惊群效应测试

怼烎@ 2022-08-18 15:27 176阅读 0赞

**概念：**

惊群效应就是当一个fd的事件被触发时，所有等待这个fd的线程或进程都被唤醒，一般都是socket的accept()会导致惊群，很多个进程都阻塞在server socket的accept()，一但有客户端连接，所有进程的accept()都会返回，但是只有一个进程会accept成功，就是惊群。

**现状：**

linux2.6内核后的版本已经没有惊群效应

**测试：**

**测试环境内核版本**

![20151228104819377][]

**客户端使用脚本 client.sh**

#!/bin/bash
    host="127.0.0.1"
    port=9527
    num=10
    for (( i=0; i<$num; i++)); do
        telnet $host $port > /dev/null 2>&1 &
    done

**非IO多路复用，测试代码 tcpserver.cpp**

#include <stdio.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <stdlib.h>
    #include <unistd.h>
    
    int main()
    {
        int listenfd;
        pid_t parentid, childid;
        struct sockaddr_in servaddr;
        int forknum = 10;
        short port = 9527;
    
        servaddr.sin_family = AF_INET;
        servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
        servaddr.sin_port = htons(port);
    
        listenfd = socket(AF_INET, SOCK_STREAM, 0);
        int res = bind(listenfd, (sockaddr *)&servaddr, sizeof(sockaddr_in));
        if (0 == res)
            printf("server bind success, port:%d\n", port);
        else {
            printf("server bind fail!\n");
            exit(-1);
        }
        res = listen(listenfd, 100);
        if (0 == res)
            printf("server listen success!\n");
        else {
            printf("server listen fail!\n");
            exit(-1);
        }
    
        parentid = getpid();
        for (int i=0; i<forknum; ++i) {
            if (getpid() == parentid) {
                childid = fork();
                if (0 == childid)
                    printf("the parentid is %d, chlidid is %d\n", getppid(), getpid());
            }
        }
    
        for (; ;) {
            int connfd = accept(listenfd, NULL, NULL);
            if (connfd != -1)
                printf("the pid is %d, connfd is %d\n", getpid(), connfd);
        }
    }

**启动进程，客户端通过telnet连接了3次，输出连接成功3次：**

**![20151228104354900][]**

**查看具体的fd:**

**![20151228104402990][]**

**epoll，测试代码 epollserver.cpp**

#include <stdio.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/epoll.h>
    
    #define MAX_EVENTS 10
    
    int main()
    {
        int listenfd;
        pid_t parentid, childid;
        struct sockaddr_in servaddr;
        int forknum = 10;
        short port = 9527;
    
        servaddr.sin_family = AF_INET;
        servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
        servaddr.sin_port = htons(port);
    
        listenfd = socket(AF_INET, SOCK_STREAM, 0);
        int res = bind(listenfd, (sockaddr *)&servaddr, sizeof(sockaddr_in));
        if (0 == res)
            printf("server bind success, 0.0.0.0:%d\n", port);
        else {
            perror("bind fail");
            exit(EXIT_FAILURE);
        }
        res = listen(listenfd, 100);
        if (0 == res)
            printf("server listen success\n");
        else {
            perror("listen fail");
            exit(EXIT_FAILURE);
        }
        
        parentid = getpid();
        for (int i=0; i<forknum; ++i) {
            if (getpid() == parentid) {
                childid = fork();
                if (0 == childid)
                    printf("the parentid is %d, chlidid is %d\n", getppid(), getpid());
            }
        }
    
        // epoll
        struct epoll_event ev, events[MAX_EVENTS];
        int epollfd = epoll_create(10);
        if (-1 == epollfd) {
            perror("epoll_create fail");
            exit(EXIT_FAILURE);
        }
    
        ev.events = EPOLLIN;
        ev.data.fd = listenfd;
        if (-1 == epoll_ctl(epollfd, EPOLL_CTL_ADD, listenfd, &ev)) {
            perror("epoll_ctl: listenfd fail");
            exit(EXIT_FAILURE);
        }
    
        for (;;) {
            int nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
            if (-1 == nfds) {
                perror("epoll_wait fail");
                exit(EXIT_FAILURE);
            }
            for (int n = 0; n < nfds; ++n) {
                if (events[n].data.fd == listenfd) {
                    int connfd = accept(listenfd, NULL, NULL);
                    if (-1 == connfd) {
                        perror("accept fail");
                        continue;
                    }
                    printf("the pid is %d, connfd is %d\n", getpid(), connfd);
                }
            }
        }
    
    }

启动进程并执行client.sh

**![20151228231331187][]**

**初步结论：**

**linux比较新的内核（2.6+）已经不存在惊群效应**

[20151228104819377]: /images/20220731/36f97f5b28784957b756307e117a7ec9.png
[20151228104354900]: /images/20220731/3dee28d36b9d4c2ab7800aeee0d2e550.png
[20151228104402990]: /images/20220731/6582858887474727b809b7cf03bd8411.png
[20151228231331187]: /images/20220731/f8ab208ab185449c817b006ab18e68a5.png