How to implement time wheel using Go language

In the field of computer programming, the timewheel is a commonly used data structure that can be used to implement time-related tasks. Due to its efficiency and portability, the time wheel is widely used in fields such as scheduled task scheduling, network delay, and expired caching. This article will introduce how to implement a time wheel using Go language.

1. Time wheel overview

The time wheel is a circular buffer based on the concept of time. It can be regarded as a circular buffer with a size of m (Power of 2). Each time the time wheel rotates by one unit, such as 1 millisecond, the contents pointed to by all buffers also change. In the time wheel, there are many marks, slots, pointers, etc. inside.

The function of the time wheel is to implement scheduled task scheduling. Essentially, a scheduled task is a structure that contains information such as the execution time of the task and the execution function of the task. We can hang these scheduled tasks in the corresponding slots of the time wheel and perform the scheduled scheduling of the time wheel.

2. Go language to implement time wheel

We use Go language to implement time wheel, which can be implemented through the following three structs:

type TimerTask struct {
    expires   int64            //任务的到期时间
    callback  func()          //任务需要执行的函数
}

type Timer struct {
    interval  int64            //时间轮转动的间隔
    slots     []*list.List    //所有的槽位
    curPos    int             //当前槽位指针
    tickCount int64           //时间轮当前tick
}

type Timewheel struct {
    timer     *Timer          //指向Timer结构体的指针
    quit      chan struct{}   //停止时间轮信号
    waitGroup sync.WaitGroup  //同步等待
}

We use the TimerTask structure The body stores information such as the execution time of the task and the execution function of the task. In the Timer structure, the time interval of the time wheel rotation, the list of all slots, the current slot pointer and the current number of ticks are saved. In the Timewheel structure, the pointer of the time wheel, the signal to stop the time wheel and the synchronization wait are stored.

The workflow of the time wheel is as follows:

1) Initialize the Timer structure and build a time list.

2) Use the addTimer function to add the specified scheduled task to the slot.

3) Start the time wheel, and the tasks added to the slot will be executed in the corresponding tick according to the specified execution time.

Below we introduce in detail how to implement each step.

2.1 Initialize the Timer structure

In order to initialize the time wheel, we need to create a list containing m (multiples of tow) slots in the Timer structure, and hang all tasks on in the corresponding slot. In order to implement a list in the Go language, we can use the linked list type provided by the container/list package. This linked list supports addition and deletion operations in O(1) time, which is very suitable for time wheels.

type Timer struct {
    interval  int64
    slots     []*list.List
    curPos    int
    tickCount int64
}

func newTimer(interval int64, m int) *Timer {
    l := make([]*list.List, m)
    for i := 0; i < m; i++ {
        l[i] = list.New()
    }
    return &Timer{
        interval:  interval,
        slots:     l,
        curPos:    0,
        tickCount: 0,
    }
}

2.2 Add a scheduled task

We use the addTimer function to add a scheduled task. This function accepts a TimerTask structure as a parameter and adds it to the corresponding time slot of the time wheel. In order to ensure that the scheduled task can be scheduled in the correct slot, we need to calculate the slot position of the task based on time and add the task to the list of the slot.

func (tw *TimerWheel) AddTimer(task *TimerTask) {
    if task.expires <= 0 {
        return
    }

    pos, round := tw.timer.getPosAndRound(task.expires)
    tw.timer.slots[pos].PushBack(task)
    task.position = &Element{
        round:       round,
        position:    pos,
        task:        task,
        nextElement: nil,
    }
}

2.3 Start the time wheel

Use the Start function to start the time wheel. The Start function uses a goroutine in the current process, which will execute the tick operation of the time wheel each time. The entire loop process is completed by the for-select statement. In the tick of each time wheel, we point the current tick to the next slot, and iterate the current slot, executing all tasks saved in it.

func (tw *TimerWheel) Start() {
    defer close(tw.quit)
    tw.timer.resetTickCount()

    ticker := time.NewTicker(time.Duration(tw.timer.interval) * time.Millisecond)
    defer ticker.Stop()

    for {
        select {
        case <-tw.quit:
            log.Println("time wheel is stop.")
            return
        case <-ticker.C:
            tw.timer.curPos = (tw.timer.curPos + 1) & (tw.timer.slotNum() - 1)
            tw.timer.tickCount++
            l := tw.timer.slots[tw.timer.curPos]
            tw.exec(l)
        }
    }
}

3. Summary

Go language is a fast and efficient programming language, which is very suitable for implementing time wheels. Task scheduling in time wheels can be easily handled using Go's container packages (such as container/heap and container/list). In order to make the time wheel more flexible and reliable, different types of tasks can be classified into multiple levels, low-priority tasks can be scheduled and retried, and high-priority tasks can be quickly scheduled through priority queues. Of course, during the implementation process, we also need to consider detailed issues such as task concurrency and memory management to ensure the efficient operation of the time wheel.

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