什么是惊群现象?
惊群（thundering herd）是指，只有一个子进程能获得连接，但所有N个子进程却都被唤醒了，这种情况将使性能受损。
举一个很简单的例子，当你往一群鸽子中间扔一块食物，虽然最终只有一个鸽子抢到食物，但所有鸽子都会被惊动来争夺，没有抢到食物的鸽子只好回去继续睡觉， 等待下一块食物到来。这样，每扔一块食物，都会惊动所有的鸽子，即为惊群。对于操作系统来说，多个进程/线程在等待同一资源时，也会产生类似的效果，其结 果就是每当资源可用，所有的进程/线程都来竞争资源，造成的后果：
1）系统对用户进程/线程频繁的做无效的调度、上下文切换，系统系能大打折扣。
2）为了确保只有一个线程得到资源，用户必须对资源操作进行加锁保护，进一步加大了系统开销。
最常见的例子就是对于socket描述符的accept操作，当多个用户进程/线程监听在同一个端口上时，由于实际只可能accept一次，因此就会产生惊群现象.这个问题是一个古老的问题，新的操作系统内核已经解决了这一问题。
在多线程情况下,每个线程都监听同一个fd,当有数据来的时候,是否会有惊群现象呢?验证如下
服务器端代码
//g++ -g libevent_server.cpp -o libevent_server -levent -lpthread//说明:服务器监听在本地19870端口, 等待udp client连接,有惊群现象: 当有数据到来时, 每个线程都被唤醒, 但是只有一个线程可以读到数据//#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#include <pthread.h>#include <event.h>#include <netinet/in.h>#include <sys/types.h>#include <sys/socket.h>#include <sys/time.h>#include <sys/types.h>#include <sys/socket.h>#include <arpa/inet.h>using namespace std;int init_count = 0;pthread_mutex_t init_lock;pthread_cond_t init_cond;typedef struct {pthread_t thread_id; /* unique ID of this thread */struct event_base *base; /* libevent handle this thread uses */struct event notify_event; /* listen event for notify pipe */} mythread;void *worker_libevent(void *arg){mythread *p = (mythread *)arg;pthread_mutex_lock(&init_lock);init_count++;pthread_cond_signal(&init_cond);pthread_mutex_unlock(&init_lock);event_base_loop(p->base, 0);}int create_worker(void*(*func)(void *), void *arg){mythread *p = (mythread *)arg;pthread_t tid;pthread_attr_t attr;pthread_attr_init(&attr);pthread_create(&tid, &attr, func, arg);p->thread_id = tid;pthread_attr_destroy(&attr);return 0;}void process(int fd, short which, void *arg){mythread *p = (mythread *)arg;printf("I am in the thread: [%lu]\n", p->thread_id);char buffer[100];memset(buffer, 0, 100);int ilen = read(fd, buffer, 100);printf("read num is: %d\n", ilen);printf("the buffer: %s\n", buffer);}//设置libevent事件回调int setup_thread(mythread *p, int fd){p->base = event_init();event_set(&p->notify_event, fd, EV_READ|EV_PERSIST, process, p);event_base_set(p->base, &p->notify_event);event_add(&p->notify_event, 0);return 0;}int main(){struct sockaddr_in in;int fd;fd = socket(AF_INET, SOCK_DGRAM, 0);//在127.0.0.1:19870处监听struct in_addr s;bzero(&in, sizeof(in));in.sin_family = AF_INET;inet_pton(AF_INET, "127.0.0.1", (void *)&s);in.sin_addr.s_addr = s.s_addr;in.sin_port = htons(19870);bind(fd, (struct sockaddr*)&in, sizeof(in));int threadnum = 10; //创建10个线程int i;pthread_mutex_init(&init_lock, NULL);pthread_cond_init(&init_cond, NULL);mythread *g_thread;g_thread = (mythread *)malloc(sizeof(mythread)*10);for(i=0; i<threadnum; i++){ //10个线程都监听同一个socket描述符, 检查是否产生惊群现象?setup_thread(&g_thread[i], fd);}for(i=0; i<threadnum; i++){create_worker(worker_libevent, &g_thread[i]);}//master线程等待worker线程池初始化完全pthread_mutex_lock(&init_lock);while(init_count < threadnum){pthread_cond_wait(&init_cond, &init_lock);}pthread_mutex_unlock(&init_lock);printf("IN THE MAIN LOOP\n");while(1){sleep(1);}//没有回收线程的代码free(g_thread);return 0;}
客户端代码
//g++ -g libevent_client.cpp -o libevent_client//#include <iostream>#include <string.h>#include <sys/types.h>#include <sys/socket.h>#include <sys/time.h>#include <netinet/in.h>#include <sys/types.h>#include <sys/socket.h>#include <arpa/inet.h>using namespace std;int main(){struct sockaddr_in in;int fd;fd = socket(AF_INET, SOCK_DGRAM, 0);struct in_addr s;bzero(&in, sizeof(in));in.sin_family = AF_INET;inet_pton(AF_INET, "127.0.0.1", (void *)&s);in.sin_addr.s_addr = s.s_addr;in.sin_port = htons(19870);string str = "I am Michael";sendto(fd, str.c_str(), str.size(), 0, (struct sockaddr *)&in, sizeof(struct sockaddr_in));return 0;}
测试效果图
参考文献
[1]./uid-26575352-id-3075103.html
[2]./nanjunxiao/article/details/9140769
[3]./leyni@126/blog/static/1622301022611523786/
[4]./blog/658012
