Shulou(Shulou.com)06/03 Report--

This article shows you how to sum up the multithreading in the java virtual machine, the content is concise and easy to understand, it will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

I remember when I first came into contact with multi-process, multi-threading, I didn't understand, why should there be multi-threading? What the heck is multithreading? I can run a program well. Why should I use multithreading? Anyway, it is very difficult for me to understand. Even after a long time, I still don't understand very well. I've heard others say it and tried it myself, but I don't understand it thoroughly.

After a long time, I feel that I have a new understanding of multithreading. Let's start with the basics. By the way, let's take a look at how multithreading allocates memory in jvm from the perspective of jvm. By the way, string it with the previous articles.

1. Multithreading in reality

For example: if you are at home alone, it takes you 5 minutes to listen to a song, 10 minutes to boil water, and 20 minutes to play a game. Now the question is, how many minutes will it take you to complete these three tasks?

If it is a primary school student will certainly answer 5: 10: 20: 35 minutes, but we are a little bit better than primary school students, a little thinking will know that it is 20 minutes, because three things can be done at the same time, playing the game can listen to music, by the way boil water, a game finished, the song is finished, the water also boiled and then can go to make tea, comfortable!

Let's take a look at these two ways with a relatively simple diagram (it is easy to understand without considering the difference between concurrency and parallelism).

We can roughly see that if you are primary school students, you have to do one thing, and the time spent in the end is the sum of the time of the three; and we are smarter, because the three things do not affect each other, we can start doing three things at the same time. This greatly reduces the unnecessary waiting time, and in the end, the time spent by the three is about the longest.

Here's a little bit about the difference between concurrency and parallelism.

Concurrency: this is under the premise of a single-core CPU in the computer, we need to know that a CPU can only do one thing at a time, but now there are three things (listening to music, boiling water, and playing games) to CPU to do, CPU is a good man, uncomplaining, suddenly go to listen to music, suddenly boil water, play games, and finally finish all three things, but what if hundreds of things are given to CPU at the same time? Emmmm, finally CPU was exhausted and went to the hospital, so our computer also jammed. To take an example closest to us: in the past, when I went to school, I had too much homework, and most of the time I didn't finish my homework for many subjects, so what should I do? Only go to copy the classmates early in the morning, but the representatives of each class come to collect the homework, so they can only copy a little of the homework of this subject and copy a little of the homework of another subject right away. Ma de, finally finished copying all of them at the specified time. But if you didn't finish your homework for 100 subjects, what would you do? You can't copy it with your life, so you get tired and sick.

Parallel: the premise of multi-core CPU, now a computer has multiple CPU, then CPU can do many things at the same time, even if there are more things, the final time for multiple CPU switching is really greatly reduced; or talk about the example of copying homework above, if you have not finished your homework for 10 subjects now, it will be too late to rely on your head and hand. So at this time you awaken the memory of the previous life, the original you are reincarnated, the fuck can be turned into three heads and six arms, this has to be able to copy how many homework ah! I finished my homework at once. Comfortable! But at this time, if you have too many homework subjects, even if you have three heads and six arms, it is not enough, and mutual coordination has become a very important issue.

Concurrency and parallelism is what it means. Let's just focus on concurrency and see how a program runs on a single-core CPU computer.

two。 Processes and threads

Think about what is called process? My understanding is that when a program enters memory, it is a process. For example, when our computer desktop double-clicks QQ, Youku, java virtual machine, etc., the operating system will load the contents of these software into memory to run, and then run the code written in a programming language, convert it into machine code and call the operating system interface, and then the operating system kernel will issue some instructions from those hardware drivers. And then our computer screen changed. Let's simply draw a picture. We mainly look at JVM.

Let's go back to JVM, where threads 1, 2, and 3 are what we are going to implement in the java code.

Process: let's take a look at Baidu's definition of process, the most important point is that process is the basic unit of operating system resource allocation, because every time a program is started, a process is created, and a piece of space is opened up in the operating system heap memory space, that is, resources are allocated.

Thread: now look at the thread, Baidu thread definition, that is to say: a process is a program, this program may simultaneously execute multiple tasks of the code, each task is a thread, and each thread in the JVM will have its own independent java stack, pc register, and CPU can only switch threads, even threads of different programs can also switch each other.

To make it clear here, if you want to compare a process with a thread, creating a process is to open up space in the memory of the operating system, which involves calling some functions of the operating system, while creating a thread (for example, in JVM) only needs to open up space in a part of the jvm. By comparison, it must cost less operating system resources to create a thread, but it is not possible to create many threads indefinitely. Otherwise there will be something wrong with jvm!

I casually checked that the maximum number of threads in a general web server cannot exceed the number of CPU cores * 50, such as 8 cores.

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