add setting urlseed_max_request_bytes to handle large web seed requests #1405 (#1423)

add setting urlseed_max_request_bytes to handle large web seed requests. change default for urlseed max request to 16MIB

ChangeLog

@@ -1,3 +1,4 @@
+	* add setting urlseed_max_request_bytes to handle large web seed requests
 	* fix python build with CC/CXX environment
 	* add trackers from add_torrent_params/magnet links to separate tiers
 	* fix resumedata check issue with files with priority 0

include/libtorrent/peer_connection.hpp

@@ -365,7 +365,7 @@ namespace libtorrent
 		int picker_options() const;
 
 		void prefer_contiguous_blocks(int num)
-		{ m_prefer_contiguous_blocks = (std::min)(num, 255); }
+		{ m_prefer_contiguous_blocks = num; }
 
 		bool request_large_blocks() const
 		{ return m_request_large_blocks; }
@@ -1134,7 +1134,7 @@ namespace libtorrent
 		// if it is 0, the download rate limit setting
 		// will be used to determine if whole pieces
 		// are preferred.
-		boost::uint8_t m_prefer_contiguous_blocks;
+		int m_prefer_contiguous_blocks;
 
 		// this is the number of times this peer has had
 		// a request rejected because of a disk I/O failure.

include/libtorrent/settings_pack.hpp

@@ -767,6 +767,21 @@ namespace libtorrent
 			// low number, like 5
 			urlseed_pipeline_size,
 
+			// The maximum request range of an url seed in bytes. This value
+			// defines the largest possible sequential web seed request. Default
+			// is 16 * 1024 * 1024. Lower values are possible but will be ignored
+			// if they are lower then piece size.
+			// This value should be related to your download speed to prevent
+			// libtorrent from creating too many expensive http requests per
+			// second. You can select a value as high as you want but keep in mind
+			// that libtorrent can't create parallel requests if the first request
+			// did already select the whole file.
+			// If you combine bittorrent seeds with web seeds and pick strategies
+			// like rarest first you may find your web seed requests split into
+			// smaller parts because we don't download already picked pieces
+			// twice.
+			urlseed_max_request_bytes,
+
 			// time to wait until a new retry of a web seed takes place
 			urlseed_wait_retry,
 

src/settings_pack.cpp

@@ -229,6 +229,7 @@ namespace libtorrent
 		SET(peer_timeout, 120, 0),
 		SET(urlseed_timeout, 20, 0),
 		SET(urlseed_pipeline_size, 5, 0),
+		SET_NOPREV(urlseed_max_request_bytes, 16 * 1024 * 1024, 0),
 		SET(urlseed_wait_retry, 30, 0),
 		SET(file_pool_size, 40, 0),
 		SET(max_failcount, 3, &session_impl::update_max_failcount),

src/web_peer_connection.cpp

@@ -86,16 +86,16 @@ web_peer_connection::web_peer_connection(peer_connection_args const& pack
 	shared_ptr<torrent> tor = pack.tor.lock();
 	TORRENT_ASSERT(tor);
 
-	// we always prefer downloading 1 MiB chunks
-	// from web seeds, or whole pieces if pieces
-	// are larger than a MiB
-	int preferred_size = 1024 * 1024;
+	// if the web server is known not to support keep-alive. request 4MiB
+	// but we want to have at least piece size to prevent block based requests
+	int const min_size = std::max((web.supports_keepalive ? 1 : 4) * 1024 * 1024,
+		tor->torrent_file().piece_length());
 
-	// if the web server is known not to support keep-alive.
-	// request even larger blocks at a time
-	if (!web.supports_keepalive) preferred_size *= 4;
+	// we prefer downloading large chunks from web seeds,
+	// but still want to be able to split requests
+	int const preferred_size = std::max(min_size, m_settings.get_int(settings_pack::urlseed_max_request_bytes));
 
-	prefer_contiguous_blocks((std::max)(preferred_size / tor->block_size(), 1));
+	prefer_contiguous_blocks(preferred_size / tor->block_size());
 
 	boost::shared_ptr<torrent> t = associated_torrent().lock();
 	bool const single_file_request = t->torrent_file().num_files() == 1;