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Chapter 4: Processes
 Process Concept
 Process Scheduling
 Operations on Processes
 Cooperating Processes
 Interprocess Communication
 Communication in Client-Server Systems
Operating System Concepts
4.1
Silberschatz, Galvin and Gagne 2002
Process Concept
 An operating system executes a variety of programs:
 Batch system – jobs
 Time-shared systems – user programs or tasks
 Textbook uses the terms job and process almost
interchangeably.
 Process – a program in execution; process execution must
progress in sequential fashion.
 A process includes:
 program counter : which contains the address of next
instruction to be executed.
 Stack: which contains temporary data (such as method
parameters, return addresses, and local variables)
 data section: which contains global variables.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
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 process.
 terminated: The process has finished
execution.
Operating System Concepts
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Diagram of Process State
Operating System Concepts
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Process Control Block (PCB)
Information associated with each process.
 Process state: The state may be new, ready, running, waiting, halted, and
SO on.
 Program counter: The counter indicates the address of the next instruction to
be executed for this process.
 CPU registers: The registers vary in number and type, depending on the
computer architecture. They include accumulators, stack pointers, and
general-purpose registers…
 CPU scheduling information: This information includes a process priority,
pointers to scheduling queues, and any other scheduling parameters.
 Memory-management information: This information may include such
information as the page tables, or the segment tables, depending on the
memory system used by the operating system
 Accounting information: This information includes the amount of CPU and
real time used, time limits, job or process numbers, and so on.
 I/O status information: The information includes the list of I/O devices
allocated to this process, a list of open files, and so on.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Process Control Block (PCB)
Operating System Concepts
4.6
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CPU Switch From Process to Process
Operating System Concepts
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Process 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.
 Process migration between the various queues.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Ready Queue And Various I/O Device Queues
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Representation of Process Scheduling
Figure 4.5
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Representation of Process
Scheduling(cont..)
 A common representation of process scheduling is a queueing
diagram, such as that in Figure 4.5. Each rectangular box
represents a queue. Two types of queues are present: the
ready queue and a set of device queues. The circles represent
the resources that serve the queues, and the arrows indicate
the flow of processes in the system.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Representation of Process
Scheduling(cont…)
 A new process is initially put in the ready queue. It waits in the
ready queue until it is selected for execution (or dispatched).
Once the process is assigned to the CPU and is executing, one
of several events could occur:
 The process could issue an I/O request, and then be placed in an
I/O queue.
 The process could create a new subprocess and wait for its
termination.
 The process could be removed forcibly from the CPU, as a result
of an interrupt, and be put back in the ready queue.
 In the first two cases, the process eventually switches from the
waiting state to the ready state, and is then put back in the
ready queue. A process continues this cycle until it terminates,
at which time it is removed from all queues and has its PCB
and resources deallocated.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Schedulers
 Long-term scheduler (or job scheduler) – selects which
processes should be brought into the ready queue.
 Short-term scheduler (or CPU scheduler) – selects which
process should be executed next and allocates CPU.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Addition of Medium Term Scheduling
Figure 4.6
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Medium-term scheduler
 Some operating systems, such as time-sharing systems, may
introduce an additional, intermediate level of scheduling.
 This medium-term scheduler, diagrammed in Figure 4.6,
removes processes from memory (and from active contention
for the CPU), and thus reduces the degree of
multiprogramming.
 At some later time, the process can be reintroduced into
memory and its execution can be continued where it left off.
 This scheme is called swapping. The process is swapped out,
and is later swapped in, by the medium-term scheduler.
Operating System Concepts
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Schedulers (Cont.)
 Short-term scheduler is invoked very frequently (milliseconds)
 (must be fast).
 Long-term scheduler is invoked very 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.
Operating System Concepts
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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.
Operating System Concepts
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Process Creation
 Parent process create 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.
 Parent and child share no resources.
 Execution
 Parent and children execute concurrently.
 Parent waits until children terminate.
Operating System Concepts
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Process Creation (Cont.)
 Address space
 Child address space is the same as it’s parent.
 Child has its own address space.
 UNIX examples
 fork system call creates new process
Operating System Concepts
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Processes Tree on a UNIX System
Operating System Concepts
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Process Termination
 Process executes last statement and asks the operating
system to decide it (exit).
 Output data from child to parent (via wait).
 Process’ resources are deallocated by operating system.
 Parent may terminate execution of children processes (abort)
because:
 Child has exceeded allocated resources.
 Task assigned to child is no longer required.
 Parent is exiting.
Operating system does not allow child to continue if its parent
terminates.
Cascading termination.
Operating System Concepts
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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: If we want a particular task to run faster,
we must break it into subtasks, each of which will be executing
in parallel with the others.
 Modularity: dividing the system functions into separate
processes or threads
 Convenience: Even an individual user may have many tasks on
which to work at one time. For instance, a user may be editing,
printing, and compiling in parallel.
Operating System Concepts
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Cooperating Processes (ProducerConsumer Problem)
 Paradigm(‫ ) نموذج‬for cooperating processes, 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. The
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.
 A producer can produce one item while the consumer is
consuming another item. The producer and consumer must be
synchronized, so that the consumer does not try to consume an
item that has not yet been produced. In this situation, the
consumer must wait until an item is produced.
Operating System Concepts
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Cooperating Processes (ProducerConsumer Problem) cont…
 The unbounded-buffer producer-consumer problem places no
practical limit on the size of the buffer. The consumer may have
to wait for new items, but the producer can always produce new
items.
 The bounded-buffer producer-consumer problem assumes a
fixed buffer size. In this case, the consumer must wait if the
buffer is empty, and the producer must wait if the buffer is full.
 The buffer may either be provided by the operating system
through the use of an interprocess-communication (IPC) facility,
or by explicitly coded by the application programmer with the
use of shared memory
Operating System Concepts
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Bounded-Buffer – Shared-Memory Solution
 Shared data
#define BUFFER_SIZE 10
Typedef struct {
...
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
 The shared buffer is implemented as a circular array with two
logical pointers: in and out. The variable in points to the next
free position in the buffer; out points to the first full position in
the buffer. The buffer is empty when in == out ; the buffer is full
when ((in + 1) % BUFFERSIZE) == out.
Operating System Concepts
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Bounded-Buffer – Producer Process
 The code for the producer and consumer processes follows.
The producer process has a local variable nextproduced in
which the new item to be produced is stored:
item nextProduced;
while (1) {
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
}
Operating System Concepts
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Bounded-Buffer – Consumer Process
 The consumer process has a local variable nextconsumed in
which the item to be consumed is stored:
item nextConsumed;
while (1) {
while (in == out)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
}
Operating System Concepts
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Interprocess Communication (IPC)
 Mechanism for processes to communicate and to synchronize
their actions.
 IPC is particularly useful in a distributed environment where the
communicating processes may reside on different computers
connected with a network. An example is a chat program used
on the World Wide Web.
 IPC is best provided by a message-passing system
 message-passing system– processes communicate with each
other without resorting(‫ )اللجوء‬to shared variables.
Operating System Concepts
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 IPC facility provides two operations:
 send(message) – message size fixed or variable
 receive(message)
 If P and Q wish to communicate, they need to:
 establish a communication link between them
 exchange messages via send/receive
 Implementation of communication link
 physical (e.g., shared memory, hardware bus)
 logical (e.g., Direct or indirect communication, Symmetric or
asymmetric communication)
Operating System Concepts
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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.
Operating System Concepts
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Indirect Communication
 Messages are directed and received from mailboxes .
 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.
 A mailbox may be owned either by a process or by the operating
system. If the mailbox is owned by a process, 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).
Operating System Concepts
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Indirect Communication (cont…)
 Since each mailbox has a unique owner, there can be no confusion
about who should receive a message sent to this mailbox. When a
process that owns a mailbox terminates, the mailbox disappears. Any
process that subsequently sends a message to this mailbox must be
notified that the mailbox no longer exists.
 a mailbox owned by the operating system is independent and is not
attached to any particular process. 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 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
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002
Indirect Communication (cont…)
 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.
Operating System Concepts
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Synchronization
 Message passing may be either blocking or non-blocking.
 Blocking send: The sending process is blocked until the





message is received by the receiving process or by the
mailbox.
Nonblocking send: The sending process sends the message
and resumes operation.
Blocking receive: The receiver blocks until a message is
available.
Nonblocking receive: The receiver retrieves either a valid
message or a null.
Blocking is considered synchronous, Non-blocking is
considered asynchronous
send and receive primitives may be either blocking or nonblocking.
Operating System Concepts
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Buffering
 Whether the communication is direct or indirect, messages
exchanged by communicating processes reside in a temporary
queue of messages attached to the link; implemented in one of
three ways.
1. Zero capacity – 0 messages
Sender must wait for receiver (the link cannot have any messages
waiting in it,( rendezvous)).
2. Bounded capacity – finite length of n messages
Sender must wait if link full.
3. Unbounded capacity – infinite length
Sender never waits.
Operating System Concepts
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Client-Server Communication
 Sockets
 Remote Procedure Calls
 Remote Method Invocation (Java)
Operating System Concepts
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Sockets

A pair of processes communicating over a network employs a
pair of sockets-one for each process.
 A socket is defined as an endpoint for communication.
 A socket is made up of an Concatenation of IP address and
port number
 The socket 161.25.19.8:1625 refers to port 1625 on host
161.25.19.8
 Communication consists between a pair of sockets.
Operating System Concepts
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Socket Communication
 All connections must be unique. Therefore, if another process
also on host X wished to establish another connection with the
same web server, it would be assigned a port number not equal
to 1625
Operating System Concepts
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Remote Procedure Calls
 Remote procedure call (RPC) abstracts procedure calls between
processes on networked systems. The semantics of RPCs allow a
client to invoke a procedure on a remote host as it would invoke a
procedure locally. The RPC system hides the necessary details
allowing the communication to take place.
 The RPC system does this by providing a stub on the client side.
Typically, a separate stub exists for each separate remote procedure.
When the client invokes a remote procedure, the RPC system calls
the appropriate stub, passing it the parameters provided to the
remote procedure.
 This stub locates the port on the server and marshalls the
parameters. Parameter marshalling involves packaging the
parameters into a form which may be transmitted over a network. The
stub then transmits a message to the server using message passing.
 A similar stub on the server side receives this message and invokes
the procedure on the server. If necessary, return values are passed
Operating System Concepts
Silberschatz, Galvin and Gagne 2002
4.39
back to the client using the same
technique.
Execution of RPC
:server
Operating System Concepts
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Remote Method Invocation
 Remote Method Invocation (RMI) is a Java mechanism
similar to RPCs.
 RMI allows a Java program on one machine to invoke a
method on a remote object.
Operating System Concepts
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Marshalling Parameters
 The skeleton is responsible for unmarshalling the parameters and
invoking the desired method on the server.
Operating System Concepts
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Silberschatz, Galvin and Gagne 2002