Java Thread States

Java Threads

• Java threads may be created by:

1. Extending Thread class
2. Implementing the Runnable interface

• Java threads are managed by the JVM.

Linux Threads

• Linux refers to them as tasks rather than threads.
• Thread creation is done through clone() system call.
• Clone() allows a child task to share the address space of the parent task (process)

Windows 2000 Threads

• Implements the one-to-one mapping.
• Each thread contains

1.    a thread id
2.    register set
3.    separate user and kernel stacks
4.    private data storage area

Solaris Process

Solaris 2 Threads

Pthreads

• a POSIX standard (IEEE 1003.1c) API for thread creation and synchronization.
• API specifies behavior of the thread library, implementation is up to development of the library.
• Common in UNIX operating systems.

Threading Issues

• Semantics of fork() and exec() system calls.
• Thread cancellation.
• Signal handling
• Thread pools
• Thread specific data

Many-To-Many

• Allows many user level threads to be mapped to many kernel threads.
• Allows the  operating system to create a sufficient number of kernel threads.
• Solaris 2
• Windows NT/2000 with the ThreadFiber package

One-To-One

• Each user-level thread maps to kernel thread.
• Examples

1.    Windows 95/98/NT/2000
2.    OS/2

Many-To-One

• Many user-level threads mapped to single kernel thread.
• Used on systems that do not support kernel threads.

Multithreading Models

• Many-to-One
• One-to-One
• Many-to-Many

Kernel Threads

• Supported by the Kernel
• Examples

1.  Windows 95/98/NT/2000
2.   Solaris
3.   Tru64 UNIX
4.   BeOS
5.   Linux

User Threads

• Thread management done by user-level threads library
• Examples

1.    POSIX Pthreads
2.    Mach C-threads
3.    Solaris threads

Benefits

• Responsiveness
• Resource Sharing
• Economy
• Utilization of MP Architectures

Single and Multithreaded Processes

Marshalling Parameters

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.

Execution of RPC

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.

Socket Communication

Sockets

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

Client-Server Communication

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

Buffering

• Queue of messages attached to the link; implemented in one of three ways.

1. Zero capacity – 0 messages
2. Sender must wait for receiver (rendezvous).
3. Bounded capacity – finite length of n messages
4. Sender must wait if link full.
5. Unbounded capacity – infinite length 
6. Sender never waits.

Synchronization

• Message passing may be either blocking or non-blocking.
• Blocking is considered synchronous
• Non-blocking is considered asynchronous
• send and receive primitives may be either blocking or non-blocking.

Indirect Communication

• Messages are directed and received from mailboxes (also referred to as ports).

1. Each mailbox has a unique id.
2. Processes can communicate only if they share a mailbox.

• Properties of communication link

1. Link established only if processes share a common mailbox
2. A link may be associated with many processes.
3. Each pair of processes may share several communication links.
4. Link may be unidirectional or bi-directional.

Direct Communication

• Processes must name each other explicitly:

1. send (P, message) – send a message to process P
2. receive(Q, message) – receive a message from process Q

• Properties of communication link

1. Links are established automatically.
2. A link is associated with exactly one pair of communicating processes.
3. Between each pair there exists exactly one link.
4. The link may be unidirectional, but is usually bi-directional.

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:

1. send(message) – message size fixed or variable
2. receive(message)

• If P and Q wish to communicate, they need to:

1. establish a communication link between them
2. exchange messages via send/receive

• Implementation of communication link

1. physical (e.g., shared memory, hardware bus)
2. logical (e.g., logical properties)

Bounded-Buffer – Consumer Process

  item nextConsumed;

  while (1) {

  while (in == out)

  ; /* do nothing */

  nextConsumed = buffer[out];

  out = (out + 1) % BUFFER_SIZE;

  }

Bounded-Buffer – Producer Process

  item nextProduced;

  while (1) {

  while (((in + 1) % BUFFER_SIZE) == out)

  ; /* do nothing */

  buffer[in] = nextProduced;

  in = (in + 1) % BUFFER_SIZE;

  }

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

Producer-Consumer Problem

• Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process.

1. unbounded-buffer places no practical limit on the size of the buffer.
2. bounded-buffer assumes that there is a fixed buffer size.

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

1. Information sharing
2. Computation speed-up
3. Modularity
4. Convenience

Process Termination

• Process executes last statement and asks the operating system to decide it (exit).

1. Output data from child to parent (via wait).
2. Process’ resources are deallocated by operating system.

• Parent may terminate execution of children processes (abort).

1. Child has exceeded allocated resources.
2. Task assigned to child is no longer required.
3. Parent is exiting.

* Operating system does not allow child to continue if its parent terminates.

* Cascading termination.

Processes Tree on a UNIX System

Process Creation

• Parent process create children processes, which, in turn create other processes, forming a tree of processes.
• Resource sharing

1. Parent and children share all resources.
2. Children share subset of parent’s resources.
3. Parent and child share no resources.

• Execution

1. Parent and children execute concurrently.
2. Parent waits until children terminate.

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.

Schedulers

• Short-term scheduler is invoked very frequently (milliseconds) Þ (must be fast).
• Long-term scheduler is invoked very infrequently (seconds, minutes) Þ (may be slow).
• The long-term scheduler controls the degree of multiprogramming.
• Processes can be described as either:

1. I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.
2. CPU-bound process – spends more time doing computations; few very long CPU bursts.

Addition of Medium Term Scheduling

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.

Representation of Process Scheduling

Ready Queue And Various I/O Device Queues

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.

CPU Switch From Process to Process

Process Control Block (PCB)

Information associated with each process.

• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

Process State

• As a process executes, it changes state

1. new:  The process is being created.
2. running:  Instructions are being executed.
3. waiting:  The process is waiting for some event to occur.
4. ready:  The process is waiting to be assigned to a process.
5. terminated:  The process has finished execution

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Process Concept

• An operating system executes a variety of programs:

1. Batch system – jobs
2. 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:

1. program counter
2. stack
3. data section

System Generation (SYSGEN)

• Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site.
• SYSGEN program obtains information concerning the specific configuration of the hardware system.
• Booting – starting a computer by loading the kernel.
• Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution.

System Implementations

• Traditionally written in assembly language, operating systems can now be written in higher-level languages.
• Code written in a high-level language:

1. can be written faster.
2. is more compact.
3. is easier to understand and debug.

• An operating system is far easier to port (move to some other hardware) if it is written in a high-level language.

Mechanisms and Policies

• Mechanisms determine how to do something, policies decide what will be done.
• The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later.

System Design Goals

• User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast.
• System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient.

Java Virtual Machine

• Compiled Java programs are platform-neutral bytecodes executed by a Java Virtual Machine (JVM).
• JVM consists of

1.    class loader
2.    class verifier
3.    runtime interpreter

• Just-In-Time (JIT) compilers increase performance

Advantages/Disadvantages of Virtual Machines

• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines.  This isolation, however, permits no direct sharing of resources.
• A virtual-machine system is a perfect vehicle for operating-systems research and development.  System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.

System Models

Virtual Machines

• A virtual machine takes the layered approach to its logical conclusion.  It treats hardware and the operating system kernel as though they were all hardware.
• A virtual machine provides an interface identical to the underlying bare hardware.
• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.
• The resources of the physical computer are shared to create the virtual machines.

1. CPU scheduling can create the appearance that users have their own processor.
2. Spooling and a file system can provide virtual card readers and virtual line printers.
3. A normal user time-sharing terminal serves as the virtual machine operator’s console.

Windows NT Client-Server Structure

Microkernel System Structure

• Moves as much from the kernel into “user” space.
• Communication takes place between user modules using message passing.
• Benefits:

1.  easier to extend a microkernel
2.   easier to port the operating system to new architectures
3.   more reliable (less code is running in kernel mode)
4.   more secure

An Operating System Layer

Layered Approach

• The operating system is divided into a number of layers (levels), each built on top of lower layers.  The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
• With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers.

UNIX System Structure

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring.  The UNIX OS consists of two separable parts.

1. Systems programs
2. The kernel

* Consists of everything below the system-call interface and above the physical hardware

* Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.

MS-DOS Layer Structure

MS-DOS System Structure

• MS-DOS – written to provide the most functionality in the least space

1. not divided into modules
2. Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

System Programs

• System programs provide a convenient environment for program development and execution.  The can be divided into:

1. File manipulation
2. Status information
3. File modification
4. Programming language support
5. Program loading and execution
6. Communications
7. Application programs

• Most users’ view of the operation system is defined by system programs, not the actual system calls. 

Communication Models

• Communication may take place using either message passing or shared memory.
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UNIX Running Multiple Programs

MS-DOS Execution

Types of System Calls

• Process control
• File management
• Device management
• Information maintenance
• Communications

Passing of Parameters as a Table

System Calls

• System calls provide the interface between a running program and the operating system.

1. Generally available as assembly-language instructions.
2. Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, C++)

• Three general methods are used to pass parameters between a running program and the operating system.

1. Pass parameters in registers.
2. Store the parameters in a table in memory, and the table address is passed as a parameter in a register.
3. Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.