Five models of I/o and three working models of web must be understood for web optimization

Five models of I/o and three working models of web must be understood for web optimization

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Five I/O models

Three working modes supported by web

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Five types of I/O:

1) block I/0

2) non-blocking I/O

3) I/O multiplexing

4) event (signal)-driven I/O

5) asynchronous I/O

Why do we need to initiate a system call?

Because the process wants to obtain data in the disk, but can only deal with the hardware is the kernel, the process notifies the kernel that I want data in the disk, this process is called by the system.

Steps for completing one I/O operation

When a process initiates a system call, the system call enters the kernel mode and starts the I/O operation.

I/O operations are divided into two steps;

1. the disk loads data to the memory space of the kernel,

2. copy the data in the kernel's memory space to the user's memory space (this process is where I/O occurs)

The following is a detailed illustration of the process's data acquisition process;

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The entire process: if this process requires operations on data in the disk, a system call is initiated to the kernel, and the process will be switched out, this process will be suspended or sleep, also known as non-disruptive sleep, because the data is not yet obtained. The process will be awakened only after the system call is complete, continue with the following steps:

① A process initiates a system call to the kernel,

② When the kernel receives a system call, it knows it is a file request, so it tells the disk to read the file

③ After the disk receives the kernel command, it loads the file into the memory space of the kernel.

④ After the kernel's memory space receives data, it copies the data to the memory space of the user's process (where I/O occurs)

⑤ Send a notification to the kernel after the process memory space obtains data

6. the kernel replies the received notifications to the process. This process is a wake-up Process. Then, the process obtains data and performs the next step.

Where I/O occurs, blocking or non-blocking occurs.

Blocking: The process initiates an I/O call and the process has to wait for the completion of I/O. At this time, the CPU switches the process out and the process is in sleep state.

How does one notify the process of blocking the I/O system?

When I/O is completed and the system directly notifies the process, the process is awakened.

Diagram of blocking I/O

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Non-blocking: The process initiates an I/O call. If I/O knows that it takes some time to complete, it immediately notifies the process to perform other operations, and the non-blocking I/O

How does the system notify the process of non-blocking I/O?

After the system completes, the process obtains the data and continues to execute the kernel data (this process is also called Blind wait)

Diagram of non-blocking I/O:

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I/O reuse diagram:

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Event (signal)-driven I/O diagram:

The event-driven mechanism triggered horizontally. The kernel notifies the process to read data. The process does not read data. The kernel needs to notify the process once;

Edge-triggered event-driven mechanism. The kernel only notifies the process to read data once, and the process can read data at any time within the time-out period.

Nginx adopts the edge-triggered event-driven mechanism, which is why nginx has better concurrency than apache. Of course, nginx has better performance than apache, and there are other aspects, for example, nginx supports asynchronous I/O and mmap (memory ing ).

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Asynchronous I/O diagram:

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The first four types of I/O are synchronous operations, and the last one is asynchronous operations.

Comparison of five I/O models:

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Three working modes of web

How Prefork works

The main process generates multiple worker processes, and the worker process responds to client requests one by one.

How Prefork works:

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How Worker works

The main process generates multiple worker processes. Each worker process generates multiple threads and each thread goes

Respond to client requests

Schematic Worker working principle:

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How Event works

The main process generates multiple worker processes. Each engineering process responds to requests from multiple clients.

After the I/O operation request to the client, the I/O operation is handed over to the kernel for execution, and the process responds to other

Client request, the process finally receives a notification from the kernel, and then replies to the client through this process

Request result, through the Event Callback Function

How Event works:

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This blog is based on reading relevant articles and writing my own understanding of I/O, which may not be accurate. I hope you can give me more advice. Thank you for your advice.

This article is from the "technical path-NLP" blog, please be sure to keep this source http://litaotao.blog.51cto.com/6224470/1289790