1. Chapter 4: Processes Process Concept Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems
Operating System Concepts

2. 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 – represents the current activity.
stack – contains the temporary data.
data section – contains global variables.
Operating System Concepts

3. 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

4. Diagram of Process State
Operating System Concepts

5. Process Control Block (PCB)
Information associated with each process.
Process state – new, ready, running, waiting, halted
Program counter – the address of the next instructions
CPU registers – accumulators, index registers, stack pointers, general-purpose registers
CPU scheduling information – process priority, pointers to scheduling queues
Memory-management information – base and limit registers, the page tables, the segment tables
Accounting information – the amount of CPU, real time used, time limits, account numbers, processes number
I/O status information – I/O devices allocated to the process
Operating System Concepts

6. Process Control Block (PCB)
Operating System Concepts

7. CPU Switch From Process to Process
Operating System Concepts

8. Process Scheduling Queues
A process could have multiple threads of execution. They allow the process to perform more than one task at a time.
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

9. Ready Queue And Various I/O Device Queues
Operating System Concepts

10. Executing Processes
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 the 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.
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

11. Representation of Process Scheduling
Operating System Concepts

12. 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.
Medium-term scheduler – removes processes from memory and introduces processes into memory. This scheme is called swapping.
Operating System Concepts

13. Addition of Medium Term Scheduling
Operating System Concepts

14. 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

15. 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.
Context switching has become such a performance bottleneck that programmers are using new structures (threads) to avoid it whenever possible.
Operating System Concepts

16. 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

17. Process Creation (Cont.)
Address space
Child duplicate of parent.
Child has a program loaded into it.
UNIX examples (Figure 4.8)
fork system call creates new process.
exec system call used after a fork to replace the process’ memory space with a new program.
wait system call used to move the parent process off the ready queue until the termination of the child.
Try the ps command.
Operating System Concepts

18. Processes Tree on a UNIX System
Operating System Concepts

19. Forking Processes /* fork processes */ main() { int i, pid;
for (i=1; i<=3; i++) {
if ((pid = fork()) == 0) {
printf("In child %d. \n", i); }
Operating System Concepts

20. Forking Processes P I = 2 I = 1 I = 3 I = 2 I = 3 I = 3 I = 3
Operating System Concepts

21. 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).
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.
If the parent terminates, the init process would be assigned as the new parent of all its children.
Operating System Concepts

22. 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
Modularity
Convenience
Operating System Concepts

23. Producer-Consumer Problem
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.
Operating System Concepts

24. Bounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
Typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
Solution is correct, but can only use BUFFER_SIZE-1 elements
Operating System Concepts

25. Bounded-Buffer – Producer Process
item nextProduced;
while (1) {
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE; }
Operating System Concepts

26. Bounded-Buffer – Consumer Process
item nextConsumed;
while (1) {
while (in == out)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE; }
Operating System Concepts

27. Interprocess Communication (IPC)
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 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., logical properties)
Operating System Concepts

28. Implementation Questions
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?
Operating System Concepts

29. Methods of Implementing a Link
Here are several methods for logically implementing a link and the send/receive operations:
Direct or indirect communication
Symmetric or asymmetric communication
Automatic or explicit buffering
Send by copy or send by reference
Fixed-sized or variable-sized messages
Operating System Concepts

30. Direct Communication
Processes must name each other explicitly (symmetry addressing):
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.
A variant of this scheme employs asymmetry in addressing:
receive(id, message) – receive a message from process id
Operating System Concepts

31. 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.
A mailbox can be viewed abstractly as an object into which messages can be placed by processes and from which messages can be removed.
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

32. Indirect Communication
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 done by a process in case of a mailbox owned by the operating system:
create a new mailbox
send and receive messages through mailbox
destroy a mailbox
Operating System Concepts

33. Indirect Communication
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

34. Synchronization
Message passing may be either blocking or non-blocking.
Blocking: The sending process is blocked until the message is received. The receiver blocks until a message is available.
Non-blocking: The sending process sends the message and resumes operation. The receiver retrieves either a valid message or a null.
When both the send and receive are blocking, we have a rendezvous between the sender and the receiver.
Blocking is considered synchronous
Non-blocking is considered asynchronous
send and receive primitives may be either blocking or non-blocking.
Operating System Concepts

35. Buffering
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.
Operating System Concepts

36. Examples: Mach and Windows 2000
The Mach operating system, developed at Carnegie Mellon University is an example of a message-based operating system.
Most communication in Mach is carried out by messages.
Refer to or for more details.
Windows 2000 provides support for multiple operating environments or subsystems, with which application programs communicate via a message-passing mechanism. Refer to or Chapter 21 for more details.
The application programs can be considered to be clients of the Windows 2000 subsystem server.
The message-passing facility in Windows 2000 is called the local procedure-call (LPC) facility.
Operating System Concepts

37. Client-Server Communication
Sockets
Remote Procedure Calls
Remote Method Invocation (Java)
Operating System Concepts

38. Sockets A socket is defined as an endpoint for communication.
Concatenation of IP address and port
The socket :1625 refers to port 1625 on host
Communication consists between a pair of sockets.
The time-of-day server/client shown in Figure 4.10/4.11 is a Java example using socket programming.
Java provides three different types of sockets:
Connection-oriented (TCP) sockets are implemented with the Socket class.
Connectionless (UDP) sockets use the DatagramSocket class.
Multicast sockets, which is a subclass of the DatagramSocket, use the MulticastSocket class.
Operating System Concepts

39. Socket Communication
Operating System Concepts

40. Sockets and Ports message agreed port any port socket
Internet address =
Internet address =
other ports
client
server
Operating System Concepts

41. Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.
Stubs – client-side proxy for the actual procedure on the server.
The client-side stub locates the server and marshalls the parameters.
The server-side stub receives this message, unpacks the marshalled parameters, and peforms the procedure on the server.
Operating System Concepts

42. Execution of RPC
Operating System Concepts

43. 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

44. Remote Method Invocation
To make remote methods transparent to both the client and the server, RMI implements the remote object using stubs and skeletons.
A stub is a proxy residing with the client for the remote object.
The client-side stub is responsible for creating a parcel consisting of the name of the method to be invoked on the server and the marshalled parameters for the method.
The skeleton is responsible for unmarshalling the parameters and invoking the designated method on the server.
The skeleton then marshalls the return value into a parcel and return it to the client.
Operating System Concepts

45. Marshalling Parameters
Operating System Concepts