1. Introduction
A thread is a single flow of control within a program. Thread is very much similar to a process. In fact, the thread is also called a lightweight process.
1.1 Thread v/s Process
Normally different processes occupy different memory space. When the CPU shifts from one process to another process, the state of the currently running process is saved and the state of another process is restored. No useful work is being done during state switch. This is referred to as context switch. The context switch time should be as less as possible to maximize the CPU utilization.
Threads are also like independent processes but they share the same memory and state. The separate threads also have separate state but the state is very small as compared to process state.
The state may contain just program counter and stack pointer. So switching from one thread to another thread takes very little time. Thus the context switch time for threads is very small as compared to the process. Hence CPU utilization is high in case of multiple threads as compared to the utilization in case of multiple processes. Threads are also referred to as light-weight process.
1.2 Multi-Threaded program
A multi-threaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution. Thus, multi-threading is a specialized form of multi-tasking.
For example, a program may have three threads:
All these threads might be running concurrently thus maximizing the CPU utilization.
1.3 Process-based Multi-tasking
Most of the operating systems allow you to run two or more programs at the same time. It is referred to as process-bases multi-tasking. For example, you can run a Java program and at the same time you may be editing a word document. The process-based multi-tasking ensures that there will be some program to be executed most of the time so it increases the CPU utilization.
In process-based multi-tasking, a program is the smallest unit of code that can be dispatched by the scheduler.
1.4 Thread-based multi-tasking
Thread is the smallest unit of execution in a thread-based multi-tasking environment. A process can be divided into a number of threads executing concurrently. This allows us to handle more than one task concurrently in a single program. For example, you can edit a word document and at the same time print a portion of it or another document.
Thread-based multi-tasking improves CPU utilization just like process-based multi-tasking. But at the same time it effectively speeds up the execution of a single program by executing its different parts concurrently in separate threads.
Thus process based multi-tasking deals with the “big picture”, and thread-based multi-tasking handles the details.
1.5 The Java Thread Model
The Java run-time system depends on threads for many things, and all the class libraries are designed with multi-threading in mind. In fact, Java uses threads to enable the entire environment to be asynchronous. This helps reduce inefficiency by preventing the waste of CPU cycles.
Single threaded systems use an approach called an event loop with polling. In this model, a single thread of control runs in an infinite loop, polling a single event queue to decide what to do next. In a single threaded environment, when a thread blocks (that is, suspends execution) because it is waiting for some resource, the entire program stops running.
The benefit of Java's multi-threading is that the main loop/polling mechanism is eliminated.
When a thread blocks in Java program, only the single thread that is blocked pauses, all other threads continue to run.
A thread is a single flow of control within a program. Thread is very much similar to a process. In fact, the thread is also called a lightweight process.
1.1 Thread v/s Process
Normally different processes occupy different memory space. When the CPU shifts from one process to another process, the state of the currently running process is saved and the state of another process is restored. No useful work is being done during state switch. This is referred to as context switch. The context switch time should be as less as possible to maximize the CPU utilization.
Threads are also like independent processes but they share the same memory and state. The separate threads also have separate state but the state is very small as compared to process state.
The state may contain just program counter and stack pointer. So switching from one thread to another thread takes very little time. Thus the context switch time for threads is very small as compared to the process. Hence CPU utilization is high in case of multiple threads as compared to the utilization in case of multiple processes. Threads are also referred to as light-weight process.
1.2 Multi-Threaded program
A multi-threaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution. Thus, multi-threading is a specialized form of multi-tasking.
For example, a program may have three threads:
- One handling the printing
- One handling the editing
- One downloading a file from the Internet.
All these threads might be running concurrently thus maximizing the CPU utilization.
1.3 Process-based Multi-tasking
Most of the operating systems allow you to run two or more programs at the same time. It is referred to as process-bases multi-tasking. For example, you can run a Java program and at the same time you may be editing a word document. The process-based multi-tasking ensures that there will be some program to be executed most of the time so it increases the CPU utilization.
In process-based multi-tasking, a program is the smallest unit of code that can be dispatched by the scheduler.
1.4 Thread-based multi-tasking
Thread is the smallest unit of execution in a thread-based multi-tasking environment. A process can be divided into a number of threads executing concurrently. This allows us to handle more than one task concurrently in a single program. For example, you can edit a word document and at the same time print a portion of it or another document.
Thread-based multi-tasking improves CPU utilization just like process-based multi-tasking. But at the same time it effectively speeds up the execution of a single program by executing its different parts concurrently in separate threads.
Thus process based multi-tasking deals with the “big picture”, and thread-based multi-tasking handles the details.
1.5 The Java Thread Model
The Java run-time system depends on threads for many things, and all the class libraries are designed with multi-threading in mind. In fact, Java uses threads to enable the entire environment to be asynchronous. This helps reduce inefficiency by preventing the waste of CPU cycles.
Single threaded systems use an approach called an event loop with polling. In this model, a single thread of control runs in an infinite loop, polling a single event queue to decide what to do next. In a single threaded environment, when a thread blocks (that is, suspends execution) because it is waiting for some resource, the entire program stops running.
The benefit of Java's multi-threading is that the main loop/polling mechanism is eliminated.
When a thread blocks in Java program, only the single thread that is blocked pauses, all other threads continue to run.
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