improved read cache memory efficiency

ChangeLog

@@ -1,3 +1,4 @@
+	* improved read cache memory efficiency
 	* added another cache flush algorithm to write the largest
 	  contiguous blocks instead of the least recently used
 	* introduced a mechanism to be lighter on the disk when checking torrents

docs/manual.rst

@@ -3448,6 +3448,8 @@ session_settings
 		{ lru, largest_contiguous };
 
 		disk_cache_algo_t disk_cache_algorithm;
+
+		int read_cache_line_size;
 	};
 
 ``user_agent`` this is the client identification to the tracker.
@@ -3839,6 +3841,10 @@ value. ``session_settings::largest_contiguous`` will flush the largest
 sequences of contiguous blocks from the write cache, regarless of the
 piece's last use time.
 
+``read_cache_line_size`` is the number of blocks to read into the read
+cache when a read cache miss occurs. Setting this to 0 is essentially
+the same thing as disabling read cache. The number of blocks read
+into the read cache is always capped by the piece boundry.
 
 pe_settings
 ===========

include/libtorrent/session_settings.hpp

@@ -168,6 +168,7 @@ namespace libtorrent
 			, optimize_hashing_for_speed(true)
 			, file_checks_delay_per_block(0)
 			, disk_cache_algorithm(lru)
+			, read_cache_line_size(16)
 		{}
 
 		// this is the user agent that will be sent to the tracker
@@ -576,6 +577,10 @@ namespace libtorrent
 		{ lru, largest_contiguous };
 
 		disk_cache_algo_t disk_cache_algorithm;
+
+		// the number of blocks that will be read ahead
+		// when reading a block into the read cache
+		int read_cache_line_size;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

src/disk_io_thread.cpp

@@ -720,6 +720,7 @@ namespace libtorrent
 		int blocks_in_piece = (piece_size + m_block_size - 1) / m_block_size;
 
 		int end_block = start_block;
+		int num_read = 0;
 		for (int i = start_block; i < blocks_in_piece
 			&& (in_use() < m_settings.cache_size
 				|| (options & ignore_cache_size)); ++i)
@@ -736,6 +737,8 @@ namespace libtorrent
 			++m_cache_stats.cache_size;
 			++m_cache_stats.read_cache_size;
 			++end_block;
+			++num_read;
+			if (num_read >= m_settings.read_cache_line_size) break;
 		}
 
 		if (end_block == start_block) return -2;

src/session_impl.cpp

@@ -641,6 +641,7 @@ namespace aux {
 			|| m_settings.optimize_hashing_for_speed != s.optimize_hashing_for_speed
 			|| m_settings.file_checks_delay_per_block != s.file_checks_delay_per_block
 			|| m_settings.disk_cache_algorithm != s.disk_cache_algorithm
+			|| m_settings.read_cache_line_size != s.read_cache_line_size
 #ifndef TORRENT_DISABLE_MLOCK
 			|| m_settings.lock_disk_cache != s.lock_disk_cache
 #endif