Sunday, December 11, 2011

Concepts related to threads & processes.



Multithreading as a widespread programming and execution model allows multiple threads to exist within the context of a single process. These threads share the process's resources but are able to execute independently. The threaded programming model provides developers with a useful abstraction of concurrent execution. However, perhaps the most interesting application of the technology is when it is applied to a single process to enable parallel execution on a multiprocessor system.

This advantage of a multithreaded program allows it to operate faster on computer systems that have multiple CPUs, CPUs with multiple cores, or across a cluster of machines — because the threads of the program naturally lend themselves to truly concurrent execution. In such a case, the programmer needs to be careful to avoid race conditions, and other non-intuitive behaviors. In order for data to be correctly manipulated, threads will often need to rendezvous in time in order to process the data in the correct order. Threads may also require mutually-exclusive operations (often implemented using semaphores) in order to prevent common data from being simultaneously modified, or read while in the process of being modified. Careless use of such primitives can lead to deadlocks.

Another use of multithreading, applicable even for single-CPU systems, is the ability for an application to remain responsive to input. In a single-threaded program, if the main execution thread blocks on a long-running task, the entire application can appear to freeze. By moving such long-running tasks to a worker thread that runs concurrently with the main execution thread, it is possible for the application to remain responsive to user input while executing tasks in the background. On the other hand, in most cases multithreading is not the only way to keep a program responsive, with non-blocking I/O and/or Unix signals being available for gaining similar results

Race conditions arise in software when separate processes or threads of execution depend on some shared state. Operations upon shared states are critical sections that must be mutually exclusive in order to avoid harmful collision between processes or threads that share those states.

Address space: Address space is seperate for process but threads of a process share the same addresss space.
Data sharing(bteween threads & processes): Self Read.
Communication: Communication between processes are through signals.

Memory protection is an issue when you are in the same address space. How do you encounter it? Through memory protection register.
Memory protection is a way to control memory access rights on a computer, and is a part of most modern operating systems. The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting other processes, or the operating system itself. Memory protection for computer security includes additional techniques such as address space layout randomization and executable space protection.

Thread Safe/Thread Safety:

In computer programming, thread-safe describes a program portion or routine that can be called from multiple programming threads without unwanted interaction between the threads. (A thread is an instance of the program running on behalf of some user or process.) Thread safety is of particular importance to Java programmers, since Java is a programming language that provides built-in support for threads. By using thread-safe routines, the risk that one thread will interfere and modify data elements of another thread is eliminated by circumventing potential data race situations with coordinated access to shared data.

It is possible to ensure that a routine is thread-safe by:

Making sure that concurrent threads use synchronized algorithms that cooperate with each other.
Confining the address of a shared object to one thread whenever an unsynchronized algorithm is active..

Thread safety is a computer programming concept applicable in the context of multi-threaded programs. A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time. There are various strategies for making thread-safe data structures. Thread safety is a property that allows code to run in multi-threaded environments by re-establishing some of the correspondences between the actual flow of control and the text of the program, by means of synchronization.

Synchronization and thread safety in Java:

As soon as we start using concurrent threads, we need to think about various issues that fall under the broad description of thread safety. Generally, we need to take steps to make sure that different threads don't interact in negative ways:

if one thread is operating on some data or structure, we don't want another thread to simultaneously operate on that same data/structure and corrupt the results;
when Thread A writes to a variable that Thread B accesses, we need to make sure that Thread B will actually see the value written by Thread A;
we don't want one thread to hog, take or lock for too long a resource that other threads need in order to make progress.


In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, so as to reach an agreement or commit to a certain sequence of action. Data synchronization refers to the idea of keeping multiple copies of a dataset in coherence with one another, or to maintain data integrity. Process synchronization primitives are commonly used to implement data synchronization.

Thread or process synchronization:

Thread synchronization or serialization, strictly defined, is the application of particular mechanisms to ensure that two concurrently-executing threads or processes do not execute specific portions of a program at the same time. If one thread has begun to execute a serialized portion of the program, any other thread trying to execute this portion must wait until the first thread finishes. Synchronization is used to control access to state both in small-scale multiprocessing systems -- in multithreaded environments and multiprocessor computers -- and in distributed computers consisting of thousands of units -- in banking and database systems, in web servers, and so on.

InterThread Communication in Java

1. Reading 1
2. Reading 2
3. Reading 3

InterProcess Communication:

In computing, Inter-process communication (IPC) is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared memory, and remote procedure calls (RPC). The method of IPC used may vary based on the bandwidth and latency of communication between the threads, and the type of data being communicated.
There are several reasons for providing an environment that allows process cooperation:
Information sharing
Privilege separation
IPC may also be referred to as inter-thread communication and inter-application communication.
The combination of IPC with the address space concept is the foundation for address space independence/isolation.

Main IPC methods

Method: Provided by (operating systems or other environments)

File: Most operating systems
Signal: Most operating systems; some systems, such as Windows, implement signals in only the C run-time library and provide no support for their use as an IPC method.
Socket: Most operating systems
Message queue: Most operating systems
Pipe: All POSIX systems, Windows
Named pipe: All POSIX systems, Windows
Semaphore: All POSIX systems, Windows
Shared memory: All POSIX systems, Windows
Message passing:
(shared nothing) Used in MPI paradigm, Java RMI, CORBA, MSMQ, MailSlots, others
Memory-mapped file: All POSIX systems, Windows; this method may carry race condition risk if a temporary file is used.

No comments:

Post a Comment