1 select
select本質上是通過設定或檢查存放fd标志位的資料結構進行下一步處理。
這帶來缺點：
單個程序可監視的fd數量被限制，即能監聽端口的數量有限
單個程序所能打開的最大連接配接數有FD_SETSIZE宏定義，其大小是32個整數的大小（在32位的機器上，大小就是3232，同理64位機器上FD_SETSIZE為3264），當然我們可以對進行修改，然後重新編譯核心，但是性能可能會受到影響，這需要進一步的測試
一般該數和系統記憶體關系很大，具體數目可以cat /proc/sys/fs/file-max察看。32位機預設1024個，64位預設2048。
對socket是線性掃描，即輪詢，效率較低：
僅知道有I/O事件發生，卻不知是哪幾個流，隻會無差異輪詢所有流，找出能讀資料或寫資料的流進行操作。同時處理的流越多，無差别輪詢時間越長 - O(n)。
當socket較多時，每次select都要通過周遊
FD_SETSIZE
個socket，不管是否活躍，這會浪費很多CPU時間。如果能給 socket 注冊某個回調函數，當他們活躍時，自動完成相關操作，即可避免輪詢，這就是epoll與kqueue。
1.1 調用過程
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. 注冊回調函數__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從使用者空間拷貝fd_set到核心空間
- 注冊回調函數__pollwait
一文搞懂select、poll和epoll差別（上）1 select - 周遊所有fd，調用其對應的poll方法（對于socket，這個poll方法是sock_poll，sock_poll根據情況會調用到tcp_poll，udp_poll或datagram_poll）
- 以tcp_poll為例，核心實作就是
__pollwait
- __pollwait，就是把current（目前程序）挂到裝置的等待隊列，不同裝置有不同等待隊列，如tcp_poll的等待隊列是sk->sk_sleep（把程序挂到等待隊列中并不代表程序已睡眠）。在裝置收到一條消息（網絡裝置）或填寫完檔案資料（磁盤裝置）後，會喚醒裝置等待隊列上睡眠的程序，這時current便被喚醒。
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;
}
/* 添加新節點 */
{
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方法傳回時會傳回一個描述讀寫操作是否就緒的mask掩碼，根據這個mask掩碼給fd_set指派
- 若周遊完所有fd，還沒傳回一個可讀寫的mask掩碼，則調schedule_timeout是調用select的程序（也就是current）進入睡眠。當裝置驅動發生自身資源可讀寫後，會喚醒其等待隊列上睡眠的程序。若超過一定逾時時間（schedule_timeout指定），還沒人喚醒，則調用select的程序會重新被喚醒獲得CPU，進而重新周遊fd，判斷有無就緒的fd
- 把fd_set從核心空間拷貝到使用者空間