java linux 线程池满 问题排查 linux c 线程池

概述

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在服务器开发过程中，使用并行/并发编程是经常会遇到的事情。

基于进程的服务器和基于线程的服务器的区别就不详述了，这里简介一下多线程的优缺点：

优点：

• 多个线程同时执行，提高了程序的执行效率
• 提高了资源利用率

缺点：

• 线程越多，cpu调度的开销越大
• 程序的复杂度上升

使用线程池的优点：

• 创建/销毁线程伴随着系统开销，过于频繁的创建/销毁线程，会很大程度上影响处理效率
• 线程并发数量过多，抢占系统资源从而导致阻塞，且操作系统对创建的线程最大数量会有一定限制
• 对线程进行一些简单的管理，如延时执行、定时循环执行等

实现原理

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程序启动之前，创建一定数量的线程，放入空闲的队列中，初始化线程池。

这些线程均处于阻塞状态，只占一点内存，不占用cpu。

当任务到达就从线程池中取出一个空闲线程，将任务传入此线程中运行。

当所有的线程都处在处理任务的时候，线程池将自动创建一定数量的新线程，用于处理更多的任务。

执行完任务的线程也并不退出，而是继续在线程池中等待下一次任务。

但大部分线程处于阻塞状态时，线程池将自动销毁一部分线程，回收系统资源。

组成部分

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• 线程管理器

• 用于创建并管理线程池。

• 工作线程

• 线程池中实际执行任务的线程。在初始化线程时会预先创建好固定数目的线程在池中，这些初始化的线程一般处于空闲状态，一般不占用CPU，占用较小的内存空间。

• 任务接口

• 每个任务必须实现的接口，当线程池的任务队列中有可执行任务时，被空闲的工作线程调去执行，把任务抽象出来形成接口，可以做到线程池与具体的任务无关。

• 任务队列

• 用来存放没有处理的任务，提供一种缓冲机制
• 实现这种结构有好几种方法，常用的是队列，主要运用先进先出原理，另外一种是链表之类的数据结构，可以动态的为它分配内存空间，应用中比较灵活

简单实现

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程序由三个文件组成，分别是thread_pool.h, thread_pool.c和test.c组成。

thread_pool.h如下：

#include <pthread.h>

struct job {
    void * (*callback_function)(void *arg);
    void *arg;
    struct job *next;
};

struct threadpool {
    int thread_num;
    int queue_max_num;
    struct job *head;
    struct job *tail;
    pthread_t *pthreads;
    pthread_mutex_t mutex;
    pthread_cond_t queue_empty;
    pthread_cond_t queue_not_empty;
    pthread_cond_t queue_not_full;
    int queue_cur_num;
    int queue_close;
    int pool_close;
};

struct threadpool *threadpool_init(int thread_num, int queue_max_num);

int threadpool_add_job(struct threadpool *pool, void *(*callback_function)(void *arg), void *arg);

int threadpool_destroy(struct threadpool *pool);

void *threadpool_function(void *arg);

thread_pool.c如下：

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#include "thread_pool.h"

struct threadpool *threadpool_init(int thread_num, int queue_max_num)
{
    struct threadpool *pool = NULL;

    do {
        pool = (struct threadpool *)calloc(1, sizeof(struct threadpool));
        if (!pool) {
            printf("calloc error: %m\n");
            break;
        }
        pool->thread_num = thread_num;
        pool->queue_max_num = queue_max_num;
        pool->queue_cur_num = 0;
        pool->head = NULL;
        pool->tail = NULL;
        if (pthread_mutex_init(&(pool->mutex), NULL)) {
            printf("init mutex error: %m\n");
            break;
        }
        if (pthread_cond_init(&(pool->queue_empty), NULL)) {
            printf("init queue_empty error: %m\n");
            break;
        }
        if (pthread_cond_init(&(pool->queue_not_empty), NULL)) {
            printf("init queue_not_empty error: %m\n");
            break;
        }
        if (pthread_cond_init(&(pool->queue_not_full), NULL)) {
            printf("init queue_not_full error: %m\n");
            break;
        }
        pool->pthreads = calloc(1, sizeof(pthread_t) * thread_num);
        if (!pool->pthreads) {
            printf("calloc pthreads error: %m\n");
            break;
        }
        pool->queue_close = 0;
        pool->pool_close = 0;
        int i;
        for (i = 0; i < pool->thread_num; i++) {
            pthread_create(&(pool->pthreads[i]), NULL, threadpool_function, (void *)pool);
        }
        return pool;
    } while (0);

    return NULL;
}

int threadpool_add_job(struct threadpool *pool, void *(*callback_function)(void *arg), void *arg)
{
    assert(pool != NULL);
    assert(callback_function != NULL);
    assert(arg != NULL);

    pthread_mutex_lock(&(pool->mutex));
    while ((pool->queue_cur_num == pool->queue_max_num) && !(pool->queue_close || pool->pool_close)) {
        pthread_cond_wait(&(pool->queue_not_full), &(pool->mutex));
    }
    if (pool->queue_close || pool->pool_close) {
        pthread_mutex_unlock(&(pool->mutex));
        return -1;
    }
    struct job *pjob = (struct job*) calloc(1, sizeof(struct job));
    if (!pjob) {
        pthread_mutex_unlock(&(pool->mutex));
        return -1;
    }

    pjob->callback_function = callback_function;
    pjob->arg = arg;
    pjob->next = NULL;
    if (pool->head == NULL) {
        pool->head = pool->tail = pjob;
        pthread_cond_broadcast(&(pool->queue_not_empty));
    } else {
        pool->tail->next = pjob;
        pool->tail = pjob;
    }

    pool->queue_cur_num++;
    pthread_mutex_unlock(&(pool->mutex));

    return 0;
}

void *threadpool_function(void *arg)
{
    struct threadpool *pool = (struct threadpool *)arg;
    struct job *pjob = NULL;

    while (1) {
        pthread_mutex_lock(&(pool->mutex));
        while ((pool->queue_cur_num == 0) && !pool->pool_close) {
            pthread_cond_wait(&(pool->queue_not_empty), &(pool->mutex));
        }

        if (pool->pool_close) {
            pthread_mutex_unlock(&(pool->mutex));
            pthread_exit(NULL);
        }
        pool->queue_cur_num--;
        pjob = pool->head;
        if (pool->queue_cur_num == 0) {
            pool->head = pool->tail = NULL;
        } else {
            pool->head = pjob->next;
        }

        if (pool->queue_cur_num == 0) {
            pthread_cond_signal(&(pool->queue_empty));
        }
        if (pool->queue_cur_num == pool->queue_max_num - 1) {
            pthread_cond_broadcast(&(pool->queue_not_full));
        }
        pthread_mutex_unlock(&(pool->mutex));

        (*(pjob->callback_function))(pjob->arg);
        free(pjob);
        pjob = NULL;
    }
}

int threadpool_destroy(struct threadpool *pool)
{
    assert(pool != NULL);
    pthread_mutex_lock(&(pool->mutex));
    if (pool->queue_close || pool->pool_close) {
        pthread_mutex_unlock(&(pool->mutex));
        return -1;
    }
    pool->queue_close = 1;
    while (pool->queue_cur_num != 0) {
        pthread_cond_wait(&(pool->queue_empty), &(pool->mutex));
    }
    pool->pool_close = 1;
    pthread_mutex_unlock(&(pool->mutex));
    pthread_cond_broadcast(&(pool->queue_not_empty));
    pthread_cond_broadcast(&(pool->queue_not_full));

    int i;
    for (i = 0; i < pool->thread_num; i++) {
        pthread_join(pool->pthreads[i], NULL);
    }

    pthread_mutex_destroy(&(pool->mutex));
    pthread_cond_destroy(&(pool->queue_empty));
    pthread_cond_destroy(&(pool->queue_not_empty));
    pthread_cond_destroy(&(pool->queue_not_full));
    free(pool->pthreads);

    struct job *p;
    while (pool->head != NULL) {
        p = pool->head;
        pool->head = p->next;
        free(p);
    }
    free(pool);

    return 0;
}

test.c用于测试，如下：

#include <stdio.h>
#include "thread_pool.h"

void* work(void* arg)
{
    char *p = (char*) arg;
    printf("threadpool callback fuction : %s.\n", p);
    sleep(1);
}

int main(void)
{
    struct threadpool *pool = threadpool_init(10, 20);
    threadpool_add_job(pool, work, "1");
    threadpool_add_job(pool, work, "2");
    threadpool_add_job(pool, work, "3");
    threadpool_add_job(pool, work, "4");
    threadpool_add_job(pool, work, "5");
    threadpool_add_job(pool, work, "6");
    threadpool_add_job(pool, work, "7");
    threadpool_add_job(pool, work, "8");
    threadpool_add_job(pool, work, "9");
    threadpool_add_job(pool, work, "10");
    threadpool_add_job(pool, work, "11");
    threadpool_add_job(pool, work, "12");
    threadpool_add_job(pool, work, "13");
    threadpool_add_job(pool, work, "14");
    threadpool_add_job(pool, work, "15");
    threadpool_add_job(pool, work, "16");
    threadpool_add_job(pool, work, "17");
    threadpool_add_job(pool, work, "18");
    threadpool_add_job(pool, work, "19");
    threadpool_add_job(pool, work, "20");
    threadpool_add_job(pool, work, "21");
    threadpool_add_job(pool, work, "22");
    threadpool_add_job(pool, work, "23");
    threadpool_add_job(pool, work, "24");
    threadpool_add_job(pool, work, "25");
    threadpool_add_job(pool, work, "26");
    threadpool_add_job(pool, work, "27");
    threadpool_add_job(pool, work, "28");
    threadpool_add_job(pool, work, "29");
    threadpool_add_job(pool, work, "30");
    threadpool_add_job(pool, work, "31");
    threadpool_add_job(pool, work, "32");
    threadpool_add_job(pool, work, "33");
    threadpool_add_job(pool, work, "34");
    threadpool_add_job(pool, work, "35");
    threadpool_add_job(pool, work, "36");
    threadpool_add_job(pool, work, "37");
    threadpool_add_job(pool, work, "38");
    threadpool_add_job(pool, work, "39");
    threadpool_add_job(pool, work, "40");

    sleep(5);
    threadpool_destroy(pool);
    return 0;
}

执行如下编译命令：
gcc test.c thread_pool.c -lpthread

程序运行结果如下：

-> % ./a.out 
threadpool callback fuction : 1.
threadpool callback fuction : 7.
threadpool callback fuction : 8.
threadpool callback fuction : 2.
threadpool callback fuction : 3.
threadpool callback fuction : 4.
threadpool callback fuction : 5.
threadpool callback fuction : 6.
threadpool callback fuction : 9.
threadpool callback fuction : 10.
threadpool callback fuction : 12.
threadpool callback fuction : 16.
threadpool callback fuction : 11.
threadpool callback fuction : 18.
threadpool callback fuction : 14.
threadpool callback fuction : 15.
threadpool callback fuction : 17.
threadpool callback fuction : 13.
threadpool callback fuction : 19.
threadpool callback fuction : 20.
threadpool callback fuction : 21.
threadpool callback fuction : 23.
threadpool callback fuction : 24.
threadpool callback fuction : 22.
threadpool callback fuction : 26.
threadpool callback fuction : 27.
threadpool callback fuction : 28.
threadpool callback fuction : 25.
threadpool callback fuction : 29.
threadpool callback fuction : 30.
threadpool callback fuction : 31.
threadpool callback fuction : 32.
threadpool callback fuction : 33.
threadpool callback fuction : 34.
threadpool callback fuction : 35.
threadpool callback fuction : 36.
threadpool callback fuction : 37.
threadpool callback fuction : 38.
threadpool callback fuction : 39.
threadpool callback fuction : 40.