TCP服务器不能接受超过1000的客户端连接？

遇到一个奇怪的问题，当我手动运行一个TCP程序(./tcpServDemo)时，tcp Server程序可以接收超过5000个客户端连接，但是当我在后台运行tcpServerDemo时(systemctl start tcpServDemo.service)，却只能接收超过 900 个客户端连接，

当我调试时，我发现 TCP recv-Q 队列已满。我修改了TCP参数(net.core.somaxconn = 65500 net.ipv4.tcp_max_syn_backlog = 409600)，但是还是不行

我已经调试了几天。真不知道问题出在哪里？谁能帮帮我，谢谢大家！

操作系统：CentOS 7.9

tcpClient.c:
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include <netdb.h>
#include <netinet/in.h>
#include <pthread.h>

//#define SERVPORT 8088
 
int ServPort;
char ServerIP[32];

pthread_mutex_t lock;
int count;

void *cTcpConn(void *arg) {
    usleep(2000);
    int sockfd,sendbytes;
    struct sockaddr_in serv_addr;//需要连接的服务器地址信息
    memset(&serv_addr, 0, sizeof(struct sockaddr_in));

    //1.创建socket
    //AF_INET 表示IPV4
    //SOCK_STREAM 表示TCP
    if((sockfd = socket(AF_INET,SOCK_STREAM,0)) < 0) {
        printf("socket create failed.");
        return NULL;
    }


    //填充服务器地址信息
    serv_addr.sin_family    = AF_INET; //网络层的IP协议: IPV4
    serv_addr.sin_port      = htons(ServPort); //传输层的端口号
    serv_addr.sin_addr.s_addr   = inet_addr(ServerIP); //网络层的IP地址: 实际的服务器IP地址

    //2.发起对服务器的连接信息
    //三次握手,需要将sockaddr_in类型的数据结构强制转换为sockaddr
    if((connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(struct sockaddr))) < 0) {
        printf("connect failed!\n");
        close(sockfd);
        return NULL;
    } else {
        pthread_mutex_lock(&lock);
        count++;
        pthread_mutex_unlock(&lock);
        printf("connect successful! count:%d\n", count);
    }


#if 0
    //3.发送消息给服务器端
    if((sendbytes = send(sockfd,"hello",5,0)) < 0) {
        perror("send");
        exit(1);
    }
#endif

    while(1) {
        sleep(10);
    }

    //4.关闭
    close(sockfd);

    return NULL;
}

int main(int argc, char **argv) {
    if (argc != 3) {
        printf("Usage: %s ServerIP ServerPort\n", argv[0]);
        return 0;
    }

    strncpy(ServerIP, argv[1], sizeof(ServerIP) - 1);
    ServPort = atoi(argv[2]);
    

    int i;
    pthread_t pid;

    for (i = 0; i < 8000; i++) {
        usleep(10000);
        if(0 != pthread_create(&pid, NULL, cTcpConn, NULL)) {
            printf("thread create failed.\n");
        }
    }

    while (1) {
        sleep(10);
    }
    return 0;
}

tcpServDemo.c:

#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/epoll.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>


int main() {
    const int EVENTS_SIZE = 4096;
    char buff[1024];
    int eNum;

    int socketFd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    struct sockaddr_in sockAddr;
    sockAddr.sin_port = htons(8088);
    sockAddr.sin_family = AF_INET;
    sockAddr.sin_addr.s_addr = htons(INADDR_ANY);

    if (bind(socketFd, (struct sockaddr *) &sockAddr, sizeof(sockAddr)) == -1) {
        return -1;
    }

    if (listen(socketFd, 10) == -1) {
        return -1;
    }

    int eFd = epoll_create(1);

    struct epoll_event epev;
    epev.events = EPOLLIN;
    epev.data.fd = socketFd;
    epoll_ctl(eFd, EPOLL_CTL_ADD, socketFd, &epev);
    
    int i;
    int count = 0;
    struct epoll_event events[EVENTS_SIZE];

    while (1) {
        eNum = epoll_wait(eFd, events, EVENTS_SIZE, -1);

        if (eNum == -1) {
            return -1;
        }
        for (i = 0; i < eNum; i++) {
            if (events[i].data.fd == socketFd) {
                if (events[i].events & EPOLLIN) {
                    struct sockaddr_in cli_addr;
                    socklen_t length = sizeof(cli_addr);
                    int fd = accept(socketFd, (struct sockaddr *) &cli_addr, &length);
                    if (fd > 0) {
                        count++;
                        epev.events = EPOLLIN | EPOLLET;
                        epev.data.fd = fd;

                        int flags = fcntl(fd, F_GETFL, 0);
                        if (flags < 0) {
                            continue;
                        }
                        if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
                            continue;
                        }

                        epoll_ctl(eFd, EPOLL_CTL_ADD, fd, &epev);
                        printf("client on line fd:%d-count:%d\n", fd, count);
                    }  else {
                        printf("accept failed.\n, fd:%d-errno:%d-strerror:%s\n", fd, errno, strerror(errno));
                    }
                }
            } else {
                if (events[i].events & EPOLLERR || events[i].events & EPOLLHUP) {
                    epoll_ctl(eFd, EPOLL_CTL_DEL, events[i].data.fd, NULL);
                    close(events[i].data.fd);
                }
            }
        }
    }
}

/usr/lib/systemd/system/tcpServDemo.service:

[Unit]
Description= application service monitor daemon
After=network.target                                                                                                                                         

[Service]
User=root
Type=forking
ExecStart=/var/workstation/testcode/C-Code/tcpServDemo
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartSec=1s

[Install]
WantedBy=multi-user.target graphic.target