The difference between multi-threading and Async

Ext.: http://www.cnblogs.com/ydhliphonedev/archive/2011/09/22/2185455.html

With the popularization of multiple hard-threading CPUs (Hyper-threading, dual-core), concurrent programming methods such as multithreading and asynchronous operations are also more concerned and discussed. This article is mainly to discuss with the experts in the park how to use concurrency to maximize the performance of the program.

　　The similarities and differences between multithreading and asynchronous operations

Both multithreading and asynchronous operations can achieve the goal of avoiding calling thread blocking, thus improving the responsiveness of the software. Even sometimes we think that multithreading and asynchronous operations are the same concept. However, there are some differences between multithreading and asynchronous operations. These differences create the difference between the timing of using multithreading and asynchronous operations.

　　The nature of asynchronous operations

All programs will eventually be executed by the computer hardware, so in order to better understand the nature of the asynchronous operation, we need to understand its hardware base. Familiar with the computer hardware friend is certainly not unfamiliar with the word DMA, hard disk, optical drive technical specifications have a clear DMA mode indicator, in fact, the network card, sound card, video card also has DMA function. DMA is the meaning of direct memory access, which means that the DMA-capable hardware does not consume CPU resources when exchanging data with the memory. As long as the CPU sends an instruction when it initiates the data transfer, the hardware begins to exchange data between itself and the memory, and after the transfer is complete, the hardware triggers an interrupt to notify the operation to complete. These I/O operations that do not consume CPU time are the hardware basis for asynchronous operations. Therefore, even in a single process such as DOS (and the Wireless Path concept) system can also initiate asynchronous DMA operation.

　　The nature of Threads
A thread is not a function of a computer's hardware, but rather a logical function provided by the operating system, which is essentially a piece of code that runs concurrently in a process, so the thread requires the operating system to run and dispatch the CPU resources.

　　Advantages and disadvantages of asynchronous operations

Because asynchronous operations do not require additional thread burdens and are handled in a callback manner, the processing function can eliminate the possibility of deadlocks by eliminating the need for shared variables (even if it is not completely unused, at least by reducing the number of shared variables). Of course, asynchronous operations are not flawless. Writing asynchronous operations is a high degree of complexity, the program mainly uses callback method to handle, and ordinary people's way of thinking some first, and difficult to debug.

　　Advantages and disadvantages of multithreading
The advantages of multithreading are obvious, the handlers in the thread are still sequential execution, in line with the normal thinking habits, so programming is simple. However, the disadvantage of multithreading is equally obvious, the use of threads (misuse) will bring the burden of context switching to the system. Also, shared variables between threads can cause deadlocks to occur.

　　Scope of application

After understanding the pros and cons of threading and asynchronous operations, we can explore the logical use of a thread and async. I think that when I am required to perform I/O operations, it is more appropriate to use asynchronous operations than to use thread + synchronous I/O operations. I/O operations include not only direct files, read and write to the network, but also database operations, Web Service, HttpRequest, and cross-process calls such as. Net Remoting.
The scope of the thread is the kind that requires long CPU operations, such as long-time graphics processing and algorithm execution. But often because of the simple and consistent use of threading programming, many friends tend to use threads to perform long-time I/O operations. This is harmless when there are only a few concurrent operations, which is not appropriate if you need to handle a large number of concurrent operations.

                                 &NB Sp                          ,         &NB Sp                          ,         &NB Sp                          ,         &NB Sp                          ,         &NB Sp                          ,         &NB Sp             ---by yuzhang2

October 16, 2014 14:40:57

Plus two a good understanding of multi-threaded and asynchronous posts, and now still do not understand, but feel very good, later can also be slowly pondering:

1, asynchronous and multi-threaded difference and contact?

2 . What is async? What is the difference from multithreading?

The difference between multi-threading and Async (GO)