fix performance bug in the file pool, evicting MRU instead of LRU. Di…

…scovered by HanabishiRecca

ChangeLog

@@ -1,6 +1,7 @@
 
 2.0.11 not released
 
+	* fix performance bug in the file pool, evicting MRU instead of LRU (HanabishiRecca)
 	* fix bug where file_progress could sometimes be reported as >100%
 	* don't hint FADV_RANDOM on posix systems. May improve seeding performance
 	* allow boost connect while checking resume data if no_verify_files flag is set

include/libtorrent/aux_/file_view_pool.hpp

@@ -162,7 +162,8 @@ namespace aux {
 			mi::indexed_by<
 			// look up files by (torrent, file) key
 			mi::ordered_unique<mi::member<file_entry, file_id, &file_entry::key>>,
-			// look up files by least recently used
+			// look up files by least recently used. New items are added to the
+			// back, and old items are removed from the front.
 			mi::sequenced<>
 #if TORRENT_HAVE_MAP_VIEW_OF_FILE
 			// look up files with dirty pages

src/file_view_pool.cpp

@@ -138,8 +138,12 @@ namespace libtorrent { namespace aux {
 			});
 
 			auto& lru_view = m_files.get<1>();
-			lru_view.relocate(m_files.project<1>(i), lru_view.begin());
+			lru_view.relocate(lru_view.end(), m_files.project<1>(i));
 
+#if TORRENT_USE_ASSERTS
+			// ensure the LRU index is maintained as we would expect it to be
+			TORRENT_ASSERT(lru_view.back().key == i->key);
+#endif
 			return i->mapping;
 		}
 
@@ -174,7 +178,7 @@ namespace libtorrent { namespace aux {
 
 			// there's an edge case where two threads are racing to insert a newly
 			// opened file, one thread is opening a file for writing and the other
-			// fore reading. If the reading thread wins, it's important that the
+			// for reading. If the reading thread wins, it's important that the
 			// thread opening for writing still overwrites the file in the pool,
 			// since a file opened for reading and writing can be used for both.
 			// So, we can't move e in here, because we may need it again of the
@@ -200,8 +204,13 @@ namespace libtorrent { namespace aux {
 				}
 
 				auto& lru_view = m_files.get<1>();
-				lru_view.relocate(m_files.project<1>(i), lru_view.begin());
+				lru_view.relocate(lru_view.end(), m_files.project<1>(i));
 			}
+#if TORRENT_USE_ASSERTS
+			// ensure the LRU index is maintained as we would expect it to be
+			auto& lru_view = m_files.get<1>();
+			TORRENT_ASSERT(lru_view.back().key == e.key);
+#endif
 			notify_file_open(ofe, i->mapping, storage_error());
 			return i->mapping;
 		}
@@ -351,11 +360,11 @@ namespace libtorrent { namespace aux {
 
 #if TRACE_FILE_VIEW_POOL
 		std::cout << std::this_thread::get_id() << " removing: ("
-			<< lru_view.back().key.first << ", " << lru_view.back().key.second << ")\n";
+			<< lru_view.front().key.first << ", " << lru_view.front().key.second << ")\n";
 #endif
 
-		auto mapping = std::move(lru_view.back().mapping);
-		lru_view.pop_back();
+		auto mapping = std::move(lru_view.front().mapping);
+		lru_view.pop_front();
 
 		// closing a file may be long running operation (mac os x)
 		// let the caller destruct it once it has released the mutex