add performance warning when disk write queue exceeds half of the write cache size

docs/manual.rst

@@ -6753,7 +6753,8 @@ upload or download rate performance.
 			upload_limit_too_low,
 			download_limit_too_low,
 			send_buffer_watermark_too_low,
-			too_many_optimistic_unchoke_slots
+			too_many_optimistic_unchoke_slots,
+			too_high_disk_queue_limit
 		};
 
 		performance_warning_t warning_code;
@@ -6816,6 +6817,12 @@ too_many_optimistic_unchoke_slots
 	If the half (or more) of all upload slots are set as optimistic unchoke slots, this
 	warning is issued. You probably want more regular (rate based) unchoke slots.
 
+too_high_disk_queue_limit
+	If the disk write queue ever grows larger than half of the cache size, this warning
+	is posted. The disk write queue eats into the total disk cache and leaves very little
+	left for the actual cache. This causes the disk cache to oscillate in evicting large
+	portions of the cache before allowing peers to download any more, onto the disk write
+	queue. Either lower ``max_queued_disk_bytes`` or increase ``cache_size``.
 
 state_changed_alert
 -------------------

include/libtorrent/alert_types.hpp

@@ -220,7 +220,7 @@ namespace libtorrent
 			send_buffer_watermark_too_low,
 			too_many_optimistic_unchoke_slots,
 			bittyrant_with_no_uplimit,
-
+			too_high_disk_queue_limit,
 
 
 

include/libtorrent/aux_/session_impl.hpp

@@ -762,6 +762,7 @@ namespace libtorrent
 			ptime m_last_second_tick;
 			// used to limit how often disk warnings are generated
 			ptime m_last_disk_performance_warning;
+			ptime m_last_disk_queue_performance_warning;
 
 			// the last time we went through the peers
 			// to decide which ones to choke/unchoke

include/libtorrent/disk_io_thread.hpp

@@ -300,7 +300,9 @@ namespace libtorrent
 
 		// aborts read operations
 		void stop(boost::intrusive_ptr<piece_manager> s);
-		void add_job(disk_io_job const& j
+
+		// returns the disk write queue size
+		int add_job(disk_io_job const& j
 			, boost::function<void(int, disk_io_job const&)> const& f
 			= boost::function<void(int, disk_io_job const&)>());
 
@@ -367,7 +369,7 @@ namespace libtorrent
 
 	private:
 
-		void add_job(disk_io_job const& j
+		int add_job(disk_io_job const& j
 			, mutex::scoped_lock& l
 			, boost::function<void(int, disk_io_job const&)> const& f
 			= boost::function<void(int, disk_io_job const&)>());

include/libtorrent/storage.hpp

@@ -234,7 +234,8 @@ namespace libtorrent
 			, boost::function<void(int, disk_io_job const&)> const& handler
 			, int cache_expiry = 0);
 
-		void async_write(
+		// returns the write queue size
+		int async_write(
 			peer_request const& r
 			, disk_buffer_holder& buffer
 			, boost::function<void(int, disk_io_job const&)> const& f);

src/alert.cpp

@@ -116,7 +116,8 @@ namespace libtorrent {
 			"download limit too low (upload rate will suffer)",
 			"send buffer watermark too low (upload rate will suffer)",
 			"too many optimistic unchoke slots",
-			"using bittyrant unchoker with no upload rate limit set"
+			"using bittyrant unchoker with no upload rate limit set",
+			"the disk queue limit is too high compared to the cache size. The disk queue eats into the cache size"
 		};
 
 		return torrent_alert::message() + ": performance warning: "

src/disk_io_thread.cpp

@@ -1362,7 +1362,7 @@ namespace libtorrent
 		return m_queue_buffer_size;
 	}
 
-	void disk_io_thread::add_job(disk_io_job const& j
+	int disk_io_thread::add_job(disk_io_job const& j
 		, mutex::scoped_lock& l
 		, boost::function<void(int, disk_io_job const&)> const& f)
 	{
@@ -1378,9 +1378,10 @@ namespace libtorrent
 				m_exceeded_write_queue = true;
 		}
 		m_signal.signal(l);
+		return m_queue_buffer_size;
 	}
 
-	void disk_io_thread::add_job(disk_io_job const& j
+	int disk_io_thread::add_job(disk_io_job const& j
 		, boost::function<void(int, disk_io_job const&)> const& f)
 	{
 		TORRENT_ASSERT(!m_abort);
@@ -1389,7 +1390,7 @@ namespace libtorrent
 			|| j.action == disk_io_job::update_settings);
 		TORRENT_ASSERT(j.buffer_size <= m_block_size);
 		mutex::scoped_lock l(m_queue_mutex);
-		add_job(j, l, f);
+		return add_job(j, l, f);
 	}
 
 	bool disk_io_thread::test_error(disk_io_job& j)

src/peer_connection.cpp

@@ -2375,12 +2375,21 @@ namespace libtorrent
 			}
 		}
 
-		fs.async_write(p, data, boost::bind(&peer_connection::on_disk_write_complete
+		int write_queue_size = fs.async_write(p, data, boost::bind(&peer_connection::on_disk_write_complete
 			, self(), _1, _2, p, t));
 		m_outstanding_writing_bytes += p.length;
 		TORRENT_ASSERT(m_channel_state[download_channel] == peer_info::bw_idle);
 		m_download_queue.erase(b);
 
+		if (write_queue_size > m_ses.m_settings.cache_size / 2
+			&& (now - m_ses.m_last_disk_queue_performance_warning) > seconds(10)
+			&& m_ses.m_alerts.should_post<performance_alert>())
+		{
+			m_ses.m_last_disk_queue_performance_warning = now;
+			t->alerts().post_alert(performance_alert(t->get_handle()
+				, performance_alert::too_high_disk_queue_limit));
+		}
+
 		if (!m_ses.can_write_to_disk()
 			&& m_ses.settings().max_queued_disk_bytes
 			&& t->alerts().should_post<performance_alert>()

src/session_impl.cpp

@@ -513,6 +513,7 @@ namespace aux {
 		, m_last_tick(m_created)
 		, m_last_second_tick(m_created - milliseconds(900))
 		, m_last_disk_performance_warning(min_time())
+		, m_last_disk_queue_performance_warning(min_time())
 		, m_last_choke(m_created)
 		, m_next_rss_update(min_time())
 #ifndef TORRENT_DISABLE_DHT

src/storage.cpp

@@ -1671,7 +1671,7 @@ ret:
 #endif
 	}
 
-	void piece_manager::async_write(
+	int piece_manager::async_write(
 		peer_request const& r
 		, disk_buffer_holder& buffer
 		, boost::function<void(int, disk_io_job const&)> const& handler)
@@ -1687,8 +1687,10 @@ ret:
 		j.offset = r.start;
 		j.buffer_size = r.length;
 		j.buffer = buffer.get();
-		m_io_thread.add_job(j, handler);
+		int queue_size = m_io_thread.add_job(j, handler);
 		buffer.release();
+
+		return queue_size;
 	}
 
 	void piece_manager::async_hash(int piece