3 process management

Processes

1. Process Concept
2. Process Scheduling
3. Operations on Processes
4. Interprocess Communication

1. Process Concept
• An operating system executes a variety of
programs:
– Batch system – jobs
– Time-shared systems – user programs or tasks
• The terms job and process are used almost
interchangeably
• 1.1 The Process
– a program in execution; process execution must
progress in sequential fashion
– A process includes:
• program counter - current activity
• Stack - temporary data(function, parameters, return
addresses, and local variables)
• data section - global variables
• heap - dynamically allocated memory during process run
time
Loganathan R, CSE, HKBKCE 2

1. Process Concept Contd…
Process in Memory
• A program by itself is not a
process
• A program is a passive
entity, a file containing a list
of instructions stored on
disk (often called an
executable file)
• A process is an active entity,
with a program counter
specifying the next
instruction to execute and a
set of associated resources

1.2 Process State
• As a process executes, it changes state, state of a process is defined
in part by the current activity of that process
– 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


1.3 Process Control Block (PCB) also called task control block contains
information associated with each process
• Process state - new, ready, running, waiting, halted
• Program counter - the address of the next instruction to be executed
• CPU registers - accumulators, index registers, stack pointers, and general-
purpose registers
• CPU scheduling information - process priority, pointers to scheduling queues
and scheduling parameters
• Memory-management information - value of the base and limit registers,
the page tables, or the segment tables
• Accounting information - the amount of CPU and real time used, time limits,
account numbers, job or process numbers
• I/O status information - list of I/O devices allocated to the process, a list of
open files
1.4 Threads
• A process is a program that performs a single thread of execution
• Single thread of control allows the process to perform only one task at one
time
• Multiple threads of execution perform more than one task at a time

Process Control CPU Switch From Process to Process
Block (PCB)


2. Process Scheduling
• Multiprogramming and Time Sharing switch the
CPU among processes so frequently that users can
interact with each program
• Process Scheduler selects an available process
(possibly from a set of several available processes)
for program execution on the CPU
2.1 Scheduling Queues
• 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 its own device queue
• Processes migrate among the various queues

2. Process Scheduling Contd…
Ready Queue And Various I/O Device Queues


• Representation of Process Scheduling is Queuing Diagram
• Rectangle represents a queue, Circle represent the resource and arrow indicate the flow
of processes
• New process is initially put in the ready queue and waits there until it is selected for
execution, or is dispatched
• Once the process is allocated the CPU and is executing, the events may occur are:
The process could issue an I/O request and then be placed in an I/O queue
The process could create a new sub process and wait for the sub process's Termination
The process could be removed forcibly from the CPU, as a result of an interrupt, and be
put back in the ready queue


2.2 Schedulers
• The selection processes from those queue is carried out by the
appropriate scheduler
• Long-term scheduler (or job scheduler) – selects which processes should
be brought into the ready queue
 Executes much less frequently, minutes may separate the creation of one
new process and the next
 controls the degree of multiprogramming (the number of processes in
memory)
 take more time to decide which process should be selected for execution
 select a good process mix of I/O-bound(spends more of its time doing I/O)
and CPU-bound (spends more of its time doing computations) processes)
 Time-sharing systems (UNIX &Windows) have no long-term scheduler but
simply put every new process in memory for the short-term scheduler
• Short-term scheduler (or CPU scheduler) – selects which process should
be executed next and allocates CPU
 select a new process for the CPU frequently i.e. executes at least once every
100 milliseconds
 Must be fast. i.e. If it takes 10 milliseconds to decide to execute a process for
100 milliseconds, then 10/(100 + 10) = 9 percent of the CPU is being wasted
simply for scheduling the work Loganathan R, CSE, HKBKCE 10

Medium Term Scheduler
• Intermediate level of scheduling, removes processes from memory (and
from active contention for the CPU) to reduce the degree of
multiprogramming
• The process can be reintroduced later into memory, and its execution
can be continued where it left off(Swapping)


2.3 Context Switch
• When CPU switches to another process, the
system must save the state(PCB) of the old
process and load the saved state for the new
process is known as context switch
• Context-switch time is overhead; the system
does no useful work while switching
• Time dependent on hardware support
– Sun UltraSPARC provide multiple sets of registers,
which requires changing the pointer to the current
register set


3. Operations on Processes
3.1 Process Creation
• Parent process create children processes via create-process
systemcall, which, in turn create other processes, forming a tree of
processes
• Processes are identified according to a unique process identifier(or
pid), which is typically an integer number

A tree of process on a
Solaris system


3. Operations on Processes Contd…
• When a Process creates new process,
• Resource sharing
• Parent and children share all resources
• Children share subset of parent’s resources
• Parent and child share no resources
• Execution possibilities
• Parent and children execute concurrently
• Parent waits until children terminate
• Address space possibilities
• Child duplicate of parent
• Child has a program loaded into it

Process Creation UNIX examples
• fork system call creates new process
• exec system call used after a fork to replace the process’s memory
space with a new program int main()
{
pid_t pid;
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 wait for the child to complete */
wait (NULL);
printf ("Child Complete");
Process Creation exit(0);
}
}
C Program Forking Separate Process

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


4. Interprocess Communication
• Processes executing concurrently in the operating system may be
either independent processes or cooperating processes
• 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 – break into subtasks, each of which will be
executing in parallel
• Modularity - dividing the system functions into separate processes or
threads
• Convenience - individual user may work on many tasks at the same
time
• Cooperating processes require an interprocess communication
(IPC) mechanism to exchange data and information

4. Interprocess Communication Contd…
• Two fundamental models of Interprocess communication
• Shared memory
• a region of memory that is shared by cooperating processes is established then
exchange information takes place by reading and writing data to the shared region
• Message passing
• communication takes place by means of messages exchanged between the
cooperating processes
Message passing

Shared memory

4.1 Shared memory Systems
• A shared-memory region resides in the address space of the process
creating the shared-memory
• Other processes that wish to communicate using this shared-memory
segment must attach it to their address space
• The form of the data and the location are determined by these
processes and are not under the operating system's control
• Example : Producer-Consumer Problem - Paradigm for cooperating
processes, producer process produces information that is consumed by
a consumer process
• Example : compiler may produce assembly code, which is consumed by an
assembler, in turn, may produce object modules, which are consumed by the
loader
• 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 (customer
may wait)
• bounded-buffer assumes that there is a fixed buffer size(both waits)

Bounded-Buffer – Shared-Memory Solution
• Shared buffer is implemented as a circular array
with two logical pointers: in and out
• The buffer is empty when in == out; the buffer is
full when ((in + 1) % BUFFER_SIZE) == out.
#define BUFFER_SIZE 10
typedef struct {
...
} item;

item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;


Code for the producer and consumer processes
• Producer process
item nextProduced;
while (true) {
/* produce an item in nextProduced */
while ( ( ( in + 1) % BUFFER-SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
}
• Consumer process
item nextConsumed;
while (true) {
while (in == out)
; //do nothing
nextConsumed = buffer[out];
out = (out + 1) % BUFFEFLSIZE;
/* consume the item in nextConsumed */
}

4.2 Message-Passing Systems
• 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) – message size fixed or variable
– receive(message)
• If ProcessP and Q wish to communicate, they need to:
– establish a communication link(Not Phsical) between them
– exchange messages via send/receive
• Implementation of communication link
– physical (e.g., shared memory, hardware bus or network)
– logical (e.g., logical properties)

• Methods for logically implementing a link and send() / receive () operations
• Direct or indirect communication
• Synchronous or asynchronous communication
• Automatic or explicit buffering
4.2.1 Naming - Processes must name each other explicitly in Direct
Communication
• 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
• In Symmetry addressing - both the sender process and the receiver process
must name the other i.e. send(P, message) and receive (Q, message)
• In asymmetry addressing - only the sender names the recipient
• send(P, message)—Send a message to process P
• receive(id, message)—-Receive a message from any process; id will be the
name of the sender process (disadvantage – Changing id of a process
necessitates all references to the old id) Loganathan R, CSE, HKBKCE
23

• Messages are sent to and received from mailboxes or ports
• Each mailbox has a unique id
• Processes can communicate only if they share a mailbox
• Primitives : send(A, message and receive(A, message) where A is a mailbox
• Properties of communication link
• Link established only if processes share a common mailbox
• A link may be associated with more than two processes
• Each pair of processes may share several communication links
• Link may be unidirectional or bi-directional
• Mailbox sharing
• P1, P2, and P3 share mailbox A and P1, sends to A; Who gets the message?
• 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.
The system may define an algorithm(RR) for selection
• A mailbox may be owned either by a process or the OS
• If the mailbox is owned by a process, it is distinguished as owner (only receive) & user (only send)
• A mailbox owned by OS has an existence of its own and allows a process to
– create a mailbox , send and receive messages through mailbox and destroy a mailbox
• The process that creates a new mailbox is the owner and it can only receive messages,
ownership and receiving privilege may be passed to other processes through appropriate
system calls

4.2.2 Synchronization
• Different design options for implementing send() and
receive () primitives
• Message passing may be either blocking or non-blocking
also known as synchronous or asynchronous.
• Blocking is considered synchronous
– Blocking send - sender block until the message is received
– Blocking receive - receiver block until a message is available
– rendezvous between the sender and the receiver
• Non-blocking is considered asynchronous
– Non-blocking send - sender send the message and continue
– Non-blocking receive - receiver receive a valid message or
null


4.2.3 Buffering
• Messages exchanged by communicating processes
reside in a temporary queue
• Queue of messages attached to the link is implemented
in one of three ways
– Zero capacity – The queue length is zero, no messages can
wait. Sender must wait for receiver (rendezvous)
– Bounded capacity – finite length of n messages. Sender must
wait if link full
– Unbounded capacity – infinite length Sender never waits


3 process management

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