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?? ?Linux中异步IO等待无非就三个系统调用：select， poll和epoll。很多人无法理解三种调用的区别，或不够了解，今天就结合Linux kernel code详细描述三个的区别！

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?? ?select:

?? ?select 的限制就是最大1024个fd，可以查看kernel中的posix_types.h，里面定义了fdset数据结构，显然select不适合poll大量fd的场景（如webserver）。?

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?? ?include/linux/posix_types.h ：

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#undef __NFDBITS
#define __NFDBITS       (8 * sizeof(unsigned long))

#undef __FD_SETSIZE
#define __FD_SETSIZE    1024

#undef __FDSET_LONGS
#define __FDSET_LONGS   (__FD_SETSIZE/__NFDBITS)

#undef __FDELT
#define __FDELT(d)      ((d) / __NFDBITS)

#undef __FDMASK
#define __FDMASK(d)     (1UL << ((d) % __NFDBITS))

typedef struct {
        unsigned long fds_bits [__FDSET_LONGS];
} __kernel_fd_set;

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??poll:

?? poll相对于select改进了fdset size的限制，poll没有再使用fdset数组结构，反而使用了pollfd，这样用户可以自定义非常大的pollfd数组，这个pollfd数组在kernel中的表现形式是poll_list链表，这样就不存在了1024的限制了，除此之外poll相比select无太大区别。

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int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
                struct timespec *end_time)
{
        struct poll_wqueues table;
        int err = -EFAULT, fdcount, len, size;
        /* Allocate small arguments on the stack to save memory and be
           faster - use long to make sure the buffer is aligned properly
           on 64 bit archs to avoid unaligned access */
        long stack_pps[POLL_STACK_ALLOC/sizeof(long)];
        struct poll_list *const head = (struct poll_list *)stack_pps;
        struct poll_list *walk = head;
        unsigned long todo = nfds;

        if (nfds > rlimit(RLIMIT_NOFILE))
                return -EINVAL;

        len = min_t(unsigned int, nfds, N_STACK_PPS);
        for (;;) {
                walk->next = NULL;
                walk->len = len;
                if (!len)
                        break;

                if (copy_from_user(walk->entries, ufds + nfds-todo,
                                        sizeof(struct pollfd) * walk->len))
                        goto out_fds;

                todo -= walk->len;
                if (!todo)
                        break;

                len = min(todo, POLLFD_PER_PAGE);
                size = sizeof(struct poll_list) + sizeof(struct pollfd) * len;
                walk = walk->next = kmalloc(size, GFP_KERNEL);
                if (!walk) {
                        err = -ENOMEM;
                        goto out_fds;
                }
        }

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?? epoll：

?? ?select与poll的共同点是fd有数据后kernel会遍历所有fd，找到有效fd后初始化相应的revents，用户空间程序须再次遍历整个fdset，以找到有效的fd，这样实际上就遍历了两次fd数组表，对于极大量fd的情况，这样的性能非常不好，请看一下do_poll代码：

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static int do_poll(unsigned int nfds,  struct poll_list *list,
                   struct poll_wqueues *wait, struct timespec *end_time)
{
        poll_table* pt = &wait->pt;
        ktime_t expire, *to = NULL;
        int timed_out = 0, count = 0;
        unsigned long slack = 0;

        /* Optimise the no-wait case */
        if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
                pt = NULL;
                timed_out = 1;
        }

        if (end_time && !timed_out)
                slack = select_estimate_accuracy(end_time);

        for (;;) {
                struct poll_list *walk;

                for (walk = list; walk != NULL; walk = walk->next) {
                        struct pollfd * pfd, * pfd_end;

                        pfd = walk->entries;
                        pfd_end = pfd + walk->len;
                        for (; pfd != pfd_end; pfd++) {
                                /*
                                 * Fish for events. If we found one, record it
                                 * and kill