1. 3.5 Interprocess Communication
Many operating systems provide mechanisms for interprocess communication (IPC)
Processes must communicate with one another in multiprogrammed and networked environments
For example, a Web browser retrieving data from a distant server
Essential for processes that must coordinate activities to achieve a common goal

2. 3.5.1 Signals
Software interrupts that notify a process that an event has occurred
Do not allow processes to specify data to exchange with other processes
Processes may catch, ignore or mask a signal
Catching a signal involves specifying a routine that the OS calls when it delivers the signal
Ignoring a signal relies on the operating system’s default action to handle the signal
Masking a signal instructs the OS to not deliver signals of that type until the process clears the signal mask

3. 3.5.2 Message Passing Message-based interprocess communication
Messages can be passed in one direction at a time
One process is the sender and the other is the receiver
Message passing can be bidirectional
Each process can act as either a sender or a receiver
Messages can be blocking or nonblocking
Blocking requires the receiver to notify the sender when the message is received
Nonblocking enables the sender to continue with other processing
Popular implementation is a pipe
A region of memory protected by the OS that serves as a buffer, allowing two or more processes to exchange data

4. 3.6 Case Study: UNIX Processes
All processes are provided with a set of memory addresses, called a virtual address space
A process’s PCB is maintained by the kernel in a protected region of memory that user processes cannot access
A UNIX PCB stores:
The contents of the processor registers
PID
The program counter
The system stack
All processes are listed in the process table

5. 3.6 Case Study: UNIX Processes
UNIX processes continued
All processes interact with the OS via system calls
A process can spawn a child process by using the fork system call, which creates a copy of the parent process
Child receives a copy of the parent’s resources as well
Process priorities are integers between -20 and 19 (inclusive)
A lower numerical priority value indicates a higher scheduling priority
UNIX provides IPC mechanisms, such as pipes, to allow unrelated processes to transfer data

6. 3.6 Case Study: UNIX Processes
Figure 3.10 UNIX system calls.

7. Group Discussion 1
1/31/08, due in class
1. What does a process’s address space look like?
2. T/F There could be a process not assigned to the processor and not waiting for I/O.
3. From where does an OS load the execution context for the process to be dispatched?
4. If no interrupts are allowed in a system, what will the system do to handle I/O?
5. What state will be assigned to a process when it is timed out?
6. T/F Division by zero is an example of asynchronous interrupt generated by processor.
7. What is an interrupt handler? What is an interrupt vector?
8. Name two types of IPC.
9. T/F A blocking send is asynchronous communication.

8. Chapter 4 – Thread Concepts
Outline Introduction 4.2 Definition of Thread
4.3 Motivation for Threads
4.4 Thread States: Life Cycle of a Thread
4.5 Thread Operations
4.6 Threading Models
4.6.1 User-Level Threads
4.6.2 Kernel-Level Threads
4.6.3 Combining User- and Kernel-Level Threads
4.7 Thread Implementation Considerations
4.7.1 Thread Signal Delivery
4.7.2 Thread Termination
4.8 POSIX and Pthreads
4.9 Linux Threads
4.10 Windows XP Threads
4.11 Java Multithreading Case Study, Part 1: Introduction to Java Threads

9. Objectives After reading this chapter, you should understand:
the motivation for creating threads.
the similarities and differences between processes and threads.
the various levels of support for threads.
the life cycle of a thread.
thread signaling and cancellation.
the basics of POSIX, Linux, Windows XP and Java threads.

10. 4.1 Introduction
General-purpose languages such as Java, C#, Visual C++ .NET, Visual Basic .NET and Python have made concurrency primitives available to applications programmer
Multithreading
Programmer specifies applications contain threads of execution
Each thread designate a portion of a program that may execute concurrently with other threads

11. 4.2 Definition of Thread Thread
Lightweight process (LWP)
Threads of instructions or thread of control
Shares address space and other global information with its process
Registers, stack, signal masks and other thread-specific data are local to each thread
Threads may be managed by the operating system or by a user application
Examples: Win32 threads, C-threads, Pthreads

12. Figure 4.1 Thread Relationship to Processes.
4.2 Definition of Thread
Figure 4.1 Thread Relationship to Processes.

13. 4.3 Motivation for Threads
Threads have become prominent due to trends in
Software design
More naturally expresses inherently parallel tasks
Performance
Scales better to multiprocessor systems
Cooperation
Shared address space incurs less overhead than IPC

14. 4.3 Motivation for Threads
Each thread transitions among a series of discrete thread states
Threads and processes have many operations in common (e.g. create, exit, resume, and suspend)
Thread creation does not require operating system to initialize resources that are shared between parent processes and its threads
Reduces overhead of thread creation and termination compared to process creation and termination

15. 4.4 Thread States: Life Cycle of a Thread
Figure 4.2 Thread life cycle.

16. 4.6.1 User-level Threads
User-level threads perform threading operations in user space
Threads are created by runtime libraries that cannot execute privileged instructions or access kernel primitives directly
User-level thread implementation
Many-to-one thread mappings
Operating system maps all threads in a multithreaded process to single execution context
Advantages
User-level libraries can schedule its threads to optimize performance
Synchronization performed outside kernel, avoids context switches
More portable
Disadvantage
Kernel views a multithreaded process as a single thread of control
Can lead to suboptimal performance if a thread issues I/O
Cannot be scheduled on multiple processors at once

17. Figure 4.3 User-level threads.

18. 4.6.2 Kernel-level Threads
Kernel-level threads attempt to address the limitations of user-level threads by mapping each thread to its own execution context
Kernel-level threads provide a one-to-one thread mapping
Advantages: Increased scalability, interactivity, and throughput
Disadvantages: Overhead due to context switching and reduced portability due to OS-specific APIs
Kernel-level threads are not always the optimal solution for multithreaded applications

19. Figure 4.4 Kernel-level threads.

20. 4.6.3 Combining User- and Kernel-level Threads
The combination of user- and kernel-level thread implementation
Many-to-many thread mapping (m-to-n thread mapping)
Number of user and kernel threads need not be equal
Can reduce overhead compared to one-to-one thread mappings by implementing thread pooling
Worker threads
Persistent kernel threads that occupy the thread pool
Improves performance in environments where threads are frequently created and destroyed
Each new thread is executed by a worker thread
Scheduler activation
Technique that enables user-level library to schedule its threads
Occurs when the operating system calls a user-level threading library that determines if any of its threads need rescheduling

21. 4.6.3 Combining User- and Kernel-level Threads
Figure 4.5 Hybrid threading model.