談一談linux下線程池

什么是線程池：
　　　　首先，顧名思義，就是把一堆開辟好的線程放在一個池子里統一管理，就是一個線程池。

　　其次，為什么要用線程池，難道來一個請求給它申請一個線程，請求處理完了釋放線程不行么？也行，但是如果創建線程和銷毀線程的時間比線程處理請求的時間長，而且請求很多的情況下，我們的CPU資源都浪費在了創建和銷毀線程上了，所以這種方法的效率比較低，於是，我們可以將若干已經創建完成的線程放在一起統一管理，如果來了一個請求，我們從線程池中取出一個線程來處理，處理完了放回池內等待下一個任務，線程池的好處是避免了繁瑣的創建和結束線程的時間，有效的利用了CPU資源。

　　按照我的理解，線程池的作用和雙緩沖的作用類似，可以完成任務處理的“魚貫”動作。

　　最后，如何才能創建一個線程池的模型呢，一般需要以下三個參與者：

　　　　1、線程池結構，它負責管理多個線程並提供任務隊列的接口

　　　　2、工作線程，它們負責處理任務

　　　　3、任務隊列，存放待處理的任務

　　有了三個參與者，下一個問題就是怎么使線程池安全有序的工作，可以使用POSIX中的信號量、互斥鎖和條件變量等同步手段。有了這些認識，我們就可以創建自己的線程池

 

 代碼示例如下：

#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include "threadpool.h"

#define DEFAULT_TIME 10                 /*10s檢測一次*/
#define MIN_WAIT_TASK_NUM 10            /*如果queue_size > MIN_WAIT_TASK_NUM 添加新的線程到線程池*/ 
#define DEFAULT_THREAD_VARY 10          /*每次創建和銷毀線程的個數*/
#define true 1
#define false 0

typedef struct
{
    void *(*function)(void *);          /*函數指針，回調函數*/
    void *arg;                          /*上面函數的參數*/
} threadpool_task_t;                    /*任務結構體*/

struct threadpool_t
{
    pthread_mutex_t lock;               /*用於鎖住當前這個結構體體taskpoll*/    
    pthread_mutex_t thread_counter;     /*記錄忙狀態線程個數*/
    pthread_cond_t queue_not_full;      /*當任務隊列滿時，添加任 務的線程阻塞，等待此條件變量*/
    pthread_cond_t queue_not_empty;     /*任務隊列里不為空時，通知等待任務的線程*/
    pthread_t *threads;                 /*保存工作線程tid的數組*/
    pthread_t adjust_tid;               /*管理線程tid*/
    threadpool_task_t *task_queue;      /*任務隊列*/
    int min_thr_num;                    /*線程組內默認最小線程數*/
    int max_thr_num;                    /*線程組內默認最大線程數*/
    int live_thr_num;                   /*當前存活線程個數*/
    int busy_thr_num;                   /*忙狀態線程個數*/
    int wait_exit_thr_num;              /*要銷毀的線程個數*/
    int queue_front;                    /*隊頭索引下標*/
    int queue_rear;                     /*隊未索引下標*/
    int queue_size;                     /*隊中元素個數*/
    int queue_max_size;                 /*隊列中最大容納個數*/
    int shutdown;                       /*線程池使用狀態，true或false*/
};

/**
 * @function void *threadpool_thread(void *threadpool)
 * @desc the worker thread
 * @param threadpool the pool which own the thread
 */
void *threadpool_thread(void *threadpool);
/**
 * @function void *adjust_thread(void *threadpool);
 * @desc manager thread
 * @param threadpool the threadpool
 */
void *adjust_thread(void *threadpool);
/**
 * check a thread is alive
 */
int is_thread_alive(pthread_t tid);

int threadpool_free(threadpool_t *pool);

threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size)
{
    int i;
    threadpool_t *pool = NULL;
    do{
        if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL)
        {
            printf("malloc threadpool fail");
            break;          /*跳出do while*/
        }

        pool->min_thr_num = min_thr_num;
        pool->max_thr_num = max_thr_num;
        pool->busy_thr_num = 0;
        pool->live_thr_num = min_thr_num;
        pool->queue_size = 0;
        pool->queue_max_size = queue_max_size;
        pool->queue_front = 0;
        pool->queue_rear = 0;
        pool->shutdown = false;

        pool->threads = (pthread_t *)malloc(sizeof(pthread_t)*max_thr_num);
        if (pool->threads == NULL)
        {
            printf("malloc threads fail");
            break;
        }
        memset(pool->threads, 0, sizeof(pool->threads));

        pool->task_queue = (threadpool_task_t *)malloc(sizeof(threadpool_task_t)*queue_max_size);
        if (pool->task_queue == NULL)
        {
            printf("malloc task_queue fail");
            break;
        }

        if (pthread_mutex_init(&(pool->lock), NULL) != 0
                || pthread_mutex_init(&(pool->thread_counter), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_empty), NULL) != 0
                || pthread_cond_init(&(pool->queue_not_full), NULL) != 0)
        {
            printf("init the lock or cond fail");
            break;
        }

        /* 啟動min_thr_num個work thread */
        for (i = 0; i < min_thr_num; i++)
        {
            pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
            printf("start thread 0x%x...\n", (unsigned int)pool->threads[i]);
        }
        pthread_create(&(pool->adjust_tid), NULL, adjust_thread, (void *)pool);
        return pool;

    }while(0);
    
    threadpool_free(pool);      /*前面代碼調用失敗時，釋放poll存儲空間*/
    return NULL;
}

int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg)
{
    pthread_mutex_lock(&(pool->lock));

    while ((pool->queue_size == pool->queue_max_size) && (!pool->shutdown))
    {
        pthread_cond_wait(&(pool->queue_not_full), &(pool->lock));
    }
    if (pool->shutdown)
    {
        pthread_mutex_unlock(&(pool->lock));
    }

    /*添加任務到任務隊列里*/
    if (pool->task_queue[pool->queue_rear].arg != NULL)
    {
        free(pool->task_queue[pool->queue_rear].arg);
        pool->task_queue[pool->queue_rear].arg = NULL;
    }
    pool->task_queue[pool->queue_rear].function = function;
    pool->task_queue[pool->queue_rear].arg = arg;
    pool->queue_rear = (pool->queue_rear + 1)%pool->queue_max_size;
    pool->queue_size++;

    /*任務隊列不為空，喚醒等待處理任務的線程*/
    pthread_cond_signal(&(pool->queue_not_empty));
    pthread_mutex_unlock(&(pool->lock));

    return 0;
}

void *threadpool_thread(void *threadpool)
{
    threadpool_t *pool = (threadpool_t *)threadpool;
    threadpool_task_t task;
    while(true)
    {
        /* Lock must be taken to wait on conditional variable */
        /*剛創建出線程，等待任務隊列里有任務，否則阻塞等待任務隊列里有任務后再喚醒接收任務*/
        pthread_mutex_lock(&(pool->lock));

        while ((pool->queue_size == 0) && (!pool->shutdown))
        {
            printf("thread 0x%x is waiting\n", (unsigned int)pthread_self());
            pthread_cond_wait(&(pool->queue_not_empty), &(pool->lock));
            /*清除指定數目的空閑線程，如果要結束的線程個數大於0，結束線程*/
            if (pool->wait_exit_thr_num > 0)
            {
                pool->wait_exit_thr_num--;
                /*如果線程池里線程個數大於最小值時可以結束當前線程*/
                if (pool->live_thr_num > pool->min_thr_num)
                {
                    printf("thread 0x%x is exiting\n", (unsigned int)pthread_self());
                    pool->live_thr_num--;
                    pthread_mutex_unlock(&(pool->lock));
                    pthread_exit(NULL);
                }
            }
        }

        /*如果指定了true，要關閉線程池里的每個線程，自行退出處理*/
        if (pool->shutdown)
        {
            pthread_mutex_unlock(&(pool->lock));
            printf("thread 0x%x is exiting\n", (unsigned int)pthread_self());
            pthread_exit(NULL);
        }
        /*從任務隊列里獲得任務*/
        task.function = pool->task_queue[pool->queue_front].function;
        task.arg = pool->task_queue[pool->queue_front].arg;
        pool->queue_front = (pool->queue_front + 1)%pool->queue_max_size;
        pool->queue_size--;

        /*通知可以有新的任務添加進來*/
        pthread_cond_broadcast(&(pool->queue_not_full));

        pthread_mutex_unlock(&(pool->lock));

        /*執行任務*/ 
        printf("thread 0x%x start working\n", (unsigned int)pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num++;                                       /*忙狀態線程數加1*/
        pthread_mutex_unlock(&(pool->thread_counter));
        (*(task.function))(task.arg);                               /*執行回調函數任務*/
        //task.function(task.arg);                               /*執行回調函數任務*/

        /*任務結束處理*/ 
        printf("thread 0x%x end working\n", (unsigned int)pthread_self());
        pthread_mutex_lock(&(pool->thread_counter));
        pool->busy_thr_num--;                                       /*忙狀態數減1*/
        pthread_mutex_unlock(&(pool->thread_counter));
    }

    pthread_exit(NULL);
    //return (NULL);
}


void *adjust_thread(void *threadpool)
{
    int i;
    threadpool_t *pool = (threadpool_t *)threadpool;
    while (!pool->shutdown)
    {
        sleep(DEFAULT_TIME);                                    /*延時10秒*/
        pthread_mutex_lock(&(pool->lock));
        int queue_size = pool->queue_size;
        int live_thr_num = pool->live_thr_num;
        pthread_mutex_unlock(&(pool->lock));

        pthread_mutex_lock(&(pool->thread_counter));
        int busy_thr_num = pool->busy_thr_num;
        pthread_mutex_unlock(&(pool->thread_counter));

        /*任務數大於最小線程池個數並且存活的線程數少於最大線程個數時，創建新線程*/
        if (queue_size >= MIN_WAIT_TASK_NUM && live_thr_num < pool->max_thr_num)
        {
            pthread_mutex_lock(&(pool->lock));
            int add = 0;
            /*一次增加DEFAULT_THREAD個線程*/
            for (i = 0; i < pool->max_thr_num && add < DEFAULT_THREAD_VARY && pool->live_thr_num < pool->max_thr_num; i++)
            {
                if (pool->threads[i] == 0 || !is_thread_alive(pool->threads[i]))
                {
                    pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
                    add++;
                    pool->live_thr_num++;
                }
            }
            pthread_mutex_unlock(&(pool->lock));
        }

        /*銷毀多余的空閑線程*/
        if ((busy_thr_num * 2) < live_thr_num
                && live_thr_num > pool->min_thr_num)
        {
            /*一次銷毀DEFAULT_THREAD個線程*/
            pthread_mutex_lock(&(pool->lock));
            pool->wait_exit_thr_num = DEFAULT_THREAD_VARY;
            pthread_mutex_unlock(&(pool->lock));

            for (i = 0; i < DEFAULT_THREAD_VARY; i++)
            {
                /*通知處在空閑狀態的線程*/
                pthread_cond_signal(&(pool->queue_not_empty));
            }
        }
    }
}

int threadpool_destroy(threadpool_t *pool)
{
    int i;
    if (pool == NULL)
    {
        return -1;
    }

    pool->shutdown = true;
    /*先銷毀管理線程*/
    pthread_join(pool->adjust_tid, NULL);
    for (i = 0; i < pool->live_thr_num; i++)
    {
        /*通知所有的空閑線程*/
        pthread_cond_broadcast(&(pool->queue_not_empty));
        pthread_join(pool->threads[i], NULL);
    }
    threadpool_free(pool);
    return 0;
}

int threadpool_free(threadpool_t *pool)
{
    if (pool == NULL)
    {
        return -1;
    }
    if (pool->task_queue)
    {
        free(pool->task_queue);
    }
    if (pool->threads)
    {
        free(pool->threads);
        pthread_mutex_lock(&(pool->lock));
        pthread_mutex_destroy(&(pool->lock));
        pthread_mutex_lock(&(pool->thread_counter));
        pthread_mutex_destroy(&(pool->thread_counter));
        pthread_cond_destroy(&(pool->queue_not_empty));
        pthread_cond_destroy(&(pool->queue_not_full));
    }
    free(pool);
    pool = NULL;
    return 0;
}

int threadpool_all_threadnum(threadpool_t *pool)
{
    int all_threadnum = -1;
    pthread_mutex_lock(&(pool->lock));
    all_threadnum = pool->live_thr_num;
    pthread_mutex_unlock(&(pool->lock));
    return all_threadnum;
}

int threadpool_busy_threadnum(threadpool_t *pool)
{
    int busy_threadnum = -1;
    pthread_mutex_lock(&(pool->thread_counter));
    busy_threadnum = pool->busy_thr_num;
    pthread_mutex_unlock(&(pool->thread_counter));
    return busy_threadnum;
}

int is_thread_alive(pthread_t tid)
{
    int kill_rc = pthread_kill(tid, 0);
    if (kill_rc == ESRCH)
    {
        return false;
    }
    return true;
}

/*測試使用，作成庫時請注釋掉下面代碼*/ 
#if 1
void *process(void *arg)
{
    printf("thread 0x%x working on task %d\n ",(unsigned int)pthread_self(),*(int *)arg);
    sleep(1);
    printf("task %d is end\n",*(int *)arg);
    return NULL;
}
int main(void)
{
    threadpool_t *thp = threadpool_create(3,100,100);    /*線程池里最小3個線程，最大100個，隊列最大值12*/
    printf("pool inited");

    int *num = (int *)malloc(sizeof(int)*20);
    //int num[20];
    int i;
    for (i=0;i<10;i++)
    {
        num[i]=i;
        printf("add task %d\n",i);
        threadpool_add(thp,process,(void*)&num[i]);
    }
    sleep(10);
    threadpool_destroy(thp);
}
#endif