The Concept of Thread
Thread is a chronological flow of control within a process. A process can include one or more threads. Threads have their own register values and program counter, but they are split the memory space and other resources of the procedure. Each process always starts with a single thread. Through the implementation of other threads may be produced as and when necessary. Like processes, each thread has an implementation state (ready, running, terminated or blocked). A thread has entrance to the memory address space and resources of its process. Threads have related life cycles as the processes do. A single processor system can maintain concurrency by switching implementation among two or more threads. A multi-processor system can hold parallel concurrency by executing a separate thread on each processor. There are three basic process in concurrent programming languages for terminating and creating threads:
Unsynchronised creation and termination: In this method threads are terminated and created using library functions such as, START_PROCESS, CREATE_THREAD, CREATE_PROCESS and START_THREAD. As a effect of these function calls a new process or thread is produced and starts consecutively independent of its parents.
Unsynchronised creation and synchronized termination: This method uses two instructions: JOIN and FORK. The FORK instruction creates a new thread or process. When the parent needs the child's(thread or process) result, it calls JOIN instruction. At this junction two threads (processes) are synchronised.
Synchronised creation and termination: The most commonly system construct to implement synchronization is
COBEGIN...COEND. The threads among the COBEGIN...COEND construct are executed in similar. The termination of parent-child is suspended until all child threads are ended.
We can think of a thread as basically a inconsequential process. However, threads present some advantages over processes. The advantages of this are:
i) It takes very less time to terminate and create a new thread than to Terminate and Create a process. The reason being that a newly terminate thread uses the current process address space.
ii) It obtains less time to switch between two threads within the similar process, partly because the recently created thread uses the current process address space.
iii) Less communication overheads -- communicating between the threads of one process is easy because the threads share among former entities the address space. So, data produced by one thread is directly available to all the other threads.