MindMap Gallery LinuxIO

LinuxIO

This is a mind map about Linux IO, which mainly includes: synchronization and asynchronous, blocking and non-blocking, what exactly IO is doing, asynchronous IO, signal driver IO, IO multiplexing, synchronous non-blocking IO model, and synchronous blocking IO model.

Edited at 2025-03-06 02:05:47

Mason·Carter

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LinuxIO

Mason·Carter

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LinuxIO

What exactly is IO doing

When a user makes a file read request

1. Disk data is directly written to the kernel buffer through DMA (no CPU participation is required)

2. Copy the file from the memory buffer to the user buffer (requires CPU participation)

The kernel needs to copy the data twice and perform 2 user-state/kernel-state switching conversions.

Synchronous and asynchronous, blocking and non-blocking

block

After the caller initiates an IO request, he must wait for the IO operation to complete or an error occurs before the subsequent code can be executed. During this period, the caller will be suspended and is in a waiting state until the data transmission is completed.

Non-blocking

After the caller initiates an IO request, the non-blocking IO will not suspend the caller if the IO condition is not ready. It will immediately return to the eagin state so that the caller can decide on subsequent processing.

Blocking/non-blocking: It is concerned whether the caller is waiting for the I/O to complete (whether it is suspended)

synchronous

It means that the execution process of the IO operation is included in this IO call. The caller gets the final data, whether it is successful data or the error returned.

asynchronous

The triggered IO call does not contain the IO execution process. The IO request initiated by the caller is just an IO submission process. The call function returns not the IO operation. However, when I execute IO asynchronously, the execution result of this IO is obtained at a certain moment in the future through a certain mechanism, such as a callback function, etc.

Synchronous/asynchronous: Focus on how I/O operations are executed

Synchronous blocking IO model

When the user program makes an IO request, the process is blocked and the subsequent logic cannot be executed. The subsequent logic can be performed after the IO is completed.

A process/thread can only handle one IO

Most of the file readings of Linux are synchronous blocking IO models

Advantages: Simple model and easy to implement Disadvantages: In high concurrency scenarios, a large number of threads/processes need to be created, resulting in waste of resources

Synchronous non-blocking IO model

When the user program makes an IO request, the kernel immediately returns to the eagin state. The user can poll the area to query the kernel's status until it is successful. During this process, the user will not stop doing so, and the subsequent logic is still executed

Advantages: The program does not need to be blocked, and the user program can execute other logic while waiting. Disadvantages: Frequent polling will take up a lot of CPU resources.

IO multiplexing

A process/thread can only process one IO. Users can listen to multiple IO events at the same time through IO multiplexing. When the IO event is ready, IO processing can be performed. Although only one can be processed at the same time, multiple can be processed within a period of time, achieving the effect of concurrency.

A process/thread can handle multiple IOs

Advantages: Improves the IO capability of the machine, and a process can handle more IO Disadvantages: The implementation is more complicated

Signal drive IO

Let the kernel send signals to the user program when the data of the file descriptor is ready, thus avoiding the process's continuous waiting for IO data preparation.

The user program registers a signal processing function through a sigaction system call and associates it with the file descriptor. When the file descriptor IO event is ready, the kernel sends a signal to the user program, and the user program performs IO operations in the signal processing function.

Advantages: The program does not need to be blocked, and the user program can execute other logic while waiting. Disadvantages: The implementation is complex, the user needs to implement the signal reception logic by himself, and when a large amount of IO occurs, signal processing will cause performance problems.

Asynchronous IO

When the user program makes an IO request, it only submits an IO task to the kernel, without blocking and polling and querying. When the kernel IO operation is completed, the callback function is triggered to notify the user program that the IO has completed

Advantages: Asynchronous IO can achieve IO request aggregation and then batch handed over to the kernel for processing, greatly improving concurrency capabilities. Disadvantages: Asynchronous I/O implementation in some systems (such as early Linux) is not perfect and requires relying on thread pool simulation.