
A history of utilization of the processor or other
resources may also be included; this information
may be needed by the scheduler.
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
48
Process Creation
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When a new process is to be added to those
currently being managed, the OS builds the
date structures that are used to manage the
process and allocates address space in main
memory to the process. These actions
constitute the creation of a new process.
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Reasons for Process Creation
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Submission of a new batch job
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Interactive logon
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A user at a terminal logs on to the system
Created by OS to provide a service such as printing
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The OS is provided with a batch job control stream usually a
tape or disk
The OS can create a process to perform a function on behalf
of a user program
Spawned by existing process
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For purposes of modularity or to exploit parallalism, a user
program can dictate the creation of a number of processes.
When one process spawns another process, the former is
called the parent and the spawned process as the child.
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Procedure for Process Creation
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Once the OS decides to create a new
process, it can proceed as follows:
(a)
(b)
(c)
(d)
Assign a unique process identifier to the new
process
Allocate space for the process.
Initialize the process control block
Set the appropriate linkage
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(e)
e.g. if the OS maintains each scheduling queue as a
linked list, then the new process must be put in the
ready or ready/suspend list
Create or expand other data structures
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e.g. the OS may maintain an accounting file on each
process to be used subsequently for billing and/or
performance assessment purpose.
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Process Creation
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Parent process create children processes,
which, in turn create other processes, forming
a tree of processes
Resource sharing
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Execution
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Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Parent and children execute concurrently
Parent waits until children terminate
Address space
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Child duplicate of parent
Child has a program loaded into it
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Process Creation Example for UNIX
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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Process Creation Example for UNIX C Program Forking Separate Process
int main()
{
Pid_t 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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A tree of processes on a typical Solaris
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Process Termination
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A batch job should include a Halt
instruction or an explicit OS service call
for termination.
User logs off
For an interactive application, there are
commands to terminate a process.
Control C will terminate a process.
Error and fault conditions
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Reasons for Process
Termination
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Normal completion
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Time limit exceeded
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The process has run longer than the specified total time
limit.
Memory unavailable
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The process executes an OS service call to indicate that it
has completed running.
The process requires more memory than the system can
provide.
Bounds violation
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The process tries to access a memory location that it is not
allowed to access.
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Reasons for Process
Termination (cont.)
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Protection error
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Arithmetic error
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process waited longer than a specified maximum for an
event
I/O failure
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The process tries a prohibited computation, such as division
by zero, or arithmetic overflow.
Time overrun
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The process attempts to use a resource or a file that it is not
allowed to use, or tries to use it in an improper fashion.
Example: write to read-only file
An error occurs during input or output.
Privileged instruction
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The process attempts to use an instruction reserved for the
OS
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Reasons for Process
Termination (cont.)
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Invalid instruction
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Data misuse
Operating system intervention
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such as when deadlock occurs
Parent terminates so child processes terminate
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The process attempts to execute a nonexistent instruction
(often as a result of branching into a data area and
attempting to execute the data)
When a parent process terminates, the OS may
automatically terminate all of the child processes.
Parent request
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A parent process may terminate any of its child process.
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Process Switching
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A running process is interrupted and the
OS assigns another process to the
Running state and turns control over to
that process
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When to Switch a Process
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Interrupt
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Clock interrupt
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I/O interrupt
Memory fault
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memory address is in virtual memory so it must be brought into
main memory
Trap
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process has executed for the maximum allowable time slice
error occurred
may cause process to be moved to Exit state
Supervisor call
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such as file open
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Comparison between interrupt,
trap and supervisor call
Mechanism
Cause
Use
Interrupt
External to the
execution of the
current instruction
Reaction to an
asynchronous
external event
Trap
Associated with the
execution of the
current instruction
Explicit request
Handling of an
error or an
exception condition
Call to an OS
function
Supervisor
call
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Change of Process State
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Save context of processor including program counter
and other registers
Update the process control block of the process that
is currently in the running state
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Change the state of the process to one of the other states
(Ready, waiting, or Exit, etc.)
Other relevant fields must also be updated, including the
reason for leaving the Running state and accounting
information
Move process control block to appropriate queue Ready, Waiting, etc.
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Change of Process State (cont.)
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Select another process for execution
Update the process control block of the process
selected
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Update memory-management data structures
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Change the state of this process to Running
This may be required, depending on how address translation
is done
Restore context of the selected process
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Contents
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Process Definition
Process States
Process Scheduling
Process Description
Process Control and Operations
Interprocess Communication
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Cooperating Processes
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Independent process cannot affect or be
affected by the execution of another process
Cooperating process can affect or be
affected by the execution of another process
Advantages of process cooperation
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Information sharing
Computation speed-up
Modularity
Convenience
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Two IPC Mechanisms
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Shared mamory
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A region of memory that is shared by cooperating
processes is established.
Processes can then exchange information by
reading and writing date to the shared region.
Message passing
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Communication takes place by means of messages
exchanged between the cooperating processes.
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Communications Models
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Example for Shared mamory:
Producer-Consumer Problem
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Paradigm for cooperating processes, producer
process produces information that is
consumed by a consumer process
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unbounded-buffer places no practical limit on the
size of the buffer
bounded-buffer assumes that there is a fixed
buffer size
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Bounded-Buffer – Shared-Memory
Solution
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Shared data
#define BUFFER_SIZE 10
Typedef struct {
...
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
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Solution is correct, but can only use
BUFFER_SIZE-1 elements
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Bounded-Buffer – Insert() Method
while (true) {
/* Produce an item */
while (((in = (in + 1) % BUFFER SIZE count) ==
out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
}
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Bounded Buffer – Remove() Method
while (true) {
while (in == out)
; // do nothing -- nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item;
}
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Message-Passing
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Mechanism for processes to communicate and to
synchronize their actions
Message system – processes communicate with each
other without resorting to shared variables
providing two operations:
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If P and Q wish to communicate, they need to:
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send(message) – message size fixed or variable
receive(message)
establish a communication link between them
exchange messages via send/receive
Implementation of communication link
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physical (e.g., shared memory, hardware bus)
logical (e.g., logical properties)
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Direct Communication
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Processes must name each other explicitly:
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send (P, message) – send a message to process P
receive(Q, message) – receive a message from process Q
Properties of communication link
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Links are established automatically
A link is associated with exactly one pair of communicating
processes
Between each pair there exists exactly one link
The link may be unidirectional, but is usually bi-directional
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Indirect Communication
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Messages are directed and received from
mailboxes (also referred to as ports)
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Each mailbox has a unique id
Processes can communicate only if they share a mailbox
Properties of communication link
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Link established only if processes share a common
mailbox
A link may be associated with many processes
Each pair of processes may share several communication
links
Link may be unidirectional or bi-directional
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Indirect Communication (cont.)
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Operations
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create a new mailbox
send and receive messages through mailbox
destroy a mailbox
Primitives are defined as:
send(A, message) – send a message to mailbox A
receive(A, message) – receive a message from
mailbox A
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Indirect Communication (cont.)
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Mailbox sharing
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P1, P2, and P3 share mailbox A
P1, sends; P2 and P3 receive
Who gets the message?
Solutions
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Allow a link to be associated with at most two processes
Allow only one process at a time to execute a receive
operation
Allow the system to select arbitrarily the receiver. Sender is
notified who the receiver was.
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Synchronization
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Message passing may be either blocking or nonblocking
Blocking is considered synchronous
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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
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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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Buffering

Queue of messages attached to the link;
implemented in one of three ways
1.Zero capacity – 0 messages
Sender must wait for receiver (rendezvous)
2.Bounded capacity – finite length of n messages
Sender must wait if link full
3.Unbounded capacity – infinite length
Sender never waits
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End of lecture 3
Thank you!
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