定时器

定时器概述

网络程序需要处理的第三类事件是定时事件，比如定期检测一个客户连接的活动状态。服务器程序通常管理着众多定时事件，因此有效的组织这些定时事件，使之能在预期的时间点被触发且不影响服务器的主要逻辑，对于服务器的性能有着至关重要的影响。

为此，我们要将每个定时事件分别封装成定时器，并使用某种容器类数据结构，比如链表、排序链表和时间轮，将所有定时器串联起来，以实现对定时事件的统一管理。

定时是指在一段时间之后触发某段代码的机制，我们可以在这段代码中依次处理所有到期的定时器。换言之，定时机制是定时器得以被处理的原动力。

Linux提供了三种定时方法：

• socket选项 SO_RCVTIMEO 和 SO_SNDTIMEO；
• SIGALRM信号；
• I/O复用系统调用的超时参数；

socket选项 SO_RCVTIMEO 和 SO_SNDTIMEO

socket选项 SO_RCVTIMEO 和 SO_SNDTIMEO，分别用来设置socket接收数据超时时间和发送数据超时时间，其仅对与数据接收和发送相关的socket专用系统调用有效，包括send、sendmsg、recv、recvmsg、accept、connect

可以根据系统调用的返回值和errno来判断超时时间是否到达，进而决定是否开始处理定时任务；

SIGALRM信号

由alarm和setitimer函数设置的实时闹钟一旦超时，将触发SIGALRM信号。因此，可以利用该信号的信号处理函数触发定时任务。但是如果要处理多个定时任务，就需要不断触发SIGALRM信号，并在其信号处理函数中执行到期的任务。

一般而言，SIGALRM信号按照固定的频率生成，即由alarm或setitimer设置的定时周期T保持不变，如果某个定时任务的超时时间不是T的整数倍，那么它实际被执行的时间和预期的时间将略有偏差，因此定时周期T反映了定时的精度；

定时器通常至少包含两个成员：一个超时时间（相对时间或绝对时间）和一个任务回调函数，有的时候还可能包含会调函数被执行时需要传入的参数，以及是否重启定时器等。

定时器升序链表实现

lst_timer.h

#ifndef LST_TIMER#define LST_TIMER#include <time.h>#define BUFFER_SIZE 64class util_timer;struct client_data{    sockaddr_in address;    int sockfd;    char buf[ BUFFER_SIZE ];    util_timer* timer;};class util_timer{public:    util_timer() : prev( NULL ), next( NULL ){}public:   time_t expire;    void (*cb_func)( client_data* );   client_data* user_data;   util_timer* prev;   util_timer* next;};class sort_timer_lst{public:    sort_timer_lst() : head( NULL ), tail( NULL ) {}    ~sort_timer_lst()    {        util_timer* tmp = head;        while( tmp )        {            head = tmp->next;            delete tmp;            tmp = head;        }    }    void add_timer( util_timer* timer )    {        if( !timer )        {            return;        }        if( !head )        {            head = tail = timer;            return;         }        if( timer->expire < head->expire )        {            timer->next = head;            head->prev = timer;            head = timer;            return;        }        add_timer( timer, head );    }    void adjust_timer( util_timer* timer )    {        if( !timer )        {            return;        }        util_timer* tmp = timer->next;        if( !tmp || ( timer->expire < tmp->expire ) )        {            return;        }        if( timer == head )        {            head = head->next;            head->prev = NULL;            timer->next = NULL;            add_timer( timer, head );        }        else        {            timer->prev->next = timer->next;            timer->next->prev = timer->prev;            add_timer( timer, timer->next );        }    }    void del_timer( util_timer* timer )    {        if( !timer )        {            return;        }        if( ( timer == head ) && ( timer == tail ) )        {            delete timer;            head = NULL;            tail = NULL;            return;        }        if( timer == head )        {            head = head->next;            head->prev = NULL;            delete timer;            return;        }        if( timer == tail )        {            tail = tail->prev;            tail->next = NULL;            delete timer;            return;        }        timer->prev->next = timer->next;        timer->next->prev = timer->prev;        delete timer;    }    void tick()    {        /*            相当于一个心搏函数，每隔一段固定时间就执行一次，以检测并处理到期的任务            判断定时任务到期的依据是定时器的expire值小于当前的系统事件        */        if( !head )        {            return;        }        printf( "timer tick\n" );        time_t cur = time( NULL );        util_timer* tmp = head;        while( tmp )        {            if( cur < tmp->expire )            {                break;            }            tmp->cb_func( tmp->user_data );            head = tmp->next;            if( head )            {                head->prev = NULL;            }            delete tmp;            tmp = head;        }    }private:    void add_timer( util_timer* timer, util_timer* lst_head )    {        util_timer* prev = lst_head;        util_timer* tmp = prev->next;        while( tmp )        {            if( timer->expire < tmp->expire )            {                prev->next = timer;                timer->next = tmp;                tmp->prev = timer;                timer->prev = prev;                break;            }            prev = tmp;            tmp = tmp->next;        }        if( !tmp )        {            prev->next = timer;            timer->prev = prev;            timer->next = NULL;            tail = timer;        }    }private:    util_timer* head;    util_timer* tail;};#endif

nonactive_conn.cpp

#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <assert.h>#include <stdio.h>#include <signal.h>#include <unistd.h>#include <errno.h>#include <string.h>#include <fcntl.h>#include <stdlib.h>#include <sys/epoll.h>#include <pthread.h>#include "lst_timer.h"#define FD_LIMIT 65535#define MAX_EVENT_NUMBER 1024#define TIMESLOT 5static int pipefd[2];static sort_timer_lst timer_lst;static int epollfd = 0;int setnonblocking( int fd ){    int old_option = fcntl( fd, F_GETFL );    int new_option = old_option | O_NONBLOCK;    fcntl( fd, F_SETFL, new_option );    return old_option;}void addfd( int epollfd, int fd ){    epoll_event event;    event.data.fd = fd;    event.events = EPOLLIN | EPOLLET;    epoll_ctl( epollfd, EPOLL_CTL_ADD, fd, &event );    setnonblocking( fd );}void sig_handler( int sig ){    int save_errno = errno;    int msg = sig;    send( pipefd[1], ( char* )&msg, 1, 0 );    errno = save_errno;}void addsig( int sig ){    struct sigaction sa;    memset( &sa, '\0', sizeof( sa ) );    sa.sa_handler = sig_handler;    sa.sa_flags |= SA_RESTART;    sigfillset( &sa.sa_mask );    assert( sigaction( sig, &sa, NULL ) != -1 );}void timer_handler(){    timer_lst.tick();    alarm( TIMESLOT );}void cb_func( client_data* user_data ){    epoll_ctl( epollfd, EPOLL_CTL_DEL, user_data->sockfd, 0 );    assert( user_data );    close( user_data->sockfd );    printf( "close fd %d\n", user_data->sockfd );}int main( int argc, char* argv[] ){    if( argc <= 2 )    {        printf( "usage: %s ip_address port_number\n", basename( argv[0] ) );        return 1;    }    const char* ip = argv[1];    int port = atoi( argv[2] );    int ret = 0;    struct sockaddr_in address;    bzero( &address, sizeof( address ) );    address.sin_family = AF_INET;    inet_pton( AF_INET, ip, &address.sin_addr );    address.sin_port = htons( port );    int listenfd = socket( PF_INET, SOCK_STREAM, 0 );    assert( listenfd >= 0 );    ret = bind( listenfd, ( struct sockaddr* )&address, sizeof( address ) );    assert( ret != -1 );    ret = listen( listenfd, 5 );    assert( ret != -1 );    epoll_event events[ MAX_EVENT_NUMBER ];    int epollfd = epoll_create( 5 );    assert( epollfd != -1 );    addfd( epollfd, listenfd );    ret = socketpair( PF_UNIX, SOCK_STREAM, 0, pipefd );    assert( ret != -1 );    setnonblocking( pipefd[1] );    addfd( epollfd, pipefd[0] );    // add all the interesting signals here    addsig( SIGALRM );    addsig( SIGTERM );    bool stop_server = false;    client_data* users = new client_data[FD_LIMIT];     bool timeout = false;    alarm( TIMESLOT );    while( !stop_server )    {        int number = epoll_wait( epollfd, events, MAX_EVENT_NUMBER, -1 );        if ( ( number < 0 ) && ( errno != EINTR ) )        {            printf( "epoll failure\n" );            break;        }        for ( int i = 0; i < number; i++ )        {            int sockfd = events[i].data.fd;            if( sockfd == listenfd )            {                struct sockaddr_in client_address;                socklen_t client_addrlength = sizeof( client_address );                int connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );                addfd( epollfd, connfd );                users[connfd].address = client_address;                users[connfd].sockfd = connfd;                util_timer* timer = new util_timer;                timer->user_data = &users[connfd];                timer->cb_func = cb_func;                time_t cur = time( NULL );                timer->expire = cur + 3 * TIMESLOT;                users[connfd].timer = timer;                timer_lst.add_timer( timer );            }            else if( ( sockfd == pipefd[0] ) && ( events[i].events & EPOLLIN ) )            {                int sig;                char signals[1024];                ret = recv( pipefd[0], signals, sizeof( signals ), 0 );                if( ret == -1 )                {                    // handle the error                    continue;                }                else if( ret == 0 )                {                    continue;                }                else                {                    for( int i = 0; i < ret; ++i )                    {                        switch( signals[i] )                        {                            case SIGALRM:                            {                                timeout = true;                                break;                            }                            case SIGTERM:                            {                                stop_server = true;                            }                        }                    }                }            }            else if(  events[i].events & EPOLLIN )            {                memset( users[sockfd].buf, '\0', BUFFER_SIZE );                ret = recv( sockfd, users[sockfd].buf, BUFFER_SIZE-1, 0 );                printf( "get %d bytes of client data %s from %d\n", ret, users[sockfd].buf, sockfd );                util_timer* timer = users[sockfd].timer;                if( ret < 0 )                {                    if( errno != EAGAIN )                    {                        cb_func( &users[sockfd] );                        if( timer )                        {                            timer_lst.del_timer( timer );                        }                    }                }                else if( ret == 0 )                {                    cb_func( &users[sockfd] );                    if( timer )                    {                        timer_lst.del_timer( timer );                    }                }                else                {                    //send( sockfd, users[sockfd].buf, BUFFER_SIZE-1, 0 );                    if( timer )                    {                        time_t cur = time( NULL );                        timer->expire = cur + 3 * TIMESLOT;                        printf( "adjust timer once\n" );                        timer_lst.adjust_timer( timer );                    }                }            }            else            {                // others            }        }        if( timeout )        {            timer_handler();            timeout = false;        }    }    close( listenfd );    close( pipefd[1] );    close( pipefd[0] );    delete [] users;    return 0;}

I/O复用系统调用的超时参数

Linux下的3组I/O复用系统调用都带有超时参数，因此它们不仅能统一处理信号和I/O事件，也能统一处理定时事件。但是由于I/O复用系统调用可能在超时时间到期之前就返回（有I/O事件发生），所以如果我们要利用它们来定时，就需要不断更新定时参数以反映剩余的时间。

#define TIMEOUT 5000int timeout = TIMEOUT;time_t start = time(NULL);time_t end = time(NULL);while(1){    printf("the timeout is now %d mil-seconds\n", timeout);    start = time(NULL);    int number = epoll_wait(epollfd, events, MAX_EVENT_NUMBER, timeout);    if((number < 0) && (errno != EINTR)){        printf("epoll failure\n");        break;    }    //说明超时时间到，但没有I/O事件    if(number == 0){        timeout = TIMEOUT;        continue;    }    //epoll_wait返回值大于0，下次超时时间为timeout    end = time(NULL);    timeout = (end - start)*1000;    //number > 0 且刚好超时，则处理I/O事件和超时事件     if(timeout <= 0){        timeout = TIMEOUT;    }    //handle connections}

时间轮

基于排序链表的定时器存在一个问题：添加定时器的效率偏低。

上图所示的时间轮内，指针指向轮子的一个槽（slot），它可以以恒定的速度顺时针转动，每转动一步就指向下一个槽，每次转动称为一个滴答（tick），一个滴答的时间间隔称为时间轮的槽间隔（si : slot interval），它实际上就是心搏时间。该时间轮一共有N个槽，因此运转一周时间是N*si，每个槽指向一条定时器链表，每条链表上的定时器具有相同的特征，他们的定时时间相隔 N* si的整数倍，时间轮正式利用这种关系将定时器散列到不同的链表中。

假设现在指针指向槽cs，要添加一个定时时间为ti 的定时器，则该定时器将被插入槽ts对应的链表中：ts = (cs + (cs / si)) % N。

时间轮采用哈希表的思想，将定时器散列到不同的链表上，这样每条链表上的定时器数目都将明显少于原来的排序链表上的定时器数目，插入操作的效率基本不受定时器数目的影响。

对时间轮而言：要提高精度，就要使si值足够小；要提高执行效率，则要求N足够大；

#ifndef TIME_WHEEL_TIMER#define TIME_WHEEL_TIMER#include <time.h>#include <netinet/in.h>#include <stdio.h>#define BUFFER_SIZE 64class tw_timer;struct client_data{    sockaddr_in address;    int sockfd;    char buf[ BUFFER_SIZE ];    tw_timer* timer;};class tw_timer{public:    tw_timer( int rot, int ts )     : next( NULL ), prev( NULL ), rotation( rot ), time_slot( ts ){}public:    int rotation;    int time_slot;    void (*cb_func)( client_data* );    client_data* user_data;    tw_timer* next;    tw_timer* prev;};class time_wheel{public:    time_wheel() : cur_slot( 0 )    {        for( int i = 0; i < N; ++i )        {            slots[i] = NULL;        }    }    ~time_wheel()    {        for( int i = 0; i < N; ++i )        {            tw_timer* tmp = slots[i];            while( tmp )            {                slots[i] = tmp->next;                delete tmp;                tmp = slots[i];            }        }    }    tw_timer* add_timer( int timeout )    {        if( timeout < 0 )        {            return NULL;        }        int ticks = 0;        if( timeout < TI )        {            ticks = 1;        }        else        {            ticks = timeout / TI;        }        int rotation = ticks / N;        int ts = ( cur_slot + ( ticks % N ) ) % N;        tw_timer* timer = new tw_timer( rotation, ts );        if( !slots[ts] )        {            printf( "add timer, rotation is %d, ts is %d, cur_slot is %d\n", rotation, ts, cur_slot );            slots[ts] = timer;        }        else        {            timer->next = slots[ts];            slots[ts]->prev = timer;            slots[ts] = timer;        }        return timer;    }    void del_timer( tw_timer* timer )    {        if( !timer )        {            return;        }        int ts = timer->time_slot;        if( timer == slots[ts] )        {            slots[ts] = slots[ts]->next;            if( slots[ts] )            {                slots[ts]->prev = NULL;            }            delete timer;        }        else        {            timer->prev->next = timer->next;            if( timer->next )            {                timer->next->prev = timer->prev;            }            delete timer;        }    }    void tick()    {        tw_timer* tmp = slots[cur_slot];        printf( "current slot is %d\n", cur_slot );        while( tmp )        {            printf( "tick the timer once\n" );            if( tmp->rotation > 0 )            {                tmp->rotation--;                tmp = tmp->next;            }            else            {                tmp->cb_func( tmp->user_data );                if( tmp == slots[cur_slot] )                {                    printf( "delete header in cur_slot\n" );                    slots[cur_slot] = tmp->next;                    delete tmp;                    if( slots[cur_slot] )                    {                        slots[cur_slot]->prev = NULL;                    }                    tmp = slots[cur_slot];                }                else                {                    tmp->prev->next = tmp->next;                    if( tmp->next )                    {                        tmp->next->prev = tmp->prev;                    }                    tw_timer* tmp2 = tmp->next;                    delete tmp;                    tmp = tmp2;                }            }        }        cur_slot = ++cur_slot % N;    }private:    static const int N = 60;    static const int TI = 1;     tw_timer* slots[N];    int cur_slot;};#endif

时间堆

前面的定时方案都是以固定频率调用心搏函数tick，并在其中一次检测到期的定时器，然后执行到期定时器上的回调函数。

设计定时器的另一种思路是：将所有定时器中超时时间最小的一个定时器的超时值作为心搏间隔，一旦心搏函数被调用，超时时间最小的定时器必然到期，就可以在tick函数中处理该定时器。然后在剩余的定时器中找出超时时间最小的一个，并将这段最小时间设置为下一次心搏间隔，如此反复，就实现了较为精确的定时。

#ifndef intIME_HEAP#define intIME_HEAP#include <iostream>#include <netinet/in.h>#include <time.h>using std::exception;#define BUFFER_SIZE 64class heap_timer;struct client_data{    sockaddr_in address;    int sockfd;    char buf[ BUFFER_SIZE ];    heap_timer* timer;};class heap_timer{public:    heap_timer( int delay )    {        expire = time( NULL ) + delay;    }public:   time_t expire;   void (*cb_func)( client_data* );   client_data* user_data;};class time_heap{public:    time_heap( int cap ) throw ( std::exception )        : capacity( cap ), cur_size( 0 )    {    array = new heap_timer* [capacity];    if ( ! array )    {            throw std::exception();    }        for( int i = 0; i < capacity; ++i )        {            array[i] = NULL;        }    }    time_heap( heap_timer** init_array, int size, int capacity ) throw ( std::exception )        : cur_size( size ), capacity( capacity )    {        if ( capacity < size )        {            throw std::exception();        }        array = new heap_timer* [capacity];        if ( ! array )        {            throw std::exception();        }        for( int i = 0; i < capacity; ++i )        {            array[i] = NULL;        }        if ( size != 0 )        {            for ( int i =  0; i < size; ++i )            {                array[ i ] = init_array[ i ];            }            for ( int i = (cur_size-1)/2; i >=0; --i )            {                percolate_down( i );            }        }    }    ~time_heap()    {        for ( int i =  0; i < cur_size; ++i )        {            delete array[i];        }        delete [] array;     }public:    void add_timer( heap_timer* timer ) throw ( std::exception )    {        if( !timer )        {            return;        }        if( cur_size >= capacity )        {            resize();        }        int hole = cur_size++;        int parent = 0;        for( ; hole > 0; hole=parent )        {            parent = (hole-1)/2;            if ( array[parent]->expire <= timer->expire )            {                break;            }            array[hole] = array[parent];        }        array[hole] = timer;    }    void del_timer( heap_timer* timer )    {        if( !timer )        {            return;        }        // lazy delelte        timer->cb_func = NULL;    }    heap_timer* top() const    {        if ( empty() )        {            return NULL;        }        return array[0];    }    void pop_timer()    {        if( empty() )        {            return;        }        if( array[0] )        {            delete array[0];            array[0] = array[--cur_size];            percolate_down( 0 );        }    }    void tick()    {        heap_timer* tmp = array[0];        time_t cur = time( NULL );        while( !empty() )        {            if( !tmp )            {                break;            }            if( tmp->expire > cur )            {                break;            }            if( array[0]->cb_func )            {                array[0]->cb_func( array[0]->user_data );            }            pop_timer();            tmp = array[0];        }    }    bool empty() const { return cur_size == 0; }private:    void percolate_down( int hole )    {        heap_timer* temp = array[hole];        int child = 0;        for ( ; ((hole*2+1) <= (cur_size-1)); hole=child )        {            child = hole*2+1;            if ( (child < (cur_size-1)) && (array[child+1]->expire < array[child]->expire ) )            {                ++child;            }            if ( array[child]->expire < temp->expire )            {                array[hole] = array[child];            }            else            {                break;            }        }        array[hole] = temp;    }    void resize() throw ( std::exception )    {        heap_timer** temp = new heap_timer* [2*capacity];        for( int i = 0; i < 2*capacity; ++i )        {            temp[i] = NULL;        }        if ( ! temp )        {            throw std::exception();        }        capacity = 2*capacity;        for ( int i = 0; i < cur_size; ++i )        {            temp[i] = array[i];        }        delete [] array;        array = temp;    }private:    heap_timer** array;    int capacity;    int cur_size;};#endif

对时间堆而言，添加一个定时器的时间复杂度为O(lg n)，删除一个定时器的时间复杂度为O(1)，执行一个定时器的时间复杂度为O(1)。