epoll监听多文件描述符时调度顺序研究

当Epoll监听多个FD时，一直很好奇如果多个FD同时有事件触发，它是如何来进行调度的，调度的顺序是否和事件触发的顺序有关系？
借助简单的代码来分析一下：

代码原理较简单：
1. 主线程中创建两个eventfd描述符，加入epoll
2. 主线程在epoll_wait前，先在子线程中循环写入10次，调eventfd_write
3. 主线程进入epoll_wait,进行事件调度，读eventfd

结论：与调度顺序没有关系，结果可以看到fd1和fd2的事件是交替被处理到的

#include <sys/eventfd.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>  
#include <sys/epoll.h>  
#include <string.h>  
#include <pthread.h>  

int g_iEvtfd1 = -1;
int g_iEvtfd2 = -1;

void *eventfd_child_Task(void *pArg)
{
    uint64_t uiWrite = 1;
    int i = 0;

    for(i=0;i<10;i++)
    {
        eventfd_write(g_iEvtfd1, uiWrite);
    }

    for(i=0;i<10;i++)
    {
        eventfd_write(g_iEvtfd2, uiWrite);
    }
    
    printf("The child task is done!
");
    return;
}

int main(int argc, char**argv[])
{
    int iEvtfd1, iEvtfd2, j;
    uint64_t uiWrite = 1;
    uint64_t uiRead;
    ssize_t s;
    int iEpfd;
    struct epoll_event stEvent;
    int iRet = 0;
    struct epoll_event stEpEvent;
    pthread_t stWthread;
    
    iEpfd = epoll_create(1);
    if (-1 == iEpfd)
    {
        printf("Create epoll failed.
");
        return 0;
    }
    
    iEvtfd1 = eventfd(0,EFD_SEMAPHORE);
    if (-1 == iEvtfd1)
    {
        printf("failed to create eventfd1
");
        return 0;
    }
    g_iEvtfd1 = iEvtfd1;
    
    iEvtfd2 = eventfd(0,EFD_SEMAPHORE);
    if (-1 == iEvtfd2)
    {
        printf("failed to create eventfd2
");
        return 0;
    }    
    
    g_iEvtfd2 = iEvtfd2;
    
    memset(&stEvent, 0, sizeof(struct epoll_event));
    stEvent.events = (unsigned long) EPOLLIN;
    stEvent.data.fd = g_iEvtfd1;
    iRet = epoll_ctl(iEpfd, EPOLL_CTL_ADD, g_iEvtfd1, &stEvent);
    if (0 != iRet)
    {
        printf("failed to add g_iEvtfd1 to epoll
");
        close(g_iEvtfd1);
        close(g_iEvtfd2);
        close(iEpfd);
        return 0;
    }
    
    memset(&stEvent, 0, sizeof(struct epoll_event));
    stEvent.events = (unsigned long) EPOLLIN;
    stEvent.data.fd = g_iEvtfd2;
    iRet = epoll_ctl(iEpfd, EPOLL_CTL_ADD, g_iEvtfd2, &stEvent);
    if (0 != iRet)
    {
        printf("failed to add g_iEvtfd2 to epoll
");
        close(g_iEvtfd1);
        close(g_iEvtfd2);
        close(iEpfd);
        return 0;
    }
    
    iRet = pthread_create(&stWthread, NULL, eventfd_child_Task, NULL);
    if (0 != iRet)
    {
        close(g_iEvtfd1);
        close(g_iEvtfd2);
        close(iEpfd);
        return;
    }
    
    for(;;)
    {
        sleep(2);
        iRet = epoll_wait(iEpfd, &stEpEvent, 1, -1);
        if (iRet > 0)
        {
            printf("epoll_wait return %d.
", iRet);
            if(stEpEvent.data.fd == g_iEvtfd1)
            {
                printf("g_iEvtfd1 event
"); 
                s = eventfd_read(g_iEvtfd1, &uiRead);
                if (s != 0)
                {
                    printf("read g_iEvtfd1 failed
");
                    break;
                }
                printf("Read %llu (0x%llx) from g_iEvtfd1
", uiRead, uiRead);                
            }
            else if (stEpEvent.data.fd == g_iEvtfd2)
            {
                printf("g_iEvtfd2 evnet
");
                s = eventfd_read(g_iEvtfd2, &uiRead);
                if (s != 0)
                {
                    printf("read g_iEvtfd2 failed
");
                    break;
                }
                printf("Read %llu (0x%llx) from g_iEvtfd2
", uiRead, uiRead);                    
            }
            else{
                printf("epoll wait error!");
            }
        }
    }
    
    close(g_iEvtfd1);
    close(g_iEvtfd2);
    close(iEpfd);
    return 0;
}