OS Processes

Chapter 3: Processes

s Process Concept
s Process Scheduling
s Operations on Processes
s Cooperating Processes
s Interprocess Communication
s Communication in Client-Server Systems

Operating System Concepts 3.2 Silberschatz, Galvin and Gagne

Process Concept
s An operating system executes a variety of programs:
q Batch system – jobs
q Time-shared systems – user programs or tasks
s Textbook uses the terms job and process almost interchangeably
s Process – a program in execution; process execution must progress in
sequential fashion
s A process includes:
q program counter which represents current activity
q Stack which holds temporary data such as function parameters,
local variables and return addresses.
q data section which contains global variables
q Heap which contains memory that is dynamically allocated during
process runtime.
q Text includes the application's code, might be shared by a number of
processes?
q The process's address space.

Process in Memory


Process State

s As a process executes, it changes state

q new: The process is being created

q running: Instructions are being executed

q waiting: The process is waiting for some event to occur

q ready: The process is waiting to be assigned to a
processor

q terminated: The process has finished execution


Diagram of Process State


Process Control Block (PCB)

Information associated with each process
s Process state
s Program counter indicates the address of next instruction.
s CPU registers
s CPU scheduling information includes process priority and pointers
to scheduling queues.
s Memory-management information includes value of limit and base
registers
s Accounting information includes amount of CPU time used, time
limits etc.
s I/O status information includes the list of I/O devices allocated to
processes, list of open files


Process Control Block (PCB)


OS Data PCB Process
Structures Address
Space
RAM CPU

Kernel User
PSW

State text
IR
Memory .
Files
Accounting PC
Priority data
User SP
CPU register
storage heap General
Purpose
Registers
stack

9

CPU Switch From Process to
Process


Process Scheduling Queues
s Job queue – set of all processes in the system

s Ready queue

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

q It is stored as a linked list. It contains pointers to the first
and final PCB’s.

q Each PCB includes a pointer field that points to the next
PCB in the ready queue.

s Device queues – set of processes waiting for an I/O device

s Processes migrate among the various queues


Queues


Representation of Process
Scheduling


Schedulers

s Long-term scheduler (or job scheduler) – selects
which processes should be brought into the ready queue
s Short-term scheduler (or CPU scheduler) – selects
which process should be executed next and allocates
CPU
s Medium term Scheduler (Swaper)


Addition of Medium Term Scheduling


Schedulers (Cont.)

s Short-term scheduler is invoked very frequently (milliseconds) ⇒
(must be fast)
s Long-term scheduler is invoked very infrequently (seconds, minutes)
⇒ (may be slow)
s The long-term scheduler controls the degree of multiprogramming
s Processes can be described as either:
q I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts
q CPU-bound process – spends more time doing computations;
few very long CPU bursts
s If all processes are I/O bound,
q The ready queue will almost always be empty
s If all processes are CPU bound,
q The I/O waiting queue will almost always be empty, devices will
go unused
q System will again be unbalanced

Context Switch

s 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

s Context-switch time is overhead; the system does

no useful work while switching

s Time dependent on hardware support

s Part of OS responsible for switching the processor

among the processes is called Dispatcher


Two-state process model

Processor
Queue
Enter Dispatch Exit

Pause

Dispatcher is now redefined:
• Moves processes to the waiting queue
• Remove completed/aborted processes
• Select the next process to run

18

How process state changes1
Ready Ready Ready

 a := 1  a := 1  a := 1
b := a + 1 read a file b := a + 1
c := b + 1 b := a + 1 c := b + 1
read a file c := b + 1 a := b - c
a := b - c a := b - c c := c * b
c := c * b c := c * b b := 0
b := 0 b := 0 c := 0

19

Running Ready Ready

 a := 1  a := 1  a := 1
b a aa 1 1
:=:=:= 1
+ read a file b := a + 1
 b := a + 1
c :=bb:= a + 1
+1 b := a + 1 c := b + 1
c :=abfile1
+
read := b + 1 c := b + 1 a := b - c
 c a file
a read - ca file
:= b a := b - c c := c * b
a readb c
:= * -
b
c :=ac:= b - c Timeout
c := c * b b := 0
b c c0 c c b b
:=:=:= * * b := 0 c := 0
b := 0
b := 0

20

Ready Running Ready

a := 1  a := 1  a := 1
b := a + 1 read a1file
a := b := a + 1
c := b + 1  read + 1
b := a a file c := b + 1
 read a file c b := a + 1
:= b + 1 a := b - c
a := b - c a := b - +
c := b c 1 c := c * b
c := c * b I/O
c a := * b c
:= c b - b := 0
b := 0 b c := c * b
:= 0 c := 0
b := 0

21

Ready Blocked Running

a := 1 a := 1  a := 1
b := a + 1  read a file ba :=a:= 1
:=a 1 1
+
c := b + 1 b := a + 1  b := a + 1
c :=bb:= a + 1
+1
 read a file c := b + 1 c := b c 1
+
a := b - b + 1
 c := - c
a := b - c a := b - c c a ac:= b - c
:=:= * b
b
c := c * b Timeout
c := c * b b c c0 c c b b
:=:=:= * *
b := 0 b := 0 c b b0:= 0
:=:= 0
c := 0
c := 0

22

Running Blocked Ready

a := 1 a := 1 a := 1
b := a + 1  read a file b := a + 1
c := b + 1 b := a + 1 c := b + 1
 read a file c := b + 1  a := b - c
a := b - c a := b - c c := c * b
c := c * b I/O
c := c * b b := 0
b := 0 b := 0 c := 0

23

Blocked Blocked Running
The Next Process to Run
cannot be simply selected
a := 1 a := 1 a := 1
b := a + 1  read a file from the front1
b := a +
c := b + 1 b := a + 1 c := b + 1
 read a file c := b + 1  a := b - c
a := b - c a := b - c c := c * b
c := c * b c := c * b b := 0
b := 0 b := 0 c := 0

24

Blocked Blocked Running

a := 1 a := 1 a := 1
b := a + 1  read a file b a := 1 1
:= a +
c := b + 1 b := a + 1 b := a + 1
c := b + 1
 read a file c := b + 1  a c := b c 1
:= b - +
a := b - c a := b - c c a := * b c
:= c b -
c :=Suppose the Green process finishesbI/O0 c * b
c*b c := c * b  c :=
:=
b := 0 b := 0 b := 0
c := 0
c := 0

25

Blocked Ready Running

a := 1 a := 1 a := 1
b := a + 1 read a file b a aa:= 1
:=:= 1 1
+
c := b + 1  b := a + 1 b := a + 1
c :=bb:= a + 1
+1
 read a file c := b + 1 c := b c 1
- +
 a :=cb:= b + 1
a := b - c a := b - c c a ac:= b c c
:=:= * b -
b-
c := c * b Timeout
c := c * b  c := c * b
b :=c0 c * b
:=
b := 0 b := 0 c b b0:= 0
:=:= 0
 := 0
c
c := 0

26

Blocked Running Ready

a := 1 a := 1 a := 1
b := a + 1 read a1file
a := b := a + 1
c := b + 1 read+ 1file
a :=a
 b :=read 1 file
a a c := b + 1
 read a file b := a + 1
c :=bb + a1 a := b - c
a := b - c  c :=:= c + 1
b+1
a :=cb - b + c := c * b
c := c * b Timeout- c 1
:=
c a ac:= b - c
:=:= * b
b b := 0
 := c * b
b := 0 b c c0 c * b
:=  c := 0
b :=:=
0
b := 0

27

Process Creation
s Reasons to create a process
 Submit a new batch job/Start program
 User logs on to the system
 OS creates on behalf of a user (printing)
 Spawned by existing process
s Parent process create children processes, which, in turn create other
processes, forming a tree of processes
s Resource sharing
q Parent and children share all resources
q Children share subset of parent’s resources
q Parent and child share no resources
s Execution
q Parent and children execute concurrently
q Parent waits until children terminate


Process Creation (Cont.)

s Address space
q Child duplicate of parent
q Child has a program loaded into it
s UNIX examples
q fork system call creates new process
q exec system call used after a fork to replace the
process’ memory space with a new program


Process Creation


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);
}
}


The fork() system call

At the end of the system call there is a new process
waiting to run once the scheduler chooses it
s A new data structure is allocated
s The new process is called the child process.
s The existing process is called the parent process.
s The parent gets the child’s pid returned to it.
s The child gets 0 returned to it.
s Both parent and child execute at the same point
after fork() returns

32

Unix Process Control

The fork syscall
returns a zero to the
child and the child
process ID to the
int pid;
int status = 0; parent Parent uses wait to
if (pid = fork()) {
sleepFork creates an
until the
/* parent */ childexact copy of the
exits; wait
…… returns child pid
parent process
pid = wait(&status); and status.
} else {
/* child */ Wait variants
…… allow wait on a
exit(status); specific child, or
} Child process of
notification
passes statusother
stops and back
to parent on exit,
signals
to report
success/failure
33

Process Termination
s Batch job issues Halt instruction
s User logs off
s Process executes a service request to terminate
s Parent terminates so child processes terminate
s Operating system intervention
q such as when deadlock occurs
s Error and fault conditions
q E.g. memory unavailable, protection error, arithmetic error, I/O
failure, invalid instruction

34

Process Termination

s Process executes last statement and asks the operating system to
delete it (exit)
q Output data from child to parent (via wait)
q Process’ resources are deallocated by operating system
s Parent may terminate execution of children processes (abort)
q Child has exceeded allocated resources
q Task assigned to child is no longer required
q If parent is exiting
 Some operating system do not allow child to continue if its
parent terminates
– All children terminated - cascading termination


Inter process Communication

Processes executing concurrently in the operating
system may be either independent processes or
cooperating processes.

s Independent process cannot affect or be affected by

the execution of another process

s Cooperating process can affect or be affected by the

execution of another process


Advantages of Process Cooperation

s Information sharing: Allow concurrent access to
same piece of information that several users may be
interested in it.
s Computation speed-up: break a task to subtasks,
each of which will be executing in parallel with the
others. (Should be multiple processing elements such
as CPUs)
s Modularity: divided the system functions into
separate processes or threads.
s Convenience: for user, user may work on many
tasks at the same time.


MODELS OF IPC

Cooperating processes require an inter
process communication mechanism that
will allow them to exchange data and
information.
2. Shared Memory
3. Message Passing


SHARED MEMORY

1. Region of memory that is shared by
cooperating processes is
established.
2. Processes can then exchange
information by reading and writing
data to shared region


MESSAGE PASSING

Communication takes place by
means of messages exchanged
between the cooperating processes.


Communications Models


Advantages & Disadvantages of Message
Passing & Shared Memory

1. Message passing is useful for exchanging
smaller amounts of data
2. Message Passing is easier to implement than
shared memory for IPC
3. Shared Memory allows maximum speed and
convenience as it can be done at memory
speeds within a computer
4. Shared memory is faster than message
passing as message passing is implemented
using system calls.

Contd….

1. Message passing requires the more
time consuming task of kernel
intervention.
2. Shared memory system calls are
required only to establish shared
memory regions, no assistance from the
kernel is required.


Producer-Consumer Problem
s Paradigm for cooperating processes, producer process
produces information that is consumed by a consumer
process
q A compiler may produce assembly code, which is consumed
by an assembler. The assembler, in turn, may produce object
modules, which are consumed by the loader
s A buffer of items that can be filled by the producer and emptied
by the consumer. should be available
s A producer can produce one item while the consumer is
consuming another item.
s The producer and consumer must be synchronized
q the consumer does not try to consume an item that has not
yet been produced.
q the consumer must wait until an item is produced

Producer-Consumer Problem

s unbounded-buffer places no practical limit on the size of the

buffer

q the consumer may have to wait for new items, but

q the producer can always produce new items

s bounded-buffer assumes that there is a fixed buffer size

q the consumer must wait if the buffer is empty, and

q the producer must wait if the buffer is full.


Bounded-Buffer – Shared-Memory
Solution

s Shared data
#define BUFFER_SIZE 10
Typedef struct {
...
} item;

item buffer[BUFFER_SIZE];
int in = 0; % in: the next free position in the buffer
int out = 0; % out: the first full position in the buffer
s The buffer is empty when in == out ;
s The buffer is full when ((in + 1) % BUFFERSIZE) == out
s Solution is correct, but can only use BUFFER_SIZE-1 elements


Bounded-Buffer – Insert() Method

The producer process has a local variable nextproduced in which
the new item to be produced is stored:
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;
{

Bounded Buffer – Remove() Method

The consumer process has a local variable nextconsumed in which the
item to be consumed is stored:

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;
{

Message Passing
s Mechanism for processes to communicate and to synchronize their
actions
s Message system – processes communicate with each other without
resorting to shared variables
s IPC facility provides two operations:
q send(message) – message size fixed or variable
q receive(message)
s If P and Q wish to communicate, they need to:
q establish a communication link between them
q exchange messages via send/receive
s Implementation of communication link
q physical (e.g., shared memory, hardware bus)
q logical (e.g., logical properties) e.g. send(), receive()


Implementation Questions

s How are links established?
s Can a link be associated with more than two processes?
s How many links can there be between every pair of communicating
processes?
s What is the capacity of a link?
s Is the size of a message that the link can accommodate fixed or
variable?
s Is a link unidirectional or bi-directional?


Methods for logical implementation of link

1. Direct or Indirect Communication

2. Synchronous and Asynchronous

Communication

3. Automatic or explicit Buffering


Direct Communication

s Processes must name each other explicitly:
q send (P, message) – send a message to process P
q receive(Q, message) – receive a message from process Q
s Properties of communication link
q Links are established automatically because the processes
need to know only the identities to communicate.
q A link is associated with exactly one pair of communicating
processes
q Between each pair there exists exactly one link
q The link may be unidirectional, but is usually bi-directional


Indirect Communication

s Messages are directed and received from mailboxes (also
referred to as ports)
q Each mailbox has a unique id
q Processes can communicate only if they share a mailbox
s Properties of communication link
q Link established only if processes share a common mailbox
q A link may be associated with many processes
q Each pair of processes may share several communication
links
q Link may be unidirectional or bi-directional



s Operations
q create a new mailbox
q send and receive messages through mailbox
q destroy a mailbox
s Primitives are defined as:
send(A, message) – send a message to mailbox A
receive(A, message) – receive a message from mailbox A



s Mailbox sharing
q P1, P2, and P3 share mailbox A
q P1, sends; P2 and P3 receive
q Who gets the message?
s Solutions
q Allow a link to be associated with at most two processes
q Allow only one process at a time to execute a receive operation
q Allow the system to select arbitrarily the receiver. Sender is
notified who the receiver was.



s If the mailbox is owned by a process (that is, the mailbox is
part of the address space of the process),
q then we distinguish between the owner (who can only receive
messages through this mailbox) and the user (who can only
send messages to the mailbox).
q Since each mailbox has a unique owner, there can be
 no confusion about who should receive a message sent to
this mailbox.
q When a process that owns a mailbox terminates, the mailbox
disappears.
q Any process that subsequently sends a message to this
mailbox must be notified that the mailbox no longer exists.



s If a mailbox owned by the operating system is
independent and is not attached to any particular
process:
q The operating system then must provide a mechanism
that allows a process to do the following:
 Create a new mailbox.
 Send and receive messages through the mailbox.
 Delete a mailbox
q Note: the
ownership and receive privilege
may be passed to other processes
through appropriate system calls.


Synchronization

s Message passing may be either blocking or non-blocking
s Blocking is considered synchronous
q Blocking send has the sender block until the message is
received
q Blocking receive has the receiver block until a message is
available
s Non-blocking is considered asynchronous
q Non-blocking send has the sender send the message and
continue
q Non-blocking receive has the receiver receive a valid
message or null


Buffering

s 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


Client-Server Communication

s Sockets
s Remote Procedure Calls
s Remote Method Invocation (Java)


Sockets

s A socket is defined as an endpoint for communication
s Concatenation of IP address and port
s The socket 161.25.19.8:1625 refers to port 1625 on host
161.25.19.8
s Communication consists between a pair of sockets


Socket Communication


Remote Procedure Calls

s Remote procedure call (RPC) abstracts procedure calls between processes on
networked systems.
s Stubs – client-side proxy for the actual procedure on the server.
s The client-side stub locates the server and marshalls the parameters.
s The server-side stub receives this message, unpacks the marshalled
parameters, and performs the procedure on the server.
s A stub is a small program routine that substitutes for a longer program,
possibly to be loaded later or that is located remotely. For example, a program
that uses Remote Procedure Calls ( RPC ) is compiled with stubs that
substitute for the program that provides a requested procedure. The stub
accepts the request and then forwards it (through another program) to the
remote procedure. When that procedure has completed its service, it returns
the results or other status to the stub which passes it back to the program that
made the request.


Execution of RPC


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