linux 条件变量与线程池
条件变量Condition Variables
概述
1. 条件变量提供了另外一种线程同步的方式。如果没有条件变量，程序需要使用线程连续轮询（可能在临界区critical section内）方式检查条件是否满足。由于线程连续忙于轮询检查，这会非常消耗资源，而条件变量是一种实现同样目标不需要轮询的方式。
2. 条件变量总是和互斥锁相结合使用。
3. 条件变量使用示例结构：
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Main Thread
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Thread A
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Thread B
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Main Thread Join / Continue |
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创建和销毁条件变量
pthread_cond_init (condition,attr)
pthread_cond_destroy (condition)
pthread_condattr_init (attr)
pthread_condattr_destroy (attr)
条件变量必须声明为pthread_cond_t，并且使用之前必须初始化。有两种方式初始化条件变量：
1）静态初始化：pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;
2）动态初始化： pthread_cond_init()。条件变量的id号通过条件变量参数返回于调用线程，这种方式允许设置条件变量的属性。然而，只有一种条件变量属性process-shared，这允许其他进程的线程可见该条件变量。如果使用条件变量属性，那么必须是pthread_condattr_t 类型（为了接受默认值可以指定为NULL）。需要注意的是，并非所有实现提供process-shared属性。
信号等待与信号通知
pthread_cond_wait (condition,mutex):阻塞调用线程直到特定的条件触发。当互斥量被锁住时该函数应当被调用；当它等待时它将自动释放互斥锁。接收到信号通知和线程被唤醒后，互斥量将自动地被线程锁住。当线程完成任务时，需要手动解锁互斥量。
pthread_cond_signal (condition)用于唤醒另外一个等待条件变量的线程。互斥量被锁住之后才可调用pthread_cond_signal并且按序解锁用于pthread_cond_wait完成。
pthread_cond_broadcast (condition)如果多于一个线程处于阻塞等待状态，那么应当使用pthread_cond_broadcast而不是pthread_cond_signal。
建议使用while循环而不是if，这样可以检查一些潜在的问题，例如：如果若干线程在等待同一个唤醒信号，它们将轮询捕获互斥量，它们中的任何一个可以修改条件；由于程序bug，线程接收到错误信号；线程库允许不违反标准的前提下虚假的唤醒一个等待线程。
使用这些函数时，必须正确地加锁解锁互斥变量。
调用pthread_cond_wait前锁定互斥量失败可能导致线程阻塞失败；
调用pthread_cond_signal后解锁互斥量失败可能不允许匹配的pthread_cond_wait完成（即阻塞掉）。
实际上pthread_cond_wait的返回不仅仅是pthread_cond_signal和pthread_cond_broadcast导致的，还会有一些假唤醒，也就是spurious wakeup。
pthread_cond_wait的通常使用方法：
pthread_mutex_lock();
while(condition_is_false)
pthread_cond_wait();
pthread_mutex_unlock();
为什么在pthread_cond_wait()前要加一个while循环来判断条件是否为假呢？
APUE中写道:
传递给pthread_cond_wait的互斥量对条件进行保护，调用者把锁住的互斥量传给函数。函数把调用线程放到等待条件的线程列表上，然后对互斥量解锁，这两个操作是原子操作。
线程释放互斥量，等待其他线程发给该条件变量的信号（唤醒一个等待者）或广播该条件变量（唤醒所有等待者）。当等待条件变量时，互斥量必须始终为释放的，这样其他线程才有机会锁住互斥量，修改条件变量。当线程从条件变量等待中醒来时，它重新继续锁住互斥量，对临界资源进行处理。
条件变量的作用是发信号，而不是互斥。
wait前检查
对于多线程程序，不能够用常规串行的思路来思考它们，因为它们是完全异步的，会出现很多临界情况。比如：pthread_cond_signal的时间早于pthread_cond_wait的时间，这样pthread_cond_wait就会一直等下去，漏掉了之前的条件变化。
对于这种情况，解决的方法是在锁住互斥量之后和等待条件变量之前，检查条件变量是否已经发生变化。
if(condition_is_false)
pthread_cond_wait();
这样在等待条件变量前检查一下条件变量的值，如果条件变量已经发生了变化，那么就没有必要进行等待了，可以直接进行处理。这种方法在并发系统中比较常见
1.等待函数里面要传入一个互斥量，这个互斥量会在这个函数调用时会发生如下变化：函数刚刚被调用时，会把这个互斥量解锁，然后让调用线程阻塞，解锁后其他线程才有机会获得这个锁。当某个线程调用通知函数时，这个函数收到通知后，又把互斥量加锁，然后继续向下操作临界区。可见这个设计是非常合理的！
2.条件变量的等待函数用while循环包围。原因：如果有多个线程都在等待这个条件变量关联的互斥量，当条件变量收到通知，它下一步就是要锁住这个互斥量，但在这个极小的时间差里面，其他线程抢先获取了这互斥量并进入临界区把某个状态改变了。此时这个条件变量应该继续判断别人刚刚抢先修改的状态，即继续执行while的判断。还有一个原因时防止虚假通知，收到虚假通知后，只要while里面的条件为真，就继续休眠.
参考资料：https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal
http://www.cnblogs.com/leaven/archive/2010/06/03/1750973.html
https://www.cnblogs.com/yuuyuu/p/5140875.html
linux下C 线程池的原理讲解和代码实现（能自行伸缩扩展线程数）
Linux C++线程池框架
Linux的多任务编程-线程池
1 #include <pthread.h> 2 #include <stdio.h> 3 #include <stdlib.h> 4 #include <unistd.h> 5 #define NUM_THREADS 3 6 #define TCOUNT 10 7 #define COUNT_LIMIT 12 8 9 int count = 0; 10 int thread_ids[3] = {0,1,2}; 11 pthread_mutex_t count_mutex; 12 pthread_cond_t count_threshold_cv; 13 14 void *inc_count(void *t) 15 { 16 int i; 17 long my_id = (long)t; 18 19 for (i=0; i<TCOUNT; i++) { 20 pthread_mutex_lock(&count_mutex); 21 count++; 22 23 /* 24 Check the value of count and signal waiting thread when condition is 25 reached. Note that this occurs while mutex is locked. 26 */ 27 if (count == COUNT_LIMIT) { 28 pthread_cond_signal(&count_threshold_cv); 29 printf("inc_count(): thread %ld, count = %d Threshold reached.\n", 30 my_id, count); 31 } 32 printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", 33 my_id, count); 34 pthread_mutex_unlock(&count_mutex); 35 36 /* Do some "work" so threads can alternate on mutex lock */ 37 sleep(1); 38 } 39 pthread_exit(NULL); 40 } 41 42 void *watch_count(void *t) 43 { 44 long my_id = (long)t; 45 46 printf("Starting watch_count(): thread %ld\n", my_id); 47 48 /* 49 Lock mutex and wait for signal. Note that the pthread_cond_wait 50 routine will automatically and atomically unlock mutex while it waits. 51 Also, note that if COUNT_LIMIT is reached before this routine is run by 52 the waiting thread, the loop will be skipped to prevent pthread_cond_wait 53 from never returning. 54 */ 55 pthread_mutex_lock(&count_mutex); 56 while (count<COUNT_LIMIT) { 57 pthread_cond_wait(&count_threshold_cv, &count_mutex); 58 printf("watch_count(): thread %ld Condition signal received.\n", my_id); 59 } 60 count += 125; 61 printf("watch_count(): thread %ld count now = %d.\n", my_id, count); 62 pthread_mutex_unlock(&count_mutex); 63 pthread_exit(NULL); 64 } 65 66 int main (int argc, char *argv[]) 67 { 68 int i, rc; 69 long t1=1, t2=2, t3=3; 70 pthread_t threads[3]; 71 pthread_attr_t attr; 72 73 /* Initialize mutex and condition variable objects */ 74 pthread_mutex_init(&count_mutex, NULL); 75 pthread_cond_init (&count_threshold_cv, NULL); 76 77 /* For portability, explicitly create threads in a joinable state */ 78 pthread_attr_init(&attr); 79 pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); 80 81 pthread_create(&threads[0], &attr, watch_count, (void *)t1); 82 pthread_create(&threads[1], &attr, inc_count, (void *)t2); 83 pthread_create(&threads[2], &attr, inc_count, (void *)t3); 84 85 /* Wait for all threads to complete */ 86 for (i=0; i<NUM_THREADS; i++) { 87 pthread_join(threads[i], NULL); 88 } 89 printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS); 90 91 /* Clean up and exit */ 92 pthread_attr_destroy(&attr); 93 pthread_mutex_destroy(&count_mutex); 94 pthread_cond_destroy(&count_threshold_cv); 95 pthread_exit(NULL); 96 97 }
翻译资料：https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal
线程池
上面之所以会谈到条件变量，有两个原因，其一线程池的实现需要条件变量方面的知识，其二因为它的实现牵涉到一些细节，理解条件变量有一定的困难，如果不理解它与互斥锁结合使用的实现原理，也就无法正确使用条件变量。
#ifndef THREADPOOL_H #define THREADPOOL_H /* *线程池包括：n个执行任务的线程，一个任务队列，一个管理线程 1、预先启动一些线程，线程负责执行任务队列中的任务，当队列空时，线程挂起。 2、调用的时候，直接往任务队列添加任务，并发信号通知线程队列非空。 3、管理线程负责监控任务队列和系统中的线程状态，当任务队列为空，线程数目多且很多处于空闲的时候，便通知一些线程退出以节约系统资源；当任务队列排队任务多且线程都在忙，便负责再多启动一些线程来执行任务，以确保任务执行效率。 * */ #include <pthread.h> typedef struct threadpool_task_t { void *(*function)(void *); void *arg; } threadpool_task_t; typedef 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;//任务队列非空的信号 pthread_t *threads;//执行任务的线程 pthread_t adjust_tid;//负责管理线程数目的线程 threadpool_task_t *task_queue;//任务队列 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; }threadpool_t; threadpool_t *threadpool_create(int min_thr_num, int max_thr_num, int queue_max_size); int threadpool_add(threadpool_t *pool, void*(*function)(void *arg), void *arg); /** * @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_destroy(threadpool_t *pool); int threadpool_free(threadpool_t *pool); int threadpool_all_threadnum(threadpool_t *pool); int threadpool_busy_threadnum(threadpool_t *pool); #endif // THREADPOOL_H
#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 <stdbool.h> #include "threadpool.h" #define DEFAULT_TIME 10 // 领导定时检查队列、线程状态的时间间隔 #define MIN_WAIT_TASK_NUM 10 // 队列中等待的任务数>这个值，便会增加线程 #define DEFAULT_THREAD_VARY 10 //每次线程加减的数目 //创建线程池 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++) { //启动任务线程 pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void *)pool); printf("start thread 0x%x...\n", pool->threads[i]); } //启动管理线程 pthread_create(&(pool->adjust_tid), NULL, adjust_thread, (void *)pool); return pool; }while(0); threadpool_free(pool); return NULL; } //把任务添加到队列中 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)); //队列满的时候，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)); } //如下是添加任务到队列，使用循环队列 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++; //每次加完任务，发个信号给线程 //若没有线程处于等待状态，此句则无效，但不影响 pthread_cond_signal(&(pool->queue_not_empty)); pthread_mutex_unlock(&(pool->lock)); return 0; } //线程执行任务 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)); //任务队列为空的时候，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)); //被唤醒后，判断是否是要退出的线程 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); } //管理线程 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)); //任务多线程少，增加线程 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)); } //任务少线程多，减少线程 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)); } } } return NULL; } 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); return 0; }
