为"MSG_MORE"标记的数据包刷新内核的TCP缓冲区

Question

send()的手册页显示了MSG_MORE断言行为的标志TCP_CORK.我有一个包装函数send():

int SocketConnection_Write(SocketConnection *this, void *buf, int len) {
    errno = 0;

    int sent = send(this->fd, buf, len, MSG_NOSIGNAL);

    if (errno == EPIPE || errno == ENOTCONN) {
        throw(exc, &SocketConnection_NotConnectedException);
    } else if (errno == ECONNRESET) {
        throw(exc, &SocketConnection_ConnectionResetException);
    } else if (sent != len) {
        throw(exc, &SocketConnection_LengthMismatchException);
    }

    return sent;
}

假设我想使用内核缓冲区,我可以使用TCP_CORK,只要有必要就启用,然后禁用它来刷新缓冲区.但另一方面,因此需要额外的系统调用.因此,使用MSG_MORE似乎更适合我.我只需将上面的send()行更改为:

int sent = send(this->fd, buf, len, MSG_NOSIGNAL | MSG_MORE);

根据lwm.net,如果数据包足够大,它们将自动刷新:

如果应用程序在套接字上设置该选项,则内核不会发送短数据包.相反,它会等到有足够的数据出现以填充最大大小的数据包,然后发送它.当TCP_CORK关闭时,任何剩余的数据都将在线路上熄灭.

但本节仅涉及TCP_CORK.现在,刷新MSG_MORE数据包的正确方法是什么？

我只能想到两种可能性:

1. 使用空缓冲区调用send()而不MSG_MORE进行设置
2. 作为描述的重新应用TCP_CORK选项此页

不幸的是,整个主题的文档很少,我在互联网上找不到多少.

我也想知道如何检查一切是否按预期工作？显然,运行服务器strace不是一种选择.那么最简单的方法是使用netcat然后查看其strace输出？或者内核是否会以不同的方式处理通过环回接口传输的流量？

Answer 1

我已经看了一下内核源代码,这两个假设似乎都是正确的.以下代码是net/ipv4/tcp.c(2.6.33.1)的摘录.

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,如果未设置标志,则肯定会刷新挂起的帧.但这只是缓冲区不为空的情况:

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);
}

该while循环的身体会不会被执行,因为psize不大于0.然后,在out一节,还有另外一个机会,tcp_push()被称为但是由于copied仍然有它的默认值,它也将失败.

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.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;
(...)

如您所见,有两种方法可以刷新.您可以设置TCP_NODELAY为1或TCP_CORK0.幸运的是,两者都不会检查标志是否已设置.因此,我重新应用TCP_CORK标志的初始计划可以优化为仅禁用它,即使它当前未设置.

我希望这可以帮助有类似问题的人.