引言
线程池是一种多线程处理形式,处理过程中将任务添加到队列,然后在创建线程后自动启动这些任务。如果一个程序任务比较多，那么我们可以使用多线程来分担任务，但是并不是线程越多越好，虽然创建的线程被称为轻量级线程，但也占用资源。因此，需要线程池机制来维持整个程序的进行。
线程池机制
- 在一开始的时候，创建若干条线程，然后使其处于睡眠状态等待任务。
- 主线程接收任务，然后唤醒线程，并然其承担任务。
- 如果线程池中的线程数目不够，可以继续增加线程。如果线程数目太多，可以减少线程。
- 使用线程池保证进程中的线程数量处于一个恰当的数目。
线程池的原理
- 首先创建若干条线程，如图TID1，TID2，TID3
- 主线程领取任务，并且投放任务，使用链表将它们连接起来
- 主线程唤醒线程去领取
- 线程取得任务后并且执行任务
代码实现
main.c：
#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;
}
thread_pool.c
#include "thread_pool.h"
void handler(void *arg)
{
printf("[%u] is ended.\n",
(unsigned)pthread_self());
pthread_mutex_unlock((pthread_mutex_t *)arg);
}
void *routine(void *arg)
{
#ifdef DEBUG
printf("[%u] is started.\n",
(unsigned)pthread_self());
#endif
thread_pool *pool = (thread_pool *)arg;
struct task *p;
while(1)
{
pthread_cleanup_push(handler, (void *)&pool->lock);
pthread_mutex_lock(&pool->lock);
// 1, no task, and is NOT shutting down, then wait
while(pool->waiting_tasks == 0 && !pool->shutdown)
{
pthread_cond_wait(&pool->cond, &pool->lock);
}
// 2, no task, and is shutting down, then exit
if(pool->waiting_tasks == 0 && pool->shutdown == true)
{
pthread_mutex_unlock(&pool->lock);
pthread_exit(NULL); // CANNOT use 'break';
}
// 3, have some task, then consume it
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->do_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->max_waiting_tasks = MAX_WAITING_TASKS;
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;
}
#ifdef DEBUG
printf("[%u]:[%s] ==> tids[%d]: [%u] is created.\n",
(unsigned)pthread_self(), __FUNCTION__,
i, (unsigned)pool->tids[i]);
#endif
}
return true;
}
bool add_task(thread_pool *pool,
void *(*do_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->do_task = do_task;
new_task->arg = arg;
new_task->next = NULL;
//============ LOCK =============//
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++;
//=========== UNLOCK ============//
pthread_mutex_unlock(&pool->lock);
//===============================//
#ifdef DEBUG
printf("[%u][%s] ==> a new task has been added.\n",
(unsigned)pthread_self(), __FUNCTION__);
#endif
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");
// no threads has been created, return fail
if(actual_increment == 0)
return -1;
break;
}
actual_increment++;
#ifdef DEBUG
printf("[%u]:[%s] ==> tids[%d]: [%u] is created.\n",
(unsigned)pthread_self(), __FUNCTION__,
i, (unsigned)pool->tids[i]);
#endif
}
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 remaining_threads = pool->active_threads - removing_threads;
remaining_threads = remaining_threads > 0 ? remaining_threads : 1;
int i;
for(i=pool->active_threads-1; i>remaining_threads-1; i--)
{
errno = pthread_cancel(pool->tids[i]);
if(errno != 0)
break;
#ifdef DEBUG
printf("[%u]:[%s] ==> cancelling tids[%d]: [%u]...\n",
(unsigned)pthread_self(), __FUNCTION__,
i, (unsigned)pool->tids[i]);
#endif
}
if(i == pool->active_threads-1)
return -1;
else
{
pool->active_threads = i+1;
return i+1;
}
}
bool destroy_pool(thread_pool *pool)
{
// 1, activate all threads
pool->shutdown = true;
pthread_cond_broadcast(&pool->cond);
// 2, wait for their exiting
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]);
}
// 3, free memories
free(pool->task_list);
free(pool->tids);
free(pool);
return true;
}
thread_pool.h:
#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 *(*do_task)(void *arg);
void *arg;
struct task *next;
};
typedef struct thread_pool
{
pthread_mutex_t lock;
pthread_cond_t cond;
bool shutdown;
struct task *task_list;
pthread_t *tids;
unsigned max_waiting_tasks;
unsigned waiting_tasks;
unsigned active_threads;
}thread_pool;
bool init_pool(thread_pool *pool, unsigned int threads_number); //初始化线程
bool add_task(thread_pool *pool, void *(*do_task)(void *arg), void *task); //增加任务
int add_thread(thread_pool *pool, unsigned int additional_threads_number); //增加线程
int remove_thread(thread_pool *pool, unsigned int removing_threads_number);//移除线程
bool destroy_pool(thread_pool *pool); //销毁线程
void *routine(void *arg);
#endif
程序运行结果：
