Operating Systems - Process Scheduling Management

Process Management
Chapter 3
Mr. C. P. Divate

Chapter 3:
Processe
s
3.2
● Process Concept
● Process Scheduling
● Operations on Processes
● Inter process Communication(IPC)

Process
Concept
3.3
● Process – a program in execution;
● Multiple parts
● The program code, also called text section
● Current activity including program counter, processor
registers
● Stack containing temporary data
4 Function parameters, return addresses, local variables
● Data section containing global variables
● Heap containing memory dynamically allocated during run
time

Process Concept
(Cont.)
3.4
● 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

Basic Elements Of Process
• Processor
• Main memory
• I/O modules
• System bus

Process
State
3.8
● 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

Process Control Block
(PCB)
3.10
Information associated with each process
(also called task control block)
● Process state – running, waiting, etc
● Program counter – location of
instruction to next execute
● CPU registers – contents of all
process-centric registers
● CPU scheduling information- priorities,
scheduling queue pointers
● Memory-management information –
memory allocated to the process
● Accounting information – CPU used,
clock time elapsed since start, time
limits
● I/O status information – I/O devices
allocated to process, list of open ﬁles

Process Control Block
(PCB)
3.11

CPU Switch From Process to
Process
3.12

Thread
s
3.13
● So far, process has a single thread of execution
● Consider having multiple program counters per process
● Multiple locations can execute at once
4 Multiple threads of control -> threads
● Must then have storage for thread details, multiple program counters in
PCB

Process
Scheduling
3.14
● Maximize CPU use, quickly switch processes onto CPU for time sharing
● Process scheduler selects among available processes for next execution
on CPU
● Maintains scheduling queues of processes
● 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

Ready Queue And Various I/O Device Queues
3.15

Ready Queue And Various I/O Device Queues
3.16

Representation of Process Scheduling
● Queueing diagram represents queues, resources, ﬂows
3.17

Scheduler
s
3.19
● Short-term scheduler (or CPU scheduler) – selects which process should
be executed next and allocates CPU
● Sometimes the only scheduler in a system
● Short-term scheduler is invoked frequently (milliseconds) ⇒ (must be
fast)
● Long-term scheduler (or job scheduler) – selects which processes should
be brought into the ready queue
● Long-term scheduler is invoked infrequently (seconds, minutes) ⇒
• (may be slow)
● The long-term scheduler controls the degree of multiprogramming
● Processes can be described as either:
● 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
● Long-term scheduler strives for good process mix

Addition of Medium Term
Scheduling
● Medium-term scheduler can be added if degree of multiple
programming needs to decrease
● Remove process from memory, store on disk, bring back
in from disk to continue execution: swapping
3.21

Context
Switch
3.22
● 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 via a context switch
● Context of a process represented in the PCB
● Context-switch time is overhead; the system does no useful
work while switching
● The more complex the OS and the PCB the longer
the context switch
● Time dependent on hardware support
● Some hardware provides multiple sets of registers per CPU
□ multiple contexts loaded at once

Operations on
Processes
3.23
● System must provide mechanisms for:
● process creation,
● process termination
● process wait

Process
Creation
3.24
● Parent process create children processes, which, in turn
create other processes, forming a tree of processes
● Generally, process identiﬁed and managed via a process
identiﬁer (pid)
● Resource sharing options
● Parent and children share all resources
● Children share subset of parent’s resources
● Parent and child share no resources
● Execution options
● Parent and children execute concurrently
● Parent waits until children terminate

Process Creation
(Cont.)
● Address space
● Child duplicate of parent
● Child has a program loaded into it
● 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
3.25

Process
Termination
3.26
● Process executes last statement and then asks the operating
system to delete it using the exit() system call.
● Returns status data from child to parent (via wait())
● Process’ resources are deallocated by operating system
● Parent may terminate the execution of children processes using
the abort() system call. Some reasons for doing so:
● Child has exceeded allocated resources
● Task assigned to child is no longer required
● The parent is exiting and the operating systems does not
allow a child to continue if its parent terminates

Process
Termination
3.27
● Some operating systems do not allow child to exists if its parent
has terminated. If a process terminates, then all its children must
also be terminated.
● cascading termination. All children, grandchildren, etc. are
terminated.
● The termination is initiated by the operating system.
● The parent process may wait for termination of a child process by
using the wait()system call. The call returns status information
and the pid of the terminated process
pid = wait(&status);
● If no parent waiting (did not invoke wait()) process is a zombie
● If parent terminated without invoking wait , process is an orphan

Interprocess
Communication
3.28
● Processes within a system may be independent or cooperating
● Cooperating process can affect or be affected by other processes,
including sharing data
● Reasons for cooperating processes:
● Information sharing
● Computation speedup
● Modularity
● Convenience
● Cooperating processes need interprocess communication (IPC)
● Two models of IPC
● Shared memory
● Message passing

Communications
Models
(a) Message passing.
3.29
(b) shared memory.

Cooperating
Processes
3.30
● 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
● Information sharing
● Computation speed-up
● Modularity
● Convenience

Producer-Consumer
Problem
3.31
● Paradigm for cooperating processes, producer process
produces information that is consumed by a consumer
process
● unbounded-buffer places no practical limit on the size
of the buffer
● bounded-buffer assumes that there is a ﬁxed buffer
size

Interprocess Communication – Shared Memory
3.32
● An area of memory shared among the processes that wish
to communicate
● The communication is under the control of the users
processes not the operating system.
● Major issues is to provide mechanism that will allow the
user processes to synchronize their actions when they
access shared memory.

Interprocess Communication – Message Passing
3.33
● Mechanism for processes to communicate and to synchronize
their actions
● Message system – processes communicate with each other
without resorting to shared variables
● IPC facility provides two operations:
● send(message)
● receive(message)
● The message size is either ﬁxed or variable

Message Passing (Cont.)
3.34
● If processes P and Q wish to communicate, they need to:
● Establish a communication link between them
● Exchange messages via send/receive
● Implementation issues:
● How are links established?
● Can a link be associated with more than two processes?
● How many links can there be between every pair of
communicating processes?
● What is the capacity of a link?
● Is the size of a message that the link can accommodate ﬁxed or
variable?
● Is a link unidirectional or bi-directional?

Message Passing (Cont.)
3.35
● Implementation of communication link
● Physical:
4 Shared memory
4 Hardware bus
4 Network
● Logical:
4 Direct or indirect
4 Synchronous or asynchronous
4 Automatic or explicit buffering

Direct
Communication
3.36
● 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

Indirect
Communication
3.37
● 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

Indirect
Communication
3.38
● Operations
● create a new mailbox (port)
● 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

Indirect
Communication
3.39
● Mailbox sharing
● P1, P2, and P3 share mailbox A
● P1, sends; P2 and P3 receive
● Who gets the message?
● Solutions
● 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.

Synchronizatio
n
3.40
● Message passing may be either blocking or non-blocking
● Blocking is considered synchronous
● Blocking send -- the sender is blocked until the message is
received
● Blocking receive -- the receiver is blocked until a message
is available
● Non-blocking is considered asynchronous
● Non-blocking send -- the sender sends the message and
continue
● Non-blocking receive -- the receiver receives:
● A valid message, or
● Null message

Buffering
3.41
● Queue of messages attached to the link.
● implemented in one of three ways
1. Zero capacity – no messages are queued on a link.
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

Operating Systems - Process Scheduling Management

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