What is a thread

Thread is the smallest unit that the operating system can perform operation scheduling. It is included in the process and is the actual operating unit in the process. A thread refers to a single sequential control flow in a process. Multiple threads can run concurrently in a process, and each thread performs different tasks in parallel.

#Threads are the basic unit of independent scheduling and dispatch. Threads can be kernel threads scheduled by the operating system kernel, such as Win32 threads; user threads scheduled by user processes themselves, such as POSIX Thread on the Linux platform; or mixed scheduling by the kernel and user processes, such as Windows 7 threads.

Multiple threads in the same process will share all system resources in the process, such as virtual address space, file descriptors, signal processing, etc. But multiple threads in the same process have their own call stacks, their own register context, and their own thread-local storage.

A process can have many threads, and each thread performs different tasks in parallel.

Features

In a multi-threaded OS, multiple threads are usually included in a process. Each thread is used as the basic unit of CPU utilization and is the cost Minimal overhead entity. Threads have the following properties.

1), Lightweight entities

The entities in the thread basically do not own system resources, but only have some indispensable resources that can ensure independent operation.

Thread entities include programs, data and TCB. Thread is a dynamic concept, and its dynamic characteristics are described by Thread Control Block (TCB).

2), the basic unit of independent scheduling and dispatch.

In a multi-threaded OS, a thread is the basic unit that can run independently, and therefore is also the basic unit of independent scheduling and dispatch. Because threads are very "light", thread switching is very fast and has little overhead (in the same process).

3), can be executed concurrently.

Multiple threads in a process can be executed concurrently, and even all threads in a process can be executed concurrently; similarly, threads in different processes can also be executed concurrently, making full use of and giving full play to The ability of the processor and peripheral devices to work in parallel.

4), shared process resources.

Each thread in the same process can share the resources owned by the process. This is first manifested in: all threads have the same address space (the address space of the process), which means that threads can Access every virtual address in this address space; in addition, you can also access open files, timers, semaphore mechanisms, etc. owned by the process. Since threads in the same process share memory and files, threads communicate with each other without calling the kernel.

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