added instrumentation for disk access. Useful to benchmark disk cache performance

disk_access.gnuplot

@@ -0,0 +1,7 @@
+set term png size 1200,700
+set output "disk_access.png"
+set xrange [0:*]
+set xlabel "time (ms)"
+set ylabel "disk offset"
+set key box
+plot  "disk_access.log" using 1:3 title "disk access locations" with steps

include/libtorrent/disk_io_thread.hpp

@@ -262,6 +262,10 @@ namespace libtorrent
 		void check_invariant() const;
 #endif
 		
+#ifdef TORRENT_DISK_STATS
+		std::ofstream m_disk_access_log;
+#endif
+
 	private:
 
 		struct cached_piece_entry

src/disk_io_thread.cpp

@@ -261,6 +261,7 @@ namespace libtorrent
 	{
 #ifdef TORRENT_DISK_STATS
 		m_log.open("disk_io_thread.log", std::ios::trunc);
+		m_disk_access_log.open("disk_access.log", std::ios::trunc);
 #endif
 	}
 

src/storage.cpp

@@ -1204,17 +1204,41 @@ ret:
 	int storage::writev(file::iovec_t const* bufs, int slot, int offset
 		, int num_bufs)
 	{
+#ifdef TORRENT_DISK_STATS
+		disk_io_thread* iothread = (disk_io_thread*)disk_pool();
+		iothread->m_disk_access_log << log_time() << " write "
+			<< (size_type(slot) * m_files.piece_length() + offset) << std::endl;
+#endif
 		fileop op = { &file::writev, &storage::write_unaligned
 			, m_settings ? settings().disk_io_write_mode : 0, file::read_write };
+#ifdef TORRENT_DISK_STATS
+		int ret = readwritev(bufs, slot, offset, num_bufs, op);
+		iothread->m_disk_access_log << log_time() << " write_end "
+			<< (size_type(slot) * m_files.piece_length() + offset + ret) << std::endl;
+		return ret;
+#else
 		return readwritev(bufs, slot, offset, num_bufs, op);
+#endif
 	}
 
 	int storage::readv(file::iovec_t const* bufs, int slot, int offset
 		, int num_bufs)
 	{
+#ifdef TORRENT_DISK_STATS
+		disk_io_thread* iothread = (disk_io_thread*)disk_pool();
+		iothread->m_disk_access_log << log_time() << " read "
+			<< (size_type(slot) * m_files.piece_length() + offset) << std::endl;
+#endif
 		fileop op = { &file::readv, &storage::read_unaligned
 			, m_settings ? settings().disk_io_read_mode : 0, file::read_only };
+#ifdef TORRENT_DISK_STATS
+		int ret = readwritev(bufs, slot, offset, num_bufs, op);
+		iothread->m_disk_access_log << log_time() << " read_end "
+			<< (size_type(slot) * m_files.piece_length() + offset + ret) << std::endl;
+		return ret;
+#else
 		return readwritev(bufs, slot, offset, num_bufs, op);
+#endif
 	}
 
 	// much of what needs to be done when reading and writing