Thread Learning Notes

Source: Internet
Author: User
Tags gcd

Multi-threaded Network learning

What is a process?

An application that is running in the system is a process.

For example, the simultaneous opening of the QQ Xcode system will start two processes respectively. And it is a relatively independent process that does not affect each other.

1 processes want to perform a task, they must have a thread, and each process has at least 1 threads.

A thread is the basic execution unit of a process, and all tasks of a process are executed in a thread.


The tasks in the 1 threads are serial (sequential execution).

If you perform multiple tasks in 1 threads, you can perform these tasks sequentially, one by one.

In other words, 1 threads can only perform one task at a time.

such as downloading tasks.


In 1 processes, multiple threads can be opened, and each thread can perform different tasks concurrently (concurrently).

For example: The computer can open the button chat, can listen to music at the same time, browse the Web and so on.

Multithreading principle:

At the same time, the CPU can only handle 1 threads, and only one thread is working (executing).

Threads execute concurrently, in fact, the CPU quickly switches between multiple threads. --that is, each thread executes a bit at a time until all processes are executed.

If the CPU schedules the thread to be fast enough, it creates the illusion of multi-threaded concurrency execution.

Think: What happens if there are a lot of threads?

CPU will switch directly on n threads, CPU will be exhausted and consume a lot of CPU resources

Advantages of multithreading

1. Improve program execution efficiency

2. Can properly improve resource utilization

Multithreading Disadvantages

1. Open threads need to occupy a certain amount of memory space (by default the main thread takes up 1m child threads to occupy 512MB) if you open a large number of threads, it consumes a lot of memory space and reduces the performance of the program.

2. The more threads, the higher the CPU overhead on the scheduling thread.

3. The programming is more complex: such as the communication between threads, multi-threaded data sharing.

What is the main thread?

When an iOS program runs, it opens 1 threads by default, becoming the main thread or UI thread. Actions related to the UI are placed on the main thread.

The main thread is mainly in the role:

1. Display/Refresh the UI interface.

2. Handling UI Events (click, scroll, drag)

Main thread Usage Note:

Do not talk more time-consuming operations into the main line approached (such as requesting a network this operation should not be placed in the main thread)

Time-consuming operation, will be stuck in the main thread, seriously affect the UI smoothness, to give users a "card" bad experience.

Prints the current location thread number = = 1 Main thread number! = 1 Child Threads threads Other threads.

NSLog (@"%@", [Nsthread CurrentThread]);

Putting time-consuming operations into sub-threads is a new thread space for time-consuming operations to run inside.  Longtime is the name of the method, which is the place where the child thread runs. Withobject is a parameter.

[ self Performselectorinbackground: @selector (longtime) withobject:nil];

Multi-threaded implementations in iOS:

1. Pthread: ———— programmer Management, almost without it is the underlying C language

    • A set of generic multi-threaded APIs
    • Suitable for Unix/linux/win and other systems
    • Cross-platform/portable
    • Very difficult to use

2. Nsthread programmer management occasionally using OC

    • Use more object-oriented
    • Easier to use, direct manipulation of thread objects

3.GCD Automatic management often uses C language

    • Designed to replace threading technology such as Nsthread
    • Make full use of the device's multi-core

4.NSOperation Automatic management often uses OC language

    • Based on gcd (bottom is GCD)
    • More simple and useful features than GCD.
    • Use more object-oriented

Using pthread must introduce import<pthread.h> this header file.

Thread Learning Notes

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