背景
传统的IO，OS只提供了对一个FD进行操作的功能，也就是BIO。
-
1对1模型
image.png -
线程池模型
image.png
传统IO模式基于OS提供的功能，就限制了一个线程同一个时刻就只能处理一个连接。
IO多路复用
基于传统IO的模式，OS提供了IO多路复用模型，一个线程可以同一个时刻处理多个连接，可以一次性给多个FD给内核进行操作。
select
select的逻辑比较简单，接受从用户态传过来的FD数组，循环遍历调用VFS中file->poll函数判断是否有准备数据可以进行操作，如果有设置标识位。
image.png
static int aeApiPoll(aeEventLoop *eventLoop, struct timeval *tvp) {
aeApiState *state = eventLoop->apidata;
int retval, j, numevents = 0;
memcpy(&state->_rfds,&state->rfds,sizeof(fd_set));
memcpy(&state->_wfds,&state->wfds,sizeof(fd_set));
retval = select(eventLoop->maxfd+1,
&state->_rfds,&state->_wfds,NULL,tvp);
if (retval > 0) {
for (j = 0; j <= eventLoop->maxfd; j++) {
int mask = 0;
aeFileEvent *fe = &eventLoop->events[j];
if (fe->mask == AE_NONE) continue;
if (fe->mask & AE_READABLE && FD_ISSET(j,&state->_rfds))
mask |= AE_READABLE;
if (fe->mask & AE_WRITABLE && FD_ISSET(j,&state->_wfds))
mask |= AE_WRITABLE;
eventLoop->fired[numevents].fd = j;
eventLoop->fired[numevents].mask = mask;
numevents++;
}
}
return numevents;
}
asmlinkage long
sys_select(int n, fd_set __user *inp, fd_set __user *outp, fd_set __user *exp, struct timeval __user *tvp)
{
fd_set_bits fds;
char *bits;
long timeout;
int ret, size, max_fdset;
timeout = MAX_SCHEDULE_TIMEOUT;
if (tvp) {
time_t sec, usec;
if ((ret = verify_area(VERIFY_READ, tvp, sizeof(*tvp)))
|| (ret = __get_user(sec, &tvp->tv_sec))
|| (ret = __get_user(usec, &tvp->tv_usec)))
goto out_nofds;
ret = -EINVAL;
if (sec < 0 || usec < 0)
goto out_nofds;
if ((unsigned long) sec < MAX_SELECT_SECONDS) {
timeout = ROUND_UP(usec, 1000000/HZ);
timeout += sec * (unsigned long) HZ;
}
}
ret = -EINVAL;
if (n < 0)
goto out_nofds;
/* max_fdset can increase, so grab it once to avoid race */
max_fdset = current->files->max_fdset;
if (n > max_fdset)
n = max_fdset;
/*
* We need 6 bitmaps (in/out/ex for both incoming and outgoing),
* since we used fdset we need to allocate memory in units of
* long-words.
*/
ret = -ENOMEM;
size = FDS_BYTES(n);
bits = select_bits_alloc(size);
if (!bits)
goto out_nofds;
fds.in = (unsigned long *) bits;
fds.out = (unsigned long *) (bits + size);
fds.ex = (unsigned long *) (bits + 2*size);
fds.res_in = (unsigned long *) (bits + 3*size);
fds.res_out = (unsigned long *) (bits + 4*size);
fds.res_ex = (unsigned long *) (bits + 5*size);
if ((ret = get_fd_set(n, inp, fds.in)) ||
(ret = get_fd_set(n, outp, fds.out)) ||
(ret = get_fd_set(n, exp, fds.ex)))
goto out;
zero_fd_set(n, fds.res_in);
zero_fd_set(n, fds.res_out);
zero_fd_set(n, fds.res_ex);
ret = do_select(n, &fds, &timeout);
if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
time_t sec = 0, usec = 0;
if (timeout) {
sec = timeout / HZ;
usec = timeout % HZ;
usec *= (1000000/HZ);
}
put_user(sec, &tvp->tv_sec);
put_user(usec, &tvp->tv_usec);
}
if (ret < 0)
goto out;
if (!ret) {
ret = -ERESTARTNOHAND;
if (signal_pending(current))
goto out;
ret = 0;
}
set_fd_set(n, inp, fds.res_in);
set_fd_set(n, outp, fds.res_out);
set_fd_set(n, exp, fds.res_ex);
out:
select_bits_free(bits, size);
out_nofds:
return ret;
}
int do_select(int n, fd_set_bits *fds, long *timeout)
{
struct poll_wqueues table;
poll_table *wait;
int retval, i;
long __timeout = *timeout;
spin_lock(¤t->files->file_lock);
retval = max_select_fd(n, fds);
spin_unlock(¤t->files->file_lock);
if (retval < 0)
return retval;
n = retval;
poll_initwait(&table);
wait = &table.pt;
if (!__timeout)
wait = NULL;
retval = 0;
for (;;) {
unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
set_current_state(TASK_INTERRUPTIBLE);
inp = fds->in; outp = fds->out; exp = fds->ex;
rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
unsigned long in, out, ex, all_bits, bit = 1, mask, j;
unsigned long res_in = 0, res_out = 0, res_ex = 0;
struct file_operations *f_op = NULL;
struct file *file = NULL;
in = *inp++; out = *outp++; ex = *exp++;
all_bits = in | out | ex;
if (all_bits == 0) {
i += __NFDBITS;
continue;
}
for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {
if (i >= n)
break;
if (!(bit & all_bits))
continue;
file = fget(i);
if (file) {
f_op = file->f_op;
mask = DEFAULT_POLLMASK;
if (f_op && f_op->poll)
mask = (*f_op->poll)(file, retval ? NULL : wait);
fput(file);
if ((mask & POLLIN_SET) && (in & bit)) {
res_in |= bit;
retval++;
}
if ((mask & POLLOUT_SET) && (out & bit)) {
res_out |= bit;
retval++;
}
if ((mask & POLLEX_SET) && (ex & bit)) {
res_ex |= bit;
retval++;
}
}
}
if (res_in)
*rinp = res_in;
if (res_out)
*routp = res_out;
if (res_ex)
*rexp = res_ex;
}
wait = NULL;
if (retval || !__timeout || signal_pending(current))
break;
if(table.error) {
retval = table.error;
break;
}
__timeout = schedule_timeout(__timeout);
}
__set_current_state(TASK_RUNNING);
poll_freewait(&table);
/*
* Up-to-date the caller timeout.
*/
*timeout = __timeout;
return retval;
}
poll
sys_poll
asmlinkage long sys_poll(struct pollfd __user * ufds, unsigned int nfds, long timeout)
{
struct poll_wqueues table;
int fdcount, err;
unsigned int i;
struct poll_list *head;
struct poll_list *walk;
/* Do a sanity check on nfds ... */
if (nfds > current->files->max_fdset && nfds > OPEN_MAX)
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;
}
poll_initwait(&table);
head = NULL;
walk = NULL;
i = nfds;
err = -ENOMEM;
while(i!=0) {
struct poll_list *pp;
pp = kmalloc(sizeof(struct poll_list)+
sizeof(struct pollfd)*
(i>POLLFD_PER_PAGE?POLLFD_PER_PAGE:i),
GFP_KERNEL);
if(pp==NULL)
goto out_fds;
pp->next=NULL;
pp->len = (i>POLLFD_PER_PAGE?POLLFD_PER_PAGE:i);
if (head == NULL)
head = pp;
else
walk->next = pp;
walk = pp;
if (copy_from_user(pp->entries, ufds + nfds-i,
sizeof(struct pollfd)*pp->len)) {
err = -EFAULT;
goto out_fds;
}
i -= pp->len;
}
fdcount = do_poll(nfds, head, &table, timeout);
/* OK, now copy the revents fields back to user space. */
walk = head;
err = -EFAULT;
while(walk != NULL) {
struct pollfd *fds = walk->entries;
int j;
for (j=0; j < walk->len; j++, ufds++) {
if(__put_user(fds[j].revents, &ufds->revents))
goto out_fds;
}
walk = walk->next;
}
err = fdcount;
if (!fdcount && signal_pending(current))
err = -EINTR;
out_fds:
walk = head;
while(walk!=NULL) {
struct poll_list *pp = walk->next;
kfree(walk);
walk = pp;
}
poll_freewait(&table);
return err;
}
do_poll
static int do_poll(unsigned int nfds, struct poll_list *list,
struct poll_wqueues *wait, long timeout)
{
int count = 0;
poll_table* pt = &wait->pt;
if (!timeout)
pt = NULL;
for (;;) {
struct poll_list *walk;
set_current_state(TASK_INTERRUPTIBLE);
walk = list;
while(walk != NULL) {
do_pollfd( walk->len, walk->entries, &pt, &count);
walk = walk->next;
}
pt = NULL;
if (count || !timeout || signal_pending(current))
break;
count = wait->error;
if (count)
break;
timeout = schedule_timeout(timeout);
}
__set_current_state(TASK_RUNNING);
return count;
}
do_pollfd
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;
}
}
- select 函数的声明比较复杂，有read，write，excpect 等参数，而poll直接封装到一个参数中了，比较优雅。
- select 中内核操作使用数组，poll使用链表，因为数组需要连续的空间，而内核中的连续内存比较昂贵。
- select和poll都需要自己遍历所有的FD集合，就算没有事件过来也需要遍历，造成资源的浪费；
epoll
image.png
关键函数
- epoll_create：创建epfd，eventpoll，并设置一些VFS相关的操作，例如：file的f_op的函数指针。
- epoll_clt: fd的CRUD操作，函数比较复杂，涉及比较多结构转化，回调函数的操作等。
- epoll_wait: 等待eventpoll的rdllist集合中是否有准备好的事件的FD。
tip epoll_clt函数部分结构图，可以自行进行分析。
epoll_ctl.png
总结
从传统IO到多路复用是操作系统IO模型的变化，像JAVA提供的NIO不过是对操作系统提供多路复用进行封装而已，所以一个很重要的观念是操作系统提供什么，上层就只能使用什么。
tip: 上面源码基于 redis-2.6，linux-2.6
