added volatile read cache option

bindings/python/src/session_settings.cpp

@@ -106,6 +106,7 @@ void bind_session_settings()
         .def_readwrite("max_suggest_pieces", &session_settings::max_suggest_pieces)
         .def_readwrite("drop_skipped_requests", &session_settings::drop_skipped_requests)
         .def_readwrite("low_prio_disk", &session_settings::low_prio_disk)
+        .def_readwrite("volatile_read_cache", &session_settings::volatile_read_cache)
     ;
 
     enum_<proxy_settings::proxy_type>("proxy_type")

docs/manual.rst

@@ -3721,6 +3721,7 @@ session_settings
 		bool drop_skipped_requests;
 
 		bool low_prio_disk;
+		bool volatile_read_cache;
 	};
 
 ``user_agent`` this is the client identification to the tracker.
@@ -4191,6 +4192,12 @@ overall responsiveness of the system while downloading in the
 background. For high-performance server setups, this might not
 be desirable.
 
+``volatile_read_cache``, if this is set to true, read cache blocks
+that are hit by peer read requests are removed from the disk cache
+to free up more space. This is useful if you don't expect the disk
+cache to create any cache hits from other peers than the one who
+triggered the cache line to be read into the cache in the first place.
+
 pe_settings
 ===========
 

include/libtorrent/session_settings.hpp

@@ -186,6 +186,7 @@ namespace libtorrent
 			, max_suggest_pieces(10)
 			, drop_skipped_requests(false)
 			, low_prio_disk(true)
+			, volatile_read_cache(false)
 		{}
 
 		// this is the user agent that will be sent to the tracker
@@ -683,6 +684,12 @@ namespace libtorrent
 		// to foreground tasks, while bittorrent runs
 		// in the background
 		bool low_prio_disk;
+
+		// if this is set to true, any block read from the
+		// disk cache will be dropped from the cache immediately
+		// following. This may be useful if the block is not
+		// expected to be hit again. It would save some memory
+		bool volatile_read_cache;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

src/disk_io_thread.cpp

@@ -711,6 +711,12 @@ namespace libtorrent
 			{
 				std::memcpy(buf.get() + offset, p.blocks[i].buf, block_size);
 				offset += m_block_size;
+				if (m_settings.volatile_read_cache)
+				{
+					free_buffer(p.blocks[i].buf);
+					p.blocks[i].buf = 0;
+					--p.num_blocks;
+				}
 			}
 			buffer_size += block_size;
 			TORRENT_ASSERT(p.num_blocks > 0);
@@ -1227,11 +1233,17 @@ namespace libtorrent
 			int to_copy = (std::min)(m_block_size
 					- block_offset, size);
 			std::memcpy(j.buffer + buffer_offset
-					, p.blocks[block].buf + block_offset
+				, p.blocks[block].buf + block_offset
-					, to_copy);
+				, to_copy);
 			size -= to_copy;
 			block_offset = 0;
 			buffer_offset += to_copy;
+			if (m_settings.volatile_read_cache)
+			{
+				free_buffer(p.blocks[block].buf);
+				p.blocks[block].buf = 0;
+				--p.num_blocks;
+			}
 			++block;
 		}
 		return j.buffer_size;

src/session_impl.cpp

@@ -1021,6 +1021,7 @@ namespace aux {
 			|| m_settings.use_read_cache != s.use_read_cache
 			|| m_settings.allow_reordered_disk_operations != s.allow_reordered_disk_operations
 			|| m_settings.file_pool_size != s.file_pool_size
+			|| m_settings.volatile_read_cache != s.volatile_read_cache
 			|| m_settings.low_prio_disk != s.low_prio_disk)
 			update_disk_io_thread = true;