OS UNIT2.ppt

3.1 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
UNIT 2
PROCESS MANAGEMENT
Objectives:
 To introduce the notion of a process -- a program in
execution, which forms the basis of all computation
 To describe the various features of processes, including
scheduling, creation and termination, and communication
 To explore interprocess communication using shared
memory and message passing
 To describe communication in client-server systems

Process Concept
 Process – a program in execution.
 A process is the unit of work in a modern time-sharing system.
 Multiple parts
 The program code, also called text section
 Stack containing temporary data
 Function parameters, return addresses, local variables
 Data section containing global variables
 Heap containing memory dynamically allocated during run time

Process in Memory

Process Concept (Cont.)
 Program is passive entity stored on disk (executable file),
process is active
 Program becomes process when executable file loaded into
memory
 Execution of program started via GUI mouse clicks, command
line entry of its name, etc
 One program can be several processes
 Consider multiple users executing the same program

Process Control Block (PCB)
Information associated with each process
(also called task control block)
 Process ID(PID) – integer
 Priority Number
 Program counter – location of
instruction to next execute
 Memory-allocation – memory allocated
to the process
 I/O status information – I/O devices
allocated to process, list of open files
 List of open files
 Accounting information – CPU used,
clock time elapsed since start, time
limits
 CPU registers – contents of all process-
centric registers
 Process state – running, waiting, etc

Process State
 As a process executes, it changes 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

Diagram of Process State

 OS maintains ready queue – ready
processes(priority order)
 Blocked queue – Blocked processes
(unordered)
 Dispatching- The process of assigning a
processor to the first process on the ready
list by dispatcher

 Exit reasons:
1. completing operation
2.due to unrecoverable error
 H/W interrupting clock – to avoid
continuous usage

State transition Remarks
1.Ready to running Process is dispatched
2.Running to ready Process time slice expires
3.Running to blocked When a process blocks
4.Blocked to ready After the I/O event
5.Ready to exit Parent process terminates child
process
6.Running to exit After completion of task

Suspended process
 I/O device is slower than Processor
 Suspended state:
- Blocked to suspended state(disk)
Characteristics:
1.The process cannot execute immediately
2.The process may or may not be waiting for
an event
3.By parent submission

Operating System
Process Scheduling:
 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.Each device has own queue
 Process is moved between the various queues.

Schedulers
 Long-term scheduler (or job scheduler):
- selects which processes should be brought into
the ready queue.
 Medium term scheduler
- Part of swapping function.
- Removes process from memory for I/O
- May lead to suspended state (disk).

 Short-term scheduler (or CPU scheduler
or Dispatcher )
– selects which process should be
executed next and allocates CPU for
execution.
- Moves process from ready queue to
processor

Context Switch
 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.
 Reasons:
1.Multitasking 2.Interrupt handling 3.User
and kernel mode switching

OPERATIONS ON PROCESSES
Process Creation:
 OS creates new process with specified
attributes and identifiers.
 The creating process is called a parent
process, where as the new processes are
called the children of that process

 Parent process creates children processes, which, in
turn create other processes, forming a tree of processes.
 Resource sharing
 Parent and children share all resources.
 Children share subset of parent’s resources.
 Execution
 Parent and children execute concurrently.
 Parent waits until children terminate.

Process Creation (Cont.)
 UNIX examples
 fork system call creates new process
Void main()
{
fork();
}
 execve system call used after a fork to
replace the process’ memory space with a
new program.

Process Termination
 Process executes last statement and asks the operating
system to decide it (exit).
 Parent may terminate execution of children processes
(abort).
1.Normal completion of operation
2.Memory is not available
3.Time slice expired
4.Parent termination
5.Failure of I/O
6.Request from parent process
7.Misuse of access rights

COOPERATING PROCESSES
 A process is independent if it cannot affect
or be affected by the other processes
executing in the system.
 A process is cooperating if it can affect or
be affected by the other processes
executing in the system.

Reason for cooperating processes
 Information sharing
 Computation speedup (By exec sub
tasks parallel)
 Modularity(dividing the system functions
into separate processes or threads)
 Convenience

Producer – Consumer Problem:
 A producer process produces information
that is consumed by a consumer process.
 For example, a print program produces
characters that are consumed by the
printer driver. A compiler may produce
assembly code, which is consumed by an
assembler.

 To allow producer and consumer
processes to run concurrently, we must
have available a buffer of items that can
be filled by the producer and emptied by
the consumer.
 unbounded-buffer: places no practical limit
on the size of the buffer.
 bounded-buffer : assumes that there is a
fixed buffer size.

Interprocess Communication (IPC)
 Operating systems provide the means for
cooperating processes to communicate with
each other via an inter process communication
(PC) facility.
 IPC provides a mechanism to allow processes to
communicate and to synchronize their actions.
 IPC is best provided by a message passing
system.
COPY

 Inter process communication is achieved
through,
1. Shared Memory - memory shared among
the processes
2. Message Passing – communicate and to
synchronize their actions
IPC facility provides two operations:
 send(message)
 receive(message)

(a) Message passing. (b) shared memory

Basic Structure:
 If processes P and Q want to
communicate, they must send messages
to and receive messages from each other,
a communication link must exist between
them.
 Physical implementation of the link is done
through a hardware bus , network etc,

Methods
1.Direct Communication:
 Processes must name each other explicitly:
 send (P, message) – send a message to process P
 receive(Q, message) – receive a message from
process Q
 Properties of communication link
 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.

2.Indirect Communication
 Messages are directed and received from mailboxes (also referred to as ports).
 Each mailbox has a unique id.
 Processes can communicate only if they share a mailbox.
 Properties of communication link
 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.
 Operations
 create a new mailbox
 send and receive messages through mailbox
 destroy a mailbox

3.Buffering:
 Queue of messages attached to the link;
implemented in one of three ways.
1. Zero capacity – 0 messages
Sender must wait for receiver.
2. Bounded capacity – finite length of n
messages, Sender must wait if link full.
3. Unbounded capacity – infinite length
Sender never waits.

4.Synchronization:
Message passing may be either blocking or non-
blocking.
 1. Blocking Send - The sender blocks itself till the
message sent by it is received by the receiver.
 2. Non-blocking Send - The sender does not block
itself after sending the message but continues with
its normal operation.
 3. Blocking Receive - The receiver blocks itself
until it receives the message.
 4. Non-blocking Receive – The receiver does not
block itself.

THREADS – OVERVIEW
 A thread is the basic unit of CPU utilization.
 It is sometimes called as a lightweight process.
 It consists of a thread ID, a program counter, a register
set and a stack.

User thread and Kernel threads
User threads:
-It uses user space for thread scheduling
-Transparent to OS
- Created by run time libraries and cannot
execute privileged instructions
- smaller and faster
EX:POSIX pthreads, Mach C-threads

 Kernel thread
-Thread management is done by kernel.
-OS supports KLT.
- Threads are constructed and controlled
by system calls.
Ex: Windows 95/98/NT

Multithreading models
 In many applications,user level threads
are mapped with kernel level threads.
1.One to one
2.Many to one
3.Many to many

One to One
 One user level thread is mapped with one
kernel level thread.
 For 3 UT,Kernel creates 3 KT
 Ex:Windows 95/XP,LINUX

Many to one
 Many user level threads are mapped with
one kernel level thread.
 Single thread of control
Ex: Green threads for SOLARIS

Many to many
 Many user level threads map with many
kernel level threads
 M-to-N thread mapping

THREADING ISSUES:
1. fork() and exec() system calls.
2. Thread cancellation.
3. Signal handling
4. Thread pools
5. Thread specific data

CPU SCHEDULING
 Selects one process among the processes in
memory that are ready to execute, and
allocates the CPU to one of them.
 CPU scheduling is the basis of multi-
programmed operating systems.
CPU-I/O Burst Cycle:
 Process execution consists of a cycle of CPU
execution and I/O wait.
 Process execution begins with a CPU burst.

CPU Scheduler:
 Whenever the CPU becomes idle, the
operating system must select one of the
processes in the ready queue to be executed.
 The selection process is carried out by the
short-term scheduler (or CPU scheduler).
 The ready queue is not necessarily a first-in,
first-out (FIFO) queue. It may be a FIFO
queue, a priority queue, a tree, or simply an
unordered linked list.

Preemptive Scheduling:
A scheduling method that interrupts the processing of a process and
transfers the CPU to another process.
CPU scheduling decisions may take place under the following four
circumstances:
1. When a process switches from the running state to the waiting
state.
2. When a process switches from the running state to the ready
state.
3. When a process switches from the waiting state to the ready
state.
4. When a process terminates.
 Under 1 & 4 scheduling scheme is non preemptive.
 Otherwise the scheduling scheme is preemptive.

Non-Preemptive Scheduling:
 In non preemptive scheduling, once the
CPU has been allocated a process, the
process keeps the CPU until it releases
the CPU either by termination or by
switching to the waiting state.

Dispatcher:
 The dispatcher is the module that gives control
of the CPU to the process selected by the short-
term scheduler.
This function involves:
1. Switching context
2. Switching to user mode
3. Jumping to the proper location in the user
program to restart that program.

SCHEDULING CRITERIA
1. CPU utilization: The CPU should be kept as busy
as possible. CPU utilization may range from 0 to
100 percent.
2. Throughput: It is the number of processes
completed per time unit.
3. Turnaround time: The interval from the time of
submission of a process to the time of completion
is the turnaround time.
Turnaround time is the sum of the periods spent
waiting to get into memory, waiting in the ready
queue, executing on the CPU, and doing I/O.

4. Waiting time: Waiting time is the sum of
the periods spent waiting in the ready
queue.
5. Response time: It is the amount of time it
takes to start responding, but not the time
that it takes to output that response.

Multilevel Queue Scheduling
 It partitions the ready queue into several
separate queues .
 The processes are permanently assigned to
one queue

Multilevel Feedback Queue Scheduling
 It allows a process to move between queues.
 If a process uses too much CPU time, it will
be moved to a lower-priority queue.

MULTIPLE-PROCESSOR SCHEDULING
 CPU scheduling more complex when multiple CPUs
are available
 Homogeneous processors within a multiprocessor
 Asymmetric multiprocessing – only one processor
accesses the system data structures.
 Symmetric multiprocessing (SMP) – each processor
is self-scheduling, all processes in common ready
queue, or each has its own private queue of ready
processes

 Load Balancing
One or more processors may sit idle
while other processors have high workloads,
along with lists of processes awaiting the
CPU.
Load balancing attempts to keep the
workload evenly distributed across all
processors in an SMP system.

REAL-TIME CPU SCHEDULING
 Composed of the scheduler, clock and the
processing hardware elements.
 Soft real-time systems- provide no guarantee
as to when a critical real-time process will be
scheduled.
 They guarantee only that the process will be
given preference over noncritical processes.
 Hard real-time systems - have stricter
requirements. A task must be serviced by its
deadline,service after the deadline has expired.

CASE STUDY: LINUX SCHEDULING
 the Completely Fair Scheduler (CFS) became
the default Linux scheduling algorithm.
 Scheduling in the Linux system is based on
scheduling classes.
 Each class is assigned a specific priority.
 Equally divides the CPU time among all the
processes

CFS PERFORMANCE :
 The Linux CFS scheduler provides an
efficient algorithm for selecting which task
to run next.
 Each runnable task is placed in a red-
black tree—a balanced binary search tree
whose key is based on the value of
vruntime

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