enable coalesce_reads and coalesce_writes by default on windows

ChangeLog

@@ -1,4 +1,5 @@
 
+	* coalesce reads and writes by default on windows
 	* fixed disk I/O performance of checking hashes and creating torrents
 	* fix race condition in part_file
 	* fix part_file open mode compatibility test

src/settings_pack.cpp

@@ -165,8 +165,17 @@ namespace libtorrent
 		DEPRECATED_SET(use_write_cache, true, 0),
 		DEPRECATED_SET(dont_flush_write_cache, false, 0),
 		DEPRECATED_SET(explicit_read_cache, false, 0),
+#ifdef TORRENT_WINDOWS
+		// the emulation of preadv/pwritev uses overlapped reads/writes to be able
+		// to issue them all back to back. However, it appears windows fail to
+		// merge them. At least for people reporting performance issues in
+		// qBittorrent
+		SET(coalesce_reads, true, 0),
+		SET(coalesce_writes, true, 0),
+#else
 		SET(coalesce_reads, false, 0),
 		SET(coalesce_writes, false, 0),
+#endif
 		SET(auto_manage_prefer_seeds, false, 0),
 		SET(dont_count_slow_torrents, true, &session_impl::update_count_slow),
 		SET(close_redundant_connections, true, 0),

test/test_transfer.cpp

@@ -447,6 +447,28 @@ TORRENT_TEST(coalesce_writes)
 	cleanup();
 }
 
+TORRENT_TEST(no_coalesce_reads)
+{
+	using namespace libtorrent;
+	// test allowed fast
+	settings_pack p;
+	p.set_int(settings_pack::read_cache_line_size, 16);
+	p.set_bool(settings_pack::coalesce_reads, false);
+	test_transfer(0, p, false);
+
+	cleanup();
+}
+
+TORRENT_TEST(no_coalesce_writes)
+{
+	using namespace libtorrent;
+	// test allowed fast
+	settings_pack p;
+	p.set_bool(settings_pack::coalesce_writes, false);
+	test_transfer(0, p, false);
+
+	cleanup();
+}
 
 TORRENT_TEST(allocate)
 {