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.

Additional Operating System Functions

Additional functions exist not for helping the user, but rather for ensuring efficient system operations.

•Resource allocation – allocating resources to multiple users or multiple jobs running at the same time.

•Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.

•Protection – ensuring that all access to system resources is controlled.

Operating System Services

• Program execution – system capability to load a program into memory and to run it.
• I/O operations –  since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.
• File-system manipulation – program capability to read, write, create, and delete files.
• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network.  Implemented via shared memory or message passing.
• Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.