1 select
select本质上是通过设置或检查存放fd标志位的数据结构进行下一步处理。
这带来缺点：
单个进程可监视的fd数量被限制，即能监听端口的数量有限
单个进程所能打开的最大连接数有FD_SETSIZE宏定义，其大小是32个整数的大小（在32位的机器上，大小就是3232，同理64位机器上FD_SETSIZE为3264），当然我们可以对进行修改，然后重新编译内核，但是性能可能会受到影响，这需要进一步的测试
一般该数和系统内存关系很大，具体数目可以cat /proc/sys/fs/file-max察看。32位机默认1024个，64位默认2048。
对socket是线性扫描，即轮询，效率较低：
仅知道有I/O事件发生，却不知是哪几个流，只会无差异轮询所有流，找出能读数据或写数据的流进行操作。同时处理的流越多，无差别轮询时间越长 - O(n)。
当socket较多时，每次select都要通过遍历
FD_SETSIZE
个socket，不管是否活跃，这会浪费很多CPU时间。如果能给 socket 注册某个回调函数，当他们活跃时，自动完成相关操作，即可避免轮询，这就是epoll与kqueue。
1.1 调用过程
asmlinkage long sys_poll(struct pollfd * ufds, unsigned int nfds, long timeout)
{
int i, j, fdcount, err;
struct pollfd **fds;
struct poll_wqueues table, *wait;
int nchunks, nleft;
/* Do a sanity check on nfds ... */
if (nfds > NR_OPEN)
return -EINVAL;
if (timeout) {
/* Careful about overflow in the intermediate values */
if ((unsigned long) timeout < MAX_SCHEDULE_TIMEOUT / HZ)
timeout = (unsigned long)(timeout*HZ+999)/1000+1;
else /* Negative or overflow */
timeout = MAX_SCHEDULE_TIMEOUT;
}
// 2. 注册回调函数__pollwait
poll_initwait(&table);
wait = &table;
if (!timeout)
wait = NULL;
err = -ENOMEM;
fds = NULL;
if (nfds != 0) {
fds = (struct pollfd **)kmalloc(
(1 + (nfds - 1) / POLLFD_PER_PAGE) * sizeof(struct pollfd *),
GFP_KERNEL);
if (fds == NULL)
goto out;
}
nchunks = 0;
nleft = nfds;
while (nleft > POLLFD_PER_PAGE) { /* allocate complete PAGE_SIZE chunks */
fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL);
if (fds[nchunks] == NULL)
goto out_fds;
nchunks++;
nleft -= POLLFD_PER_PAGE;
}
if (nleft) { /* allocate last PAGE_SIZE chunk, only nleft elements used */
fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL);
if (fds[nchunks] == NULL)
goto out_fds;
}
err = -EFAULT;
for (i=0; i < nchunks; i++)
//
if (copy_from_user(fds[i], ufds + i*POLLFD_PER_PAGE, PAGE_SIZE))
goto out_fds1;
if (nleft) {
if (copy_from_user(fds[nchunks], ufds + nchunks*POLLFD_PER_PAGE,
nleft * sizeof(struct pollfd)))
goto out_fds1;
}
fdcount = do_poll(nfds, nchunks, nleft, fds, wait, timeout);
/* OK, now copy the revents fields back to user space. */
for(i=0; i < nchunks; i++)
for (j=0; j < POLLFD_PER_PAGE; j++, ufds++)
__put_user((fds[i] + j)->revents, &ufds->revents);
if (nleft)
for (j=0; j < nleft; j++, ufds++)
__put_user((fds[nchunks] + j)->revents, &ufds->revents);
err = fdcount;
if (!fdcount && signal_pending(current))
err = -EINTR;
out_fds1:
if (nleft)
free_page((unsigned long)(fds[nchunks]));
out_fds:
for (i=0; i < nchunks; i++)
free_page((unsigned long)(fds[i]));
if (nfds != 0)
kfree(fds);
out:
poll_freewait(&table);
return err;
} static int do_poll(unsigned int nfds, unsigned int nchunks, unsigned int nleft,
struct pollfd *fds[], struct poll_wqueues *wait, long timeout)
{
int count;
poll_table* pt = &wait->pt;
for (;;) {
unsigned int i;
set_current_state(TASK_INTERRUPTIBLE);
count = 0;
for (i=0; i < nchunks; i++)
do_pollfd(POLLFD_PER_PAGE, fds[i], &pt, &count);
if (nleft)
do_pollfd(nleft, fds[nchunks], &pt, &count);
pt = NULL;
if (count || !timeout || signal_pending(current))
break;
count = wait->error;
if (count)
break;
timeout = schedule_timeout(timeout);
}
current->state = TASK_RUNNING;
return count;
} - 使用copy_from_user从用户空间拷贝fd_set到内核空间
- 注册回调函数__pollwait
一文搞懂select、poll和epoll区别（上）1 select - 遍历所有fd，调用其对应的poll方法（对于socket，这个poll方法是sock_poll，sock_poll根据情况会调用到tcp_poll，udp_poll或datagram_poll）
- 以tcp_poll为例，核心实现就是
__pollwait
- __pollwait，就是把current（当前进程）挂到设备的等待队列，不同设备有不同等待队列，如tcp_poll的等待队列是sk->sk_sleep（把进程挂到等待队列中并不代表进程已睡眠）。在设备收到一条消息（网络设备）或填写完文件数据（磁盘设备）后，会唤醒设备等待队列上睡眠的进程，这时current便被唤醒。
void __pollwait(struct file *filp, wait_queue_head_t *wait_address, poll_table *_p)
{
struct poll_wqueues *p = container_of(_p, struct poll_wqueues, pt);
struct poll_table_page *table = p->table;
if (!table || POLL_TABLE_FULL(table)) {
struct poll_table_page *new_table;
new_table = (struct poll_table_page *) __get_free_page(GFP_KERNEL);
if (!new_table) {
p->error = -ENOMEM;
__set_current_state(TASK_RUNNING);
return;
}
new_table->entry = new_table->entries;
new_table->next = table;
p->table = new_table;
table = new_table;
}
/* 添加新节点 */
{
struct poll_table_entry * entry = table->entry;
table->entry = entry+1;
get_file(filp);
entry->filp = filp;
entry->wait_address = wait_address;
init_waitqueue_entry(&entry->wait, current);
add_wait_queue(wait_address,&entry->wait);
}
} static void do_pollfd(unsigned int num, struct pollfd * fdpage,
poll_table ** pwait, int *count)
{
int i;
for (i = 0; i < num; i++) {
int fd;
unsigned int mask;
struct pollfd *fdp;
mask = 0;
fdp = fdpage+i;
fd = fdp->fd;
if (fd >= 0) {
struct file * file = fget(fd);
mask = POLLNVAL;
if (file != NULL) {
mask = DEFAULT_POLLMASK;
if (file->f_op && file->f_op->poll)
mask = file->f_op->poll(file, *pwait);
mask &= fdp->events | POLLERR | POLLHUP;
fput(file);
}
if (mask) {
*pwait = NULL;
(*count)++;
}
}
fdp->revents = mask;
}
} - poll方法返回时会返回一个描述读写操作是否就绪的mask掩码，根据这个mask掩码给fd_set赋值
- 若遍历完所有fd，还没返回一个可读写的mask掩码，则调schedule_timeout是调用select的进程（也就是current）进入睡眠。当设备驱动发生自身资源可读写后，会唤醒其等待队列上睡眠的进程。若超过一定超时时间（schedule_timeout指定），还没人唤醒，则调用select的进程会重新被唤醒获得CPU，进而重新遍历fd，判断有无就绪的fd
- 把fd_set从内核空间拷贝到用户空间