Linux条件变量&线程池详解
一、条件变量
【互斥量】解决了线程间同步的问题,避免了多线程对同一块临界资源访问产生的冲突,但同一时刻对临界资源的访问,不论是生产者还是消费者,都需要竞争互斥锁,由此也带来了竞争的问题。即生产者和消费者、消费者和消费者之间时刻都在竞争这把锁,而临界资源是有限的,当临界资源为空的时候,消费者之间的竞争便没有意义,反而降低了运行效率。
有没有什么办法可以等生产者线程生产出资源,消费者线程再去竞争锁消费呢?这就是条件变量的作用。
正如互斥量保护了【临界资源】,条件变量也保护【条件】资源,当条件不符合时即【条件】资源空缺,消费者线程休眠等待;当【条件】资源产生,消费者线程被唤醒然后去消费资源。这样便解决了线程间等待的问题,提高了运行效率。
pthread_cond_t cond //定义条件变量
pthread_cond_init(&cond) //动态初始化
pthread_cond_t cond = PTHREAD_COND_INITIALIZER //静态创建并初始化
/*消费者线程休眠等待生产者线程通知*/
int pthread_cond_wait(pthread_cond_t *restrict cond,pthread_mutex_t *restrict mutex)
/*生产者线程生产完资源后发送通知*/
int pthread_cond_signal(pthread_cond_t *cond) //唤醒一个休眠的线程
int pthread_cond_broadcast(pthread_cond_t *cond) //广播唤醒所有休眠的线程
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条件变量的使用要绑定互斥锁
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因为条件变量的使用过程中,对于生产者线程,需要产生资源当然要上锁;对于消费者线程,需要消费资源甚至还要判断资源是否为空,也要上锁
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【wait】函数的具体动作
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进入等待的线程列表中休眠并释放锁
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被唤醒后返回,同时上锁
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wait函数的每个动作都是【原子操作】,动作连续并且不会被别的程序干扰,意味着CPU调度一定能保证动作粒一气呵成
消费线程不判断资源是否为空直接等待的话容易丢失signal信号(假设生产线程提前生产出资源也signal了)
唤醒也可以用broadcast,这种情况消费线程一定要判断资源是否为空,否则链表形式的资源容易出现【段错误】(因为多个线程去争抢资源时,总有抢不到的)
二、线程池
若干线程的集合,可用结构体来构造
必要性:当会出现大量执行时间短的任务时,甚至这个时间比线程创建+销毁的时间还短,这时候再创建多个线程就不划算了,可以未雨绸缪,构建线程池,提前创建多个线程来节省开销
线程池的实现过程
1.创建任务队列、线程池的结构体、定义一个线程池
#define pool_num (10)
typedef struct Task{
struct Task *next;
void *(*func)(void *);
void *arg;
}Task;
typedef struct{
pthread_mutex_t mutex;
pthread_cond_t cond;
Task *task_head;
int busywork;
pthread_t tid[pool_num];
}ThreadPool;
ThreadPool *pool;
2.初始化线程池、线程的工作
void pool_init()
{
pool = (ThreadPool *)malloc(sizeof(ThreadPool));
pthread_mutex_init(&pool->mutex,NULL);
pthread_cond_init(&pool->cond,NULL);
pool->task_head = NULL;
pool->busywork = 0;
for(i=0;i<pool_num;i++)
{
pthread_create(&pool->tid[i],NULL,workthread,NULL);
}
}
void *workthread(void *arg)
{
while(1)
{
//usleep(10000); //avoid other process couldn't rob the lock and several process repeatedly rob the lock
pthread_mutex_lock(&pool->mutex);
while(pool->task_head == NULL)
{
pthread_cond_wait(&pool->cond,&pool->mutex);
}
Task *ptask = pool->task_head;//任务取走线程池的头个线程
pool->task_head = ptask->next;//重置线程池的头个线程为下一个
pool->busywork--;//
pthread_mutex_unlock(&pool->mutex);
printf("%ld start task %d\n",pthread_self(),(int)ptask->arg);
ptask->func(ptask->arg);
}
}
3.添加任务
void *realwork(void *arg)
{
usleep(100000);
printf("finish task %d\n",(int)arg);
}
void add_task(int arg)
{
pthread_mutex_lock(&pool->mutex);
while(pool->busywork >= pool_num)
{
pthread_mutex_unlock(&pool->mutex);
usleep(10000);
pthread_mutex_lock(&pool->mutex);
}
pthread_mutex_unlock(&pool->mutex);
//以上为判断任务数量是否超过线程池数量,超过则不再允许添加任务
Task *newtask;
newtask = (Task *)malloc(sizeof(Task));
newtask->func = realwork;
newtask->arg = (void *)arg;
pthread_mutex_lock(&pool->mutex);
Task *p = pool->task_head;
if(p == NULL)
{
pool->task_head = newtask;
}else
{
while(p->next != NULL)
{
p = p->next;
}
p->next = newtask;
pthread_cond_signal(&pool->cond);
pool->busywork++;
}
pthread_mutex_unlock(&pool->mutex);
}
4.销毁线程池、任务队列、互斥锁、条件变量
void pool_destroy()
{
Task *p;
while(pool->task_head != NULL)
{
p = pool->task_head;
pool->task_head = p->next;
free(p);
}
pthread_mutex_destroy(&pool->mutex);
pthread_cond_destroy(&pool->cond);
free(pool);
}
示例
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#define pool_num (10)
int i = 0;
typedef struct Task{
struct Task *next;
void *(*func)(void *);
void *arg;
}Task;
typedef struct{
pthread_mutex_t mutex;
pthread_cond_t cond;
Task *task_head;
int busywork;
pthread_t tid[pool_num];
}ThreadPool;
ThreadPool *pool;
void *workthread(void *arg)
{
while(1)
{
//usleep(10000); //avoid other process couldn't rob the lock and several process repeatedly rob the lock
pthread_mutex_lock(&pool->mutex);
while(pool->task_head == NULL)
{
pthread_cond_wait(&pool->cond,&pool->mutex);
}
Task *ptask = pool->task_head;//任务取走线程池的头个线程
pool->task_head = ptask->next;//重置线程池的头个线程为下一个
pool->busywork--;//
pthread_mutex_unlock(&pool->mutex);
printf("%ld start task %d\n",pthread_self(),(int)ptask->arg);
ptask->func(ptask->arg);
}
}
void *realwork(void *arg)
{
usleep(100000);
printf("finish task %d\n",(int)arg);
}
void add_task(int arg)
{
pthread_mutex_lock(&pool->mutex);
while(pool->busywork >= pool_num)
{
pthread_mutex_unlock(&pool->mutex);
usleep(10000);
pthread_mutex_lock(&pool->mutex);
}
pthread_mutex_unlock(&pool->mutex);
Task *newtask;
newtask = (Task *)malloc(sizeof(Task));
newtask->func = realwork;
newtask->arg = (void *)arg;
pthread_mutex_lock(&pool->mutex);
Task *p = pool->task_head;
if(p == NULL)
{
pool->task_head = newtask;
}else
{
while(p->next != NULL)
{
p = p->next;
}
p->next = newtask;
pthread_cond_signal(&pool->cond);
pool->busywork++;
}
pthread_mutex_unlock(&pool->mutex);
}
void pool_init()
{
pool = (ThreadPool *)malloc(sizeof(ThreadPool));
pthread_mutex_init(&pool->mutex,NULL);
pthread_cond_init(&pool->cond,NULL);
pool->task_head = NULL;
pool->busywork = 0;
for(i=0;i<pool_num;i++)
{
pthread_create(&pool->tid[i],NULL,workthread,NULL);
}
}
void pool_destroy()
{
Task *p;
while(pool->task_head != NULL)
{
p = pool->task_head;
pool->task_head = p->next;
free(p);
}
pthread_mutex_destroy(&pool->mutex);
pthread_cond_destroy(&pool->cond);
free(pool);
}
int main()
{
printf("mypid is %d\n",getpid());
pool_init();
sleep(5);
for(i=1;i<=40;i++)
{
add_task(i);
}
sleep(5);
pool_destroy();
while(1)
{
printf("now im alone\n");
sleep(2);
}
}
原文地址:https://blog.csdn.net/2301_79532069/article/details/144222098
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