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Process Description
z Process
y Process description
y Key concept of the process
y Examples of the process
z
z
z
z
z
Two-state Process Model
Uniprogramming
Mutliprogramming
Process Creation
Process Termination
Major Requirements of an
Operating System
z Interleave the execution of the number of
processes to maximize processor
utilization while providing reasonable
response time
z Allocate resources to processes
z Support interprocess communication and
user creation of processes
Process
z Execution of an individual program (a
sequence of instructions) in the context of
a particular process state
z Can be traced
y list the sequence of instructions that execute
z Also called a task
Key Concept of Process
z processes are separated:
no process can directly affect the state of
another process.
Process is a Key OS
Abstraction that Users
See
z The environment you interact with when
you use a computer is built up out of
processes.
Process Examples
z The shell you type stuff into is a process.
z When you execute a program you have
just compiled, the OS generates a
process to run the program.
z Your WWW browser is a process.
Dispatcher
z Program that moves the processor from
one process to another
z Prevents a single process from
monopolizing processor time
Two-State Process Model
Dispatch
Enter
Not
Running
Running
Exit
Pause
(a) State transition diagram
Queue
Enter
Dispatch
Exit
Processor
Pause
(a) Queuing diagram
Uniprogramming
z Uniprogramming:
only one process at a time
z Typical uniprogramming example: DOS.
Problem:
users often wish to perform
more than one activity at a
time and uniprogramming
does not allow this.
z Uniprogrammed systems put in things like
memory-resident programs that invoked
asynchronously. One key problem with DOS
is that there is no memory protection - one
program may write the memory of another
program, causing bugs.
Multiprogramming
z Multiprogramming:
multiple processes at a time
z Typical multiprogramming example: Unix
Problem :
which processes get to use
the physical resources of the
machine and when?
z Standard solution:
To use preemptive multitasking - OS runs
one process for a while, then takes the
CPU away from that process and lets
another process run. Must save and
restore process state.
Key issue:
Fairness for Multiprogramming
z Must ensure that:
all processes get their fair share of the CPU.
Process Creation
z Submission of a batch job
z User logs on
z Create to provide a service such as
printing
z Spawned by an existing process
Process Termination
z Batch job issues Halt instruction
z User logs off
z Process executes a service request to
terminate
z Error and fault conditions
Reasons for Process
Termination
z
z
z
z
z
Normal completion
Time limit exceeded
Memory unavailable
Bounds violation
Protection error
y example write to read-only file
z Arithmetic error
z Time overrun
y process waited longer than a specified
maximum for an event
Reasons for Process
Termination
z I/O failure
z Invalid instruction
y happens when try to execute data
z Data misuse
z Operating system intervention
y such as when deadlock occurs
z Parent terminates so child processes
terminate
z Parent request
Processes
z Not-running
y ready to execute
z Blocked
y waiting for I/O
z Dispatcher cannot just select the process
that has been in the queue the longest
because it may be blocked
A Five-State Model (1)
z
z
z
z
z
Running
Ready
Blocked
New
Exit
Five-State Process Model
(2)
Dispatch
New
Admit
Ready
Release
Running
Time-out
Event
Occurs
Blocked
Event
Wait
Exit
A Five-State Model Running (3)
z Running: the process that is currently
being executed.
z We will assume a computer with a single
processor, so at most one process can be
in this state at a time
A Five-State Model Running (4)
z Running > Exit: the currently running
process is terminated by the operating
system if the process indicated that it has
completed, or if it aborts.
z Running > Ready: the running process
has reached the maximum allowable time
for uninterrupted execution.
z Running > Blocked: a process requests
something for which it must wait - I/O
operation.
A Five-State Model - Ready
(5)
z Ready
A process that is prepared to execute
when given the opportunity.
z Ready > Running
When it is time to select a new process to
run, the Operating system choose one of
the processes in the ready state. The
issue is which process to choose.
A Five-State Model Blocked (6)
z Blocked
A process that can not execute until some
event occurs, such as the completion of
an I/O operation.
z Blocked > Ready
A process in the Blocked state is moved to
the Ready state when the event for which
it has been waiting occurs
A Five-State Model - New
(7)
z New: a process that has just been created
but has not yet been admitted to the pool
of executable processed by the operating
system.
z Null > New: a new process is created to
execute a program. This event occurs for
new job, logon, and create child process.
z New > Ready: When the process is
prepared to take on an additional process.
A Five-State Model - Exit
(8)
z Exit: A process that has been released
from the pool of executable processes by
the operating system, either because it
halted or because it aborted for some
reason.
Single Blocked Queue
Ready Queue
Release
Dispatch
Admit
Processor
Time-out
Event Wait
Event
Occurs
Blocked Queue
Multiple Blocked Queues
Ready Queue
Release
Dispatch
Admit
Processor
Time-out
Event 1 Wait
Event 1
Occurs
Event 1 Queue
Event 2 Wait
Event 2
Occurs
Event 2 Queue
Suspended Processes - Do
Better than Five-State
Process Model
z Processor is faster than I/O so all
processes could be waiting for I/O
z Swap these processes to disk to free up
more memory
z Blocked state becomes suspend state
when swapped to disk
z Two new states
y Blocked, suspend
y Ready, suspend
Process State Transition Diagram
with Two Suspend States - SevenState Process Model (1)
Ready
Admit
Admit
Suspend
Dispatch
Activate
Ready,
suspend
Ready
Suspend
Running
Time out
Event
Occurs
Event
Occurs
Activate
Blocked,
suspend
Blocked
Suspend
Event
Wait
Exit
A Seven-State Model Assumption (2)
z The discussion so far has assumed that
virtual memory is not in use and that a
process is either all in main memory or all
out of memory.
z With a virtual memory scheme, it is
possible to execute a process that is only
partially in main memory.
A Seven-State Model States (3)
z Ready: The process is in main memory
and available for execution.
z Blocked: The process is in main memory
and awaiting an event.
z Blocked, suspend: The process in in
secondary memory and awaiting an event.
z Ready, suspend: The process is in
secondary memory but is available for
execution as soon as it is loaded into main
memory.
A Seven-State Model (4)
z Blocked > Blocked, suspend
If there are no ready processes, then at
least one blocked process is swapped out
to make room for another process that is
not blocked.
A Seven-State Model (5)
z Blocked, suspend > Ready, suspend
A process in the Blocked, suspend state is
moved to the Ready, suspend state when
the event for which it has been waiting
occurs.
A Seven-State Model (6)
z Ready, suspend > Ready
When there are no ready processes in
main memory, the operating system will
need to bring one in to continue
execution.
z It might be the case that a process in the
ready, suspend state has higher priority
than any of the processes in the Ready
state. In that case, the operating system
designer may dedicate that it is more
A Seven-State Model (7)
z Ready > Ready,suspend
It may be necessary to suspend a ready
process if that is the only way to free a
sufficiently large block of main memory.
z The operating system may choose to
suspend a lower-priority ready process
rather a higher-priority blocked process if
it believes that the blocked process will be
ready soon.
A Seven-State Model (8)
z New > Ready, suspend and New > Ready
When a new process is created, it can
either be added to the Ready queue or
the Ready, suspend queue.
z There would always be insufficient room
in main memory for a new process; hence
the use of the New > Ready, suspend
transition.
A Seven-State Model (9)
z Blocked, suspend > Blocked
A process terminates, freeing some main
memory. There is a process in the
Block,suspend queue with a higher
priority than any of the processes in the
Ready, suspend queue and the operating
system has reason to believe that the
blocking event for that process will occur
soon.
A Seven-State Model (10)
z Running > Ready, suspend
If, however, the operating system is
preempting the process because a higherpriority process on the Blocked, suspend
queue has just become unblocked, the
operating system could move the running
process directly to the Ready, suspend
queue and free some main memory.
A Seven-State Model (11)
z Various > Exit
*Typically, a process terminates while it is
running, either because it has completed
or because of some fatal fault condition
*However, in some operating systems, a
process may be terminated by the process
that created it or when the parent process
is itself terminated (think about fork
system call). If this is allowed, then a
process in any state can be moved to the
Reasons for process
Suspension (1)
z Swapping
The operating system needs to release
sufficient main memory to bring in a
process that is ready to execute.
z Other OS reason
The operating system may suspend a
background or utility process or a process
that is suspected of causing a problem.
z Interactive user request
A user may wish to suspend execution of
a program for purpose of debugging or in
connection with the use of a resource.
Reasons for process
Suspension (2)
z Timing
A process may be executed periodically
(e.g., an accounting or system monitoring
process) and may be suspended while
waiting for the next time interval.
Parent process request
A parent process may wish to suspend
execution of a descendent to examine or
modify the suspended process, or to
coordinate the activity of various
descendents.