linux系统线程池

简述

一个进程中的线程就好比是一家公司里的员工，员工的数目应该根据公司的业务多少来定，太少了忙不过来，但是太多了也浪费资源。最理想的情况是让进程有一些初始数目的线程（线程池），当没有任务时这些线程自动进入睡眠，有了任务它们会立即执行任务，不断循环。进程还应根据自身任务的繁重与否来增删线程的数目，当所有的任务都完成之后，所有的线程还能妥当地收官。

如下图所示是一个处于初始状态的线程池。

​ 图1 线程池示意图

有以下几点需要注意：

（1）任务队列中刚开始没有任何任务，是一个具有头结点的空链队列

（2）使用互斥锁来保护这个队列

（3）使用条件变量来代表任务队列中的任务个数的变化，将来如果主线程向队列中投放任务，那么可以通过条件变量来唤醒那么睡着了的线程

（4）通过一个公共开关----shutdown，来控制线程退出，进而销毁整个线程池

接口设计

线程池相关结构体如下表所示。

下面是线程池的接口说明

（1）线程池初始化：init_pool()

（2）投放任务：add_task()

（3） 增加活跃线程：add_thread()

（4）删除活跃线程：remove_thread()

（5）销毁线程池：destroy_pool()

源代码实现

头文件thread_pool.h

//
//  
//  Description: 本文件包含了线程池基本结构体定义，以及各个操作函
//               数的声明
//
//

#ifndef _THREAD_POOL_H_
#define _THREAD_POOL_H_

#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>

#include <errno.h>
#include <pthread.h>

#define MAX_WAITING_TASKS	1000
#define MAX_ACTIVE_THREADS	20

struct task
{
	void *(*task)(void *arg);
	void *arg;

	struct task *next;
};

typedef struct thread_pool
{
	pthread_mutex_t lock;
	pthread_cond_t  cond;
	struct task *task_list;

	pthread_t *tids;

	unsigned waiting_tasks;
	unsigned active_threads;

	bool shutdown;
}thread_pool;


bool
init_pool(thread_pool *pool,
          unsigned int threads_number);

bool
add_task(thread_pool *pool,
         void *(*task)(void *arg),
         void *arg);

int 
add_thread(thread_pool *pool,
           unsigned int additional_threads_number);

int 
remove_thread(thread_pool *pool,
              unsigned int removing_threads_number);

bool destroy_pool(thread_pool *pool);
void *routine(void *arg);

#endif

接口实现：thread_pool.c

//
//  Description: 本文件包含了线程池操作函数的定义
//

#include "thread_pool.h"

void handler(void *arg)
{

	pthread_mutex_unlock((pthread_mutex_t *)arg);
}

void *routine(void *arg)
{
	thread_pool *pool = (thread_pool *)arg;
	struct task *p;

	while(1)
	{

		pthread_cleanup_push(handler, (void *)&pool->lock);
		pthread_mutex_lock(&pool->lock);


		while(pool->waiting_tasks == 0 && !pool->shutdown)
		{
			pthread_cond_wait(&pool->cond, &pool->lock);
		}


		if(pool->waiting_tasks == 0 && pool->shutdown == true)
		{
			pthread_mutex_unlock(&pool->lock);
			pthread_exit(NULL);
		}


		p = pool->task_list->next;
		pool->task_list->next = p->next;
		pool->waiting_tasks--;


		pthread_mutex_unlock(&pool->lock);
		pthread_cleanup_pop(0);


		pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
		(p->task)(p->arg);
		pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);

		free(p);
	}

	pthread_exit(NULL);
}

bool init_pool(thread_pool *pool, unsigned int threads_number)
{
	pthread_mutex_init(&pool->lock, NULL);
	pthread_cond_init(&pool->cond, NULL);

	pool->shutdown = false;
	pool->task_list = malloc(sizeof(struct task));
	pool->tids = malloc(sizeof(pthread_t) * MAX_ACTIVE_THREADS);

	if(pool->task_list == NULL || pool->tids == NULL)
	{
		perror("allocate memory error");
		return false;
	}

	pool->task_list->next = NULL;

	pool->waiting_tasks = 0;
	pool->active_threads = threads_number;

	int i;
	for(i=0; i<pool->active_threads; i++)
	{
		if(pthread_create(&((pool->tids)[i]), NULL,
					routine, (void *)pool) != 0)
		{
			perror("create threads error");
			return false;
		}
	}

	return true;
}

bool add_task(thread_pool *pool,
			void *(*task)(void *arg), void *arg)
{
	struct task *new_task = malloc(sizeof(struct task));
	if(new_task == NULL)
	{
		perror("allocate memory error");
		return false;
	}
	new_task->task = task;
	new_task->arg = arg;
	new_task->next = NULL;


	pthread_mutex_lock(&pool->lock);
	if(pool->waiting_tasks >= MAX_WAITING_TASKS)
	{
		pthread_mutex_unlock(&pool->lock);

		fprintf(stderr, "too many tasks.\n");
		free(new_task);

		return false;
	}
	
	struct task *tmp = pool->task_list;
	while(tmp->next != NULL)
		tmp = tmp->next;

	tmp->next = new_task;
	pool->waiting_tasks++;


	pthread_mutex_unlock(&pool->lock);
	pthread_cond_signal(&pool->cond);

	return true;
}

int add_thread(thread_pool *pool, unsigned additional_threads)
{
	if(additional_threads == 0)
		return 0;

	unsigned total_threads =
		     pool->active_threads + additional_threads;

	int i, actual_increment = 0;
	for(i = pool->active_threads;
	    i < total_threads && i < MAX_ACTIVE_THREADS;
	    i++)
	{
		if(pthread_create(&((pool->tids)[i]),
				NULL, routine, (void *)pool) != 0)
		{
			perror("add threads error");

			if(actual_increment == 0)
				return -1;

			break;
		}
		actual_increment++; 
	}

	pool->active_threads += actual_increment;
	return actual_increment;
}

int remove_thread(thread_pool *pool, unsigned int removing_threads)
{
	if(removing_threads == 0)
		return pool->active_threads;

	int remain_threads = pool->active_threads - removing_threads;
	remain_threads = remain_threads>0 ? remain_threads:1;

	int i;
	for(i=pool->active_threads-1; i>remain_threads-1; i--)
	{
		errno = pthread_cancel(pool->tids[i]);
		if(errno != 0)
			break;
	}

	if(i == pool->active_threads-1)
		return -1;
	else
	{
		pool->active_threads = i+1;
		return i+1;
	}
}

bool destroy_pool(thread_pool *pool)
{

	pool->shutdown = true;
	pthread_cond_broadcast(&pool->cond);

	int i;
	for(i=0; i<pool->active_threads; i++)
	{
		errno = pthread_join(pool->tids[i], NULL);
		if(errno != 0)
		{
			printf("join tids[%d] error: %s\n",
					i, strerror(errno));
		}
		else
			printf("[%u] is joined\n", (unsigned)pool->tids[i]);
		
	}

	free(pool->task_list);
	free(pool->tids);
	free(pool);

	return true;
}

测试程序

main.c

//

//  Description: 一个使用了线程池的示例

//

#include "thread_pool.h"

void *mytask(void *arg)
{
	int n = (int)arg;

	printf("[%u][%s] ==> job will be done in %d sec...\n",
		(unsigned)pthread_self(), __FUNCTION__, n);

	sleep(n);

	printf("[%u][%s] ==> job done!\n",
		(unsigned)pthread_self(), __FUNCTION__);

	return NULL;
}

void *count_time(void *arg)
{
	int i = 0;
	while(1)
	{
		sleep(1);
		printf("sec: %d\n", ++i);
	}
}

int main(void)
{
	pthread_t a;
	pthread_create(&a, NULL, count_time, NULL);

	// 1, initialize the pool
	thread_pool *pool = malloc(sizeof(thread_pool));
	init_pool(pool, 2);

	// 2, throw tasks
	printf("throwing 3 tasks...\n");
	add_task(pool, mytask, (void *)(rand()%10));
	add_task(pool, mytask, (void *)(rand()%10));
	add_task(pool, mytask, (void *)(rand()%10));

	// 3, check active threads number
	printf("current thread number: %d\n",
			remove_thread(pool, 0));
	sleep(9);

	// 4, throw tasks
	printf("throwing another 2 tasks...\n");
	add_task(pool, mytask, (void *)(rand()%10));
	add_task(pool, mytask, (void *)(rand()%10));

	// 5, add threads
	add_thread(pool, 2);

	sleep(5);

	// 6, remove threads
	printf("remove 3 threads from the pool, "
	       "current thread number: %d\n",
			remove_thread(pool, 3));

	// 7, destroy the pool
	destroy_pool(pool);
	return 0;
}

运行结果

zzc@zzc-virtual-machine:~/share/example/thread$ ./test 
throwing 3 tasks...
current thread number: 2
[3981727488][mytask] ==> job will be done in 3 sec...
[3990120192][mytask] ==> job will be done in 6 sec...
sec: 1
sec: 2
[3981727488][mytask] ==> job done!
[3981727488][mytask] ==> job will be done in 7 sec...
sec: 3
sec: 4
sec: 5
[3990120192][mytask] ==> job done!
sec: 6
sec: 7
sec: 8
throwing another 2 tasks...
[3990120192][mytask] ==> job will be done in 5 sec...
[3892311808][mytask] ==> job will be done in 3 sec...
sec: 9
[3981727488][mytask] ==> job done!
sec: 10
sec: 11
[3892311808][mytask] ==> job done!
sec: 12
sec: 13
remove 3 threads from the pool, current thread number: 1
[3990120192][mytask] ==> job done!
[3990120192] is joined

延伸

实际使用的时候，只需将上述代码中的mytask函数改成我们需要实现的功能函数即可。

总结

本文简单介绍了线程池的概念和特点，对线程池的结构体和相关操作接口进行了设计，并提供了接口的具体实现，最后通过示例程序演示了线程池的运行过程。