allow each peer have at least 2 allocated disk blocks at any given time, to avoid stalling when cache_size setting is small. also deprecate use_write_cache

include/libtorrent/disk_io_job.hpp

@@ -136,11 +136,7 @@ namespace libtorrent
 			in_progress = 0x20,
 
 			// turns into file::coalesce_buffers in the file operation
-			coalesce_buffers = 0x40,
-
-			// the disk cache was enabled when this job was issued, it should use
-			// the disk cache once it's handled by a disk thread
-			use_disk_cache = 0x80
+			coalesce_buffers = 0x40
 		};
 
 		// for write jobs, returns true if its block

include/libtorrent/disk_io_thread.hpp

@@ -1,6 +1,6 @@
 /*
 
-Copyright (c) 2007-2016, Arvid Norberg
+Copyright (c) 2007-2016, Arvid Norberg, Steven Siloti
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without

include/libtorrent/settings_pack.hpp

@@ -259,14 +259,16 @@ namespace libtorrent
 			// passes the hash check, it is taken out of parole mode.
 			use_parole_mode,
 
-			// enable and disable caching of read blocks and blocks to be written
-			// to disk respsectively. the purpose of the read cache is partly
-			// read-ahead of requests but also to avoid reading blocks back from
-			// the disk multiple times for popular pieces. the write cache purpose
-			// is to hold off writing blocks to disk until they have been hashed,
-			// to avoid having to read them back in again.
+			// enable and disable caching of blocks read from disk. the purpose of
+			// the read cache is partly read-ahead of requests but also to avoid
+			// reading blocks back from the disk multiple times for popular
+			// pieces.
 			use_read_cache,
+#ifndef TORRENT_NO_DEPRECATED
 			use_write_cache,
+#else
+			deprecated7,
+#endif
 
 			// this will make the disk cache never flush a write piece if it would
 			// cause is to have to re-read it once we want to calculate the piece

src/disk_io_thread.cpp

@@ -1,6 +1,6 @@
 /*
 
-Copyright (c) 2007-2016, Arvid Norberg
+Copyright (c) 2007-2016, Arvid Norberg, Steven Siloti
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -1208,14 +1208,6 @@ namespace libtorrent
 
 	int disk_io_thread::do_read(disk_io_job* j, jobqueue_t& completed_jobs)
 	{
-		if ((j->flags & disk_io_job::use_disk_cache) == 0)
-		{
-			// we're not using a cache. This is the simple path
-			// just read straight from the file
-			int ret = do_uncached_read(j);
-			return ret;
-		}
-
 		int block_size = m_disk_cache.block_size();
 		int piece_size = j->storage->files()->piece_size(j->piece);
 		int blocks_in_piece = (piece_size + block_size - 1) / block_size;
@@ -1232,26 +1224,8 @@ namespace libtorrent
 		cached_piece_entry* pe = m_disk_cache.find_piece(j);
 		if (pe == NULL)
 		{
-			// this isn't supposed to happen. The piece is supposed
-			// to be allocated when the read job is posted to the
-			// queue, and have 'outstanding_read' set to 1
-			TORRENT_ASSERT(false);
-
-			int cache_state = (j->flags & disk_io_job::volatile_read)
-				? cached_piece_entry::volatile_read_lru
-				: cached_piece_entry::read_lru1;
-			pe = m_disk_cache.allocate_piece(j, cache_state);
-			if (pe == NULL)
-			{
-				j->error.ec = error::no_memory;
-				j->error.operation = storage_error::alloc_cache_piece;
-				m_disk_cache.free_iovec(iov, iov_len);
-				return -1;
-			}
-#if TORRENT_USE_ASSERTS
-			pe->piece_log.push_back(piece_log_t(piece_log_t::set_outstanding_jobs));
-#endif
-			pe->outstanding_read = 1;
+			l.unlock();
+			return do_uncached_read(j);
 		}
 		TORRENT_PIECE_ASSERT(pe->outstanding_read == 1, pe);
 
@@ -1633,7 +1607,6 @@ namespace libtorrent
 			j->error.operation = storage_error::read;
 			return 0;
 		}
-		j->flags |= disk_io_job::use_disk_cache;
 		if (pe->outstanding_read)
 		{
 			TORRENT_PIECE_ASSERT(j->piece == pe->piece, pe);

src/peer_connection.cpp

@@ -2611,7 +2611,13 @@ namespace libtorrent
 			return;
 		}
 
-		if (exceeded)
+		// every peer is entitled to have two disk blocks allocated at any given
+		// time, regardless of whether the cache size is exceeded or not. If this
+		// was not the case, when the cache size setting is very small, most peers
+		// would be blocked most of the time, because the disk cache would
+		// continously be in exceeded state. Only rarely would it actually drop
+		// down to 0 and unblock all peers.
+		if (exceeded && m_outstanding_writing_bytes > 0)
 		{
 			if ((m_channel_state[download_channel] & peer_info::bw_disk) == 0)
 				m_counters.inc_stats_counter(counters::num_peers_down_disk);
@@ -4541,7 +4547,9 @@ namespace libtorrent
 			return false;
 		}
 
-		if (exceeded)
+		// to understand why m_outstanding_writing_bytes is here, see comment by
+		// the other call to allocate_disk_buffer()
+		if (exceeded && m_outstanding_writing_bytes > 0)
 		{
 #ifndef TORRENT_DISABLE_LOGGING
 			peer_log(peer_log_alert::info, "DISK", "exceeded disk buffer watermark");

src/settings_pack.cpp

@@ -150,7 +150,7 @@ namespace libtorrent
 		SET(upnp_ignore_nonrouters, false, 0),
 		SET(use_parole_mode, true, 0),
 		SET(use_read_cache, true, 0),
-		SET(use_write_cache, true, 0),
+		DEPRECATED_SET(use_write_cache, true, 0),
 		SET(dont_flush_write_cache, false, 0),
 		SET(explicit_read_cache, false, 0),
 		SET(coalesce_reads, false, 0),
@@ -603,14 +603,14 @@ namespace libtorrent
 				&& std::find(callbacks.begin(), callbacks.end(), sa.fun) == callbacks.end())
 				callbacks.push_back(sa.fun);
 		}
-	
+
 		for (std::vector<std::pair<boost::uint16_t, int> >::const_iterator i = pack->m_ints.begin()
 			, end(pack->m_ints.end()); i != end; ++i)
 		{
 			// disregard setting indices that are not int types
 			if ((i->first & settings_pack::type_mask) != settings_pack::int_type_base)
 				continue;
-		
+
 			// ignore settings that are out of bounds
 			int index = i->first & settings_pack::index_mask;
 			if (index < 0 || index >= settings_pack::num_int_settings)
@@ -629,7 +629,7 @@ namespace libtorrent
 			// disregard setting indices that are not bool types
 			if ((i->first & settings_pack::type_mask) != settings_pack::bool_type_base)
 				continue;
-		
+
 			// ignore settings that are out of bounds
 			int index = i->first & settings_pack::index_mask;
 			if (index < 0 || index >= settings_pack::num_bool_settings)