optimize sending buffers on peer sockets by corking and uncorking

examples/client_test.cpp

@@ -1367,6 +1367,8 @@ int main(int argc, char* argv[])
 	settings.disk_cache_algorithm = session_settings::avoid_readback;
 	settings.volatile_read_cache = false;
 
+	settings.max_allowed_in_request_queue = 1000;
+	settings.send_buffer_watermark = 10 * 1024 * 1024;
 	ses.set_settings(settings);
 
 	for (std::vector<add_torrent_params>::iterator i = magnet_links.begin()

include/libtorrent/peer_connection.hpp

@@ -538,11 +538,14 @@ namespace libtorrent
 
 		// these functions are virtual to let bt_peer_connection hook into them
 		// and encrypt the content
-		enum message_type_flags { message_type_request = 1, cork_message = 2 };
+		enum message_type_flags { message_type_request = 1 };
 		virtual void send_buffer(char const* begin, int size, int flags = 0
 			, void (*fun)(char*, int, void*) = 0, void* userdata = 0);
 		virtual void setup_send();
 
+		void cork_socket() { TORRENT_ASSERT(!m_corked); m_corked = true; }
+		void uncork_socket();
+
 #ifdef TORRENT_DISK_STATS
 		void log_buffer_usage(char* buffer, int size, char const* label);
 #endif
@@ -1139,6 +1142,13 @@ namespace libtorrent
 		// is not included in the torrent or session stats
 		bool m_ignore_stats:1;
 
+		// when this is set, the peer_connection socket is
+		// corked, similar to the linux TCP feature TCP_CORK.
+		// we won't send anything to the actual socket, just
+		// buffer messages up in the application layer send
+		// buffer, and send it once we're uncorked.
+		bool m_corked:1;
+		
 		template <std::size_t Size>
 		struct handler_storage
 		{
@@ -1232,6 +1242,14 @@ namespace libtorrent
 		int m_received_in_piece;
 #endif
 	};
+
+	struct cork
+	{
+		cork(peer_connection& p): m_pc(p) { m_pc.cork_socket(); }
+		~cork() { m_pc.uncork_socket(); }
+		peer_connection& m_pc;
+	};
+
 }
 
 #endif // TORRENT_PEER_CONNECTION_HPP_INCLUDED

src/bt_peer_connection.cpp

@@ -2339,12 +2339,12 @@ namespace libtorrent
 			char* ptr = msg;
 			detail::write_int32(r.length + 1 + 4 + 4 + 4 + piece_list_buf.size(), ptr);
 
-			send_buffer(msg, 17, cork_message);
-			send_buffer(&piece_list_buf[0], piece_list_buf.size(), cork_message);
+			send_buffer(msg, 17);
+			send_buffer(&piece_list_buf[0], piece_list_buf.size());
 		}
 		else
 		{
-			send_buffer(msg, 13, cork_message);
+			send_buffer(msg, 13);
 		}
 
 		append_send_buffer(buffer.get(), r.length

src/peer_connection.cpp

@@ -162,6 +162,7 @@ namespace libtorrent
 		, m_sent_suggests(false)
 		, m_holepunch_mode(false)
 		, m_ignore_stats(false)
+		, m_corked(false)
 #if defined TORRENT_DEBUG || TORRENT_RELEASE_ASSERTS
 		, m_in_constructor(true)
 		, m_disconnect_started(false)
@@ -312,6 +313,7 @@ namespace libtorrent
 		, m_sent_suggests(false)
 		, m_holepunch_mode(false)
 		, m_ignore_stats(false)
+		, m_corked(false)
 #if defined TORRENT_DEBUG || TORRENT_RELEASE_ASSERTS
 		, m_in_constructor(true)
 		, m_disconnect_started(false)
@@ -2550,6 +2552,11 @@ namespace libtorrent
 #endif
 		TORRENT_ASSERT(m_ses.is_network_thread());
 
+		// flush send buffer at the end of this scope
+		// TODO: peers should really be corked/uncorked outside of
+		// all completed disk operations
+		cork _c(*this);
+
 		INVARIANT_CHECK;
 
 		m_outstanding_writing_bytes -= p.length;
@@ -4405,6 +4412,11 @@ namespace libtorrent
 
 	void peer_connection::on_disk_read_complete(int ret, disk_io_job const& j, peer_request r)
 	{
+		// flush send buffer at the end of this scope
+		// TODO: peers should really be corked/uncorked outside of
+		// all completed disk operations
+		cork _c(*this);
+
 #ifdef TORRENT_STATS
 		++m_ses.m_num_messages[aux::session_impl::on_disk_read_counter];
 #endif
@@ -4570,6 +4582,13 @@ namespace libtorrent
 			, priority , bwc1, bwc2, bwc3, bwc4);
 	}
 
+	void peer_connection::uncork_socket()
+	{
+		if (!m_corked) return;
+		m_corked = false;
+		setup_send();
+	}
+
 	void peer_connection::setup_send()
 	{
 		if (m_channel_state[upload_channel] != peer_info::bw_idle
@@ -4676,6 +4695,14 @@ namespace libtorrent
 
 		TORRENT_ASSERT(amount_to_send > 0);
 
+		if (m_corked)
+		{
+#ifdef TORRENT_VERBOSE_LOGGING
+			peer_log(">>> CORKED WRITE [ bytes: %d ]", amount_to_send);
+#endif
+			return;		
+		}
+
 #ifdef TORRENT_VERBOSE_LOGGING
 		peer_log(">>> ASYNC_WRITE [ bytes: %d ]", amount_to_send);
 #endif
@@ -5004,7 +5031,7 @@ namespace libtorrent
 				, boost::bind(&session_impl::free_buffer, boost::ref(m_ses), _1));
 			++i;
 		}
-		if ((flags & cork_message) == 0) setup_send();
+		setup_send();
 	}
 
 	template<class T>
@@ -5042,6 +5069,9 @@ namespace libtorrent
 		// to keep the object alive through the exit check
 		boost::intrusive_ptr<peer_connection> me(self());
 
+		// flush the send buffer at the end of this function
+		cork _c(*this);
+
 		INVARIANT_CHECK;
 
 #ifdef TORRENT_VERBOSE_LOGGING