add send_buffer_low_watermark in an attempt to improve seed ramp-up time

src/peer_connection.cpp

@@ -4525,7 +4525,10 @@ namespace libtorrent
 		int buffer_size_watermark = upload_rate
 			* m_ses.settings().send_buffer_watermark_factor / 100;
 
-		if (buffer_size_watermark < 512) buffer_size_watermark = 512;
+		if (buffer_size_watermark < m_ses.settings().send_buffer_low_watermark)
+		{
+			buffer_size_watermark = m_ses.settings().send_buffer_low_watermark;
+		}
 		else if (buffer_size_watermark > m_ses.settings().send_buffer_watermark)
 		{
 			buffer_size_watermark = m_ses.settings().send_buffer_watermark;

src/session.cpp

@@ -274,13 +274,17 @@ namespace libtorrent
 		// the bandwidth delay product. Assuming an RTT
 		// of 500 ms, and a send rate of 20 MB/s, the upper
 		// limit should be 10 MB
-		set.send_buffer_watermark = 2 * 1024 * 1024;
+		set.send_buffer_watermark = 3 * 1024 * 1024;
 
 		// put 1.5 seconds worth of data in the send buffer
 		// this gives the disk I/O more heads-up on disk
 		// reads, and can maximize throughput
 		set.send_buffer_watermark_factor = 150;
 
+		// always stuff at least 1 MiB down each peer
+		// pipe, to quickly ramp up send rates
+ 		set.send_buffer_low_watermark = 1 * 1024 * 1024;
+
 		// don't retry peers if they fail once. Let them
 		// connect to us if they want to
 		set.max_failcount = 1;

src/session_impl.cpp

@@ -320,6 +320,7 @@ namespace aux {
 #endif
 		TORRENT_SETTING(boolean, free_torrent_hashes)
 		TORRENT_SETTING(boolean, upnp_ignore_nonrouters)
+ 		TORRENT_SETTING(integer, send_buffer_low_watermark)
  		TORRENT_SETTING(integer, send_buffer_watermark)
 #ifndef TORRENT_NO_DEPRECATE
 		TORRENT_SETTING(boolean, auto_upload_slots)