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    <p>I have taken a look at the kernel source and both assumptions seem to be true. The following code are extracts from <code>net/ipv4/tcp.c</code> (2.6.33.1).</p> <pre><code>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 &amp; MSG_MORE) || forced_push(tp)) tcp_mark_push(tp, skb); tcp_mark_urg(tp, flags, skb); __tcp_push_pending_frames(sk, mss_now, (flags &amp; MSG_MORE) ? TCP_NAGLE_CORK : nonagle); } } </code></pre> <p>Hence, if the flag is <em>not</em> set, the pending frames will definitely be flushed. But this is be <strong>only the case when the buffer is not empty</strong>:</p> <pre><code>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 &gt; 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, &amp;sk-&gt;sk_socket-&gt;flags); wait_for_memory: if (copied) tcp_push(sk, flags &amp; ~MSG_MORE, mss_now, TCP_NAGLE_PUSH); if ((err = sk_stream_wait_memory(sk, &amp;timeo)) != 0) goto do_error; mss_now = tcp_send_mss(sk, &amp;size_goal, flags); } out: if (copied) tcp_push(sk, flags, mss_now, tp-&gt;nonagle); return copied; do_error: if (copied) goto out; out_err: return sk_stream_error(sk, flags, err); } </code></pre> <p>The <code>while</code> loop's body will never be executed because <code>psize</code> is not greater 0. Then, in the <code>out</code> section, there is another chance, <code>tcp_push()</code> gets called but because <code>copied</code> still has its default value, it will fail as well.</p> <p>So sending a packet with the length 0 will never result in a flush.</p> <p>The next theory was to re-apply <code>TCP_CORK</code>. Let's take a look at the code first:</p> <pre><code>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-&gt;nonagle |= TCP_NAGLE_OFF|TCP_NAGLE_PUSH; tcp_push_pending_frames(sk); } else { tp-&gt;nonagle &amp;= ~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-&gt;nonagle |= TCP_NAGLE_CORK; } else { tp-&gt;nonagle &amp;= ~TCP_NAGLE_CORK; if (tp-&gt;nonagle&amp;TCP_NAGLE_OFF) tp-&gt;nonagle |= TCP_NAGLE_PUSH; tcp_push_pending_frames(sk); } break; (...) </code></pre> <p>As you can see, there are two ways to flush. You can either set <code>TCP_NODELAY</code> to 1 or <code>TCP_CORK</code> to 0. Luckily, both won't check whether the flag is already set. Thus, my initial plan to re-apply the <code>TCP_CORK</code> flag can be optimized to just disable it, even if it's currently not set.</p> <p>I hope this helps someone with similar issues.</p>
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    1. COThank you for this research. Very helpful.
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