什麼線程池，爲什麼要使用線程池？下面是一個比喻。
階段一、一個醫院，每天面對成千上萬的病人，處理方式是：來一個病人找來一個醫生處理，處理完了醫生也走了。當看病時間較短的時候，醫生來去的時間，顯得尤爲費時了。
階段二、醫院引進了線程池的概念。設置門診，把醫生全派出去坐診，病人來看病先掛號排隊，醫生根據病人隊列順序依次處理各個病人，這樣就省去醫生來來去去的時間了。但是，很多時候病人不多，醫生卻很多導致很多醫生空閒浪費水電資源撒。
階段三、醫院引進了可伸縮性線程池的概念，如階段二，但是門診一開始只派出了部分醫生，但是增加了一個領導，病人依舊是排隊看病，領導負責協調整個醫院的醫生。當病人很多醫生忙不過來的時候，領導就去多叫幾個醫生來幫忙；當病人不多醫生太多的時候，領導就叫一些醫生回家休息去免得浪費醫院資源。
階段三就是一個線程池的例子。
線程池包括：n個執行任務的線程，一個任務隊列，一個管理線程
1、預先啓動一些線程，線程負責執行任務隊列中的任務，當隊列空時，線程掛起。
2、調用的時候，直接往任務隊列添加任務，並發信號通知線程隊列非空。
3、管理線程負責監控任務隊列和系統中的線程狀態，當任務隊列爲空，線程數目多且很多處於空閒的時候，便通知一些線程退出以節約系統資源；當任務隊列排隊任務多且線程都在忙，便負責再多啓動一些線程來執行任務，以確保任務執行效率。
下面是代碼（下載附件）：運行環境Ubuntu 12.04
#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 // 領導定時檢查隊列、線程狀態的時間間隔
#define MIN_WAIT_TASK_NUM 10 // 隊列中等待的任務數>這個值，便會增加線程
#define DEFAULT_THREAD_VARY 10 //每次線程加減的數目
typedef struct
{
void *(*function)(void *);
void *arg;
} threadpool_task_t;
struct threadpool_t
{
pthread_mutex_t lock;// mutex for the taskpool
pthread_mutex_t thread_counter;//mutex for count the busy thread
pthread_cond_t queue_not_full;
pthread_cond_t queue_not_empty;//任務隊列非空的信號
pthread_t *threads;//執行任務的線程
pthread_t adjust_tid;//負責管理線程數目的線程
threadpool_task_t *task_queue;//任務隊列
int min_thr_num;
int max_thr_num;
int live_thr_num;
int busy_thr_num;
int wait_exit_thr_num;
int queue_front;
int queue_rear;
int queue_size;
int queue_max_size;
bool shutdown;
};
/**
* @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
*/
bool is_thread_alive(pthread_t tid);
int threadpool_free(threadpool_t *pool);
//創建線程池
threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size)
{
threadpool_t *pool = NULL;
do{
if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL)
{
printf("malloc threadpool fail");
break;
}
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;
}
/**
* start work thread min_thr_num
*/
for (int i = 0; i < min_thr_num; i++)
{
//啓動任務線程
pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool);
printf("start thread 0x%x...\n", pool->threads[i]);
}
//啓動管理線程
pthread_create(&(pool->adjust_tid), NULL, adjust_thread, (void *)pool);
return pool;
}while(0);
threadpool_free(pool);
return NULL;
}
//把任務添加到隊列中
int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg)
{
assert(pool != NULL);
assert(function != NULL);
assert(arg != NULL);
pthread_mutex_lock(&(pool->lock));
//隊列滿的時候，等待
while ((pool->queue_size == pool->queue_max_size) && (!pool->shutdown))
{
//queue full wait
pthread_cond_wait(&(pool->queue_not_full), &(pool->lock));
}
if (pool->shutdown)
{
pthread_mutex_unlock(&(pool->lock));
}
//如下是添加任務到隊列，使用循環隊列
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++;
//每次加完任務，發個信號給線程
//若沒有線程處於等待狀態，此句則無效，但不影響
pthread_cond_signal(&(pool->queue_not_empty));
pthread_mutex_unlock(&(pool->lock));
return 0;
}
//線程執行任務
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 */
pthread_mutex_lock(&(pool->lock));
//任務隊列爲空的時候，等待
while ((pool->queue_size == 0) && (!pool->shutdown))
{
printf("thread 0x%x is waiting\n", pthread_self());
pthread_cond_wait(&(pool->queue_not_empty), &(pool->lock));
//被喚醒後，判斷是否是要退出的線程
if (pool->wait_exit_thr_num > 0)
{
pool->wait_exit_thr_num--;
if (pool->live_thr_num > pool->min_thr_num)
{
printf("thread 0x%x is exiting\n", pthread_self());
pool->live_thr_num--;
pthread_mutex_unlock(&(pool->lock));
pthread_exit(NULL);
}
}
}
if (pool->shutdown)
{
pthread_mutex_unlock(&(pool->lock));
printf("thread 0x%x is exiting\n", pthread_self());
pthread_exit(NULL);
}
//get a task from queue
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--;
//now queue must be not full
pthread_cond_broadcast(&(pool->queue_not_full));
pthread_mutex_unlock(&(pool->lock));
// Get to work
printf("thread 0x%x start working\n", pthread_self());
pthread_mutex_lock(&(pool->thread_counter));
pool->busy_thr_num++;
pthread_mutex_unlock(&(pool->thread_counter));
(*(task.function))(task.arg);
// task run over
printf("thread 0x%x end working\n", pthread_self());
pthread_mutex_lock(&(pool->thread_counter));
pool->busy_thr_num--;
pthread_mutex_unlock(&(pool->thread_counter));
}
pthread_exit(NULL);
return (NULL);
}
//管理線程
void *adjust_thread(void *threadpool)
{
threadpool_t *pool = (threadpool_t *)threadpool;
while (!pool->shutdown)
{
sleep(DEFAULT_TIME);
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));
//任務多線程少，增加線程
if (queue_size >= MIN_WAIT_TASK_NUM
&& live_thr_num < pool->max_thr_num)
{
//need add thread
pthread_mutex_lock(&(pool->lock));
int add = 0;
for (int 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));
}
//任務少線程多，減少線程
if ((busy_thr_num * 2) < live_thr_num
&& live_thr_num > pool->min_thr_num)
{
//need del thread
pthread_mutex_lock(&(pool->lock));
pool->wait_exit_thr_num = DEFAULT_THREAD_VARY;
pthread_mutex_unlock(&(pool->lock));
//wake up thread to exit
for (int i = 0; i < DEFAULT_THREAD_VARY; i++)
{
pthread_cond_signal(&(pool->queue_not_empty));
}
}
}
}
int threadpool_destroy(threadpool_t *pool)
{
if (pool == NULL)
{
return -1;
}
pool->shutdown = true;
//adjust_tid exit first
pthread_join(pool->adjust_tid, NULL);
// wake up the waiting thread
pthread_cond_broadcast(&(pool->queue_not_empty));
for (int i = 0; i < pool->min_thr_num; i++)
{
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;
}
bool is_thread_alive(pthread_t tid)
{
int kill_rc = pthread_kill(tid, 0);
if (kill_rc == ESRCH)
{
return false;
}
return true;
}
// for test
//void *process(void *arg)
//{
//printf("thread 0x%x working on task %d\n ",pthread_self(),*(int *)arg);
//sleep(1);
//printf("task %d is end\n",*(int *)arg);
//return NULL;
//}
//int main()
//{
//threadpool_t *thp = threadpool_create(3,100,12);
//printf("pool inited");
//
//int *num = (int *)malloc(sizeof(int)*20);
//for (int 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);
//}