Explore the operating principle of the Swoole coroutine scheduler

Swoole is a high-performance network communication framework developed based on PHP language. It can provide a variety of programming methods such as asynchronous, coroutine, and concurrency, allowing developers to write network applications more efficiently.

In Swoole, the coroutine scheduler is one of its core components. It is responsible for the creation, scheduling and recycling of coroutines, and is one of the keys to achieving high performance in Swoole.

So, how does the Swoole coroutine scheduler work? In this article, we will delve into how the Swoole coroutine scheduler operates.

1. Coroutine

Before we understand the Swoole coroutine scheduler in depth, let’s first understand what a coroutine is.

Coroutine is a lightweight thread. Compared with processes and threads, it has the following characteristics:

1. The switching of coroutines does not depend on the kernel and does not require The overhead of context switching, switching speed is very fast.

2. The memory consumption of coroutines is smaller than that of threads and processes, and more coroutines can be created at the same time.

3. In the asynchronous I/O model, coroutines are more suitable for IO-intensive application scenarios and can better utilize resources.

4. The coroutine is non-preemptive. Only one coroutine can run at the same time. Once the coroutine performs an IO operation or actively gives up control, it will switch to other coroutines for execution.

In coroutines in PHP language, it is usually necessary to use the yield keyword, which means that the coroutine yields, gives up control, and waits for the execution of other coroutines or the triggering of IO events.

2. Coroutine Scheduler

Swoole coroutine scheduler is a green thread that runs in user mode and can achieve multi-task scheduling by switching coroutines and scheduling thread resources very efficiently. and high-performance web applications.

The coroutine scheduler will create a coroutine stack (coroutine_stack) every time it is executed, and will save the context (context) to the coroutine stack, and then reload the context when the coroutine needs to resume execution. .

The scheduling method of the coroutine scheduler is non-preemptive. Only when it can be switched to other coroutines for execution, the current coroutine will actively give up control, and the scheduler will perform coroutine switching operations.

It should be noted that Swoole's coroutine scheduler wraps the underlying network IO through coroutines to achieve the effect of coroutine switching, rather than multi-threading in the true sense. Swoole uses Epoll or Kqueue at the bottom level for event polling and IO reuse, and cooperates with the coroutine switching mechanism to greatly improve the performance of I/O-intensive network applications.

3. Operating Principle

The operating principle of the Swoole coroutine scheduler can be divided into three steps: coroutine creation, coroutine switching and coroutine recycling.

1. Coroutine creation process

When the Swoole program starts executing, the coroutine scheduler also starts working. When a coroutine is created, coroutine scheduling will first allocate a coroutine_id to represent the ID of the coroutine, and then open up a space to store the context information of the coroutine, including the currently executed file, current line number, etc., and then This context information is saved into a coroutine_stack data structure.

2. Coroutine switching process

The coroutine scheduler is responsible for switching between multiple coroutines. Once a coroutine is activated or needs to wait for other events, the coroutine scheduler Will switch to another coroutine for execution. This switching process seems very cumbersome. In fact, the Swoole coroutine scheduler has been optimized. You only need to save the context to the current running stack, and then start the running stack from another coroutine. Load the context in and you can switch coroutines.

3. Coroutine recycling process

After the coroutine is finished running, the memory needs to be released in time. The recycling process of the coroutine scheduler is to detect the usage of the coroutine stack. If the coroutine is If the memory in the process stack is not occupied, it means that the coroutine can be recycled. When recycling, the context of the coroutine will first be removed from the coroutine scheduler, then the memory in the coroutine stack will be destroyed, and resources such as underlying file descriptors will be released.

4. Summary

This article introduces the operating principle of the Swoole coroutine scheduler, including three aspects: coroutine creation, coroutine switching and coroutine recycling. By having an in-depth understanding of the working principle of the Swoole coroutine scheduler, you can not only better understand the asynchronous, coroutine, event and other mechanisms of the Swoole framework, but also use Swoole more efficiently in actual development to improve application performance.

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