我看了一下内核源码,两个假设似乎都是真的。以下代码摘自net/ipv4/tcp.c
(2.6.33.1)。
static inline void tcp_push(struct sock *sk, int flags, int mss_now,
int nonagle)
{
struct tcp_sock *tp = tcp_sk(sk);
if (tcp_send_head(sk)) {
struct sk_buff *skb = tcp_write_queue_tail(sk);
if (!(flags & MSG_MORE) || forced_push(tp))
tcp_mark_push(tp, skb);
tcp_mark_urg(tp, flags, skb);
__tcp_push_pending_frames(sk, mss_now,
(flags & MSG_MORE) ? TCP_NAGLE_CORK : nonagle);
}
}
因此,如果标志是不组,因此处理中的帧一定会被冲洗。但是,这是只是情况下,当缓冲区不为空:
static ssize_t do_tcp_sendpages(struct sock *sk, struct page **pages, int poffset,
size_t psize, int flags)
{
(...)
ssize_t copied;
(...)
copied = 0;
while (psize > 0) {
(...)
if (forced_push(tp)) {
tcp_mark_push(tp, skb);
__tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_PUSH);
} else if (skb == tcp_send_head(sk))
tcp_push_one(sk, mss_now);
continue;
wait_for_sndbuf:
set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
wait_for_memory:
if (copied)
tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);
if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)
goto do_error;
mss_now = tcp_send_mss(sk, &size_goal, flags);
}
out:
if (copied)
tcp_push(sk, flags, mss_now, tp->nonagle);
return copied;
do_error:
if (copied)
goto out;
out_err:
return sk_stream_error(sk, flags, err);
}
的while
循环体将永远不会被执行,因为psize
不大于0。然后,在out
部分,另外还有一个机会, tcp_push()
被调用,但因为copied
仍然有其默认值,所以它也会失败。
因此,发送长度为0的数据包永远不会导致刷新。
下一个理论是重新申请TCP_CORK
。让我们先来看看代码:
static int do_tcp_setsockopt(struct sock *sk, int level,
int optname, char __user *optval, unsigned int optlen)
{
(...)
switch (optname) {
(...)
case TCP_NODELAY:
if (val) {
/* TCP_NODELAY is weaker than TCP_CORK, so that
* this option on corked socket is remembered, but
* it is not activated until cork is cleared.
*
* However, when TCP_NODELAY is set we make
* an explicit push, which overrides even TCP_CORK
* for currently queued segments.
*/
tp->nonagle |= TCP_NAGLE_OFF|TCP_NAGLE_PUSH;
tcp_push_pending_frames(sk);
} else {
tp->nonagle &= ~TCP_NAGLE_OFF;
}
break;
case TCP_CORK:
/* When set indicates to always queue non-full frames.
* Later the user clears this option and we transmit
* any pending partial frames in the queue. This is
* meant to be used alongside sendfile() to get properly
* filled frames when the user (for example) must write
* out headers with a write() call first and then use
* sendfile to send out the data parts.
*
* TCP_CORK can be set together with TCP_NODELAY and it is
* stronger than TCP_NODELAY.
*/
if (val) {
tp->nonagle |= TCP_NAGLE_CORK;
} else {
tp->nonagle &= ~TCP_NAGLE_CORK;
if (tp->nonagle&TCP_NAGLE_OFF)
tp->nonagle |= TCP_NAGLE_PUSH;
tcp_push_pending_frames(sk);
}
break;
(...)
正如你所看到的,有两种方法可以刷新。您可以将TCP_NODELAY
设置为1或TCP_CORK
设置为0.幸运的是,两者都不会检查标志是否已设置。因此,我最初的计划重新应用TCP_CORK
标志可以优化,只是禁用它,即使它目前没有设置。
我希望这可以帮助有类似问题的人。
sendfile()保留'MSG_MORE'标志。当sendfile()返回时,缓存将被刷新。 – user206268 2010-03-31 14:20:40