Rework server stepping and dtime calculation

src/CMakeLists.txt

@@ -314,6 +314,13 @@ else()
 	endif()
 endif()
 
+# On clang and gcc, some functionalities of std::atomic require -latomic.
+# See <https://en.cppreference.com/w/cpp/atomic/atomic#Notes>.
+if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
+		OR CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+	set(PLATFORM_LIBS ${PLATFORM_LIBS} atomic)
+endif()
+
 check_include_files(endian.h HAVE_ENDIAN_H)
 
 configure_file(

src/client/game.cpp

@@ -1304,8 +1304,8 @@ void Game::run()
 		updatePauseState();
 		if (m_is_paused)
 			dtime = 0.0f;
-		else
-			step(dtime);
+
+		step(dtime);
 
 		processClientEvents(&cam_view_target);
 		updateDebugState();
@@ -1454,7 +1454,7 @@ bool Game::createSingleplayerServer(const std::string &map_dir,
 	} else {
 		bind_str = g_settings->get("bind_address");
 	}
-	
+
 	Address bind_addr(0, 0, 0, 0, port);
 
 	if (g_settings->getBool("ipv6_server"))
@@ -1682,10 +1682,7 @@ bool Game::connectToServer(const GameStartData &start_data,
 			fps_control.limit(device, &dtime);
 
 			// Update client and server
-			client->step(dtime);
-
-			if (server != NULL)
-				server->step(dtime);
+			step(dtime);
 
 			// End condition
 			if (client->getState() == LC_Init) {
@@ -1744,10 +1741,7 @@ bool Game::getServerContent(bool *aborted)
 		fps_control.limit(device, &dtime);
 
 		// Update client and server
-		client->step(dtime);
-
-		if (server != NULL)
-			server->step(dtime);
+		step(dtime);
 
 		// End condition
 		if (client->mediaReceived() && client->itemdefReceived() &&
@@ -2765,10 +2759,23 @@ void Game::updatePauseState()
 
 inline void Game::step(f32 dtime)
 {
-	if (server)
-		server->step(dtime);
+	if (server) {
+		float fps_max = (!device->isWindowFocused() || g_menumgr.pausesGame()) ?
+				g_settings->getFloat("fps_max_unfocused") :
+				g_settings->getFloat("fps_max");
+		fps_max = std::max(fps_max, 1.0f);
+		float steplen = 1.0f / fps_max;
+
+		server->setStepSettings(Server::StepSettings{
+				steplen,
+				m_is_paused
+			});
 
-	client->step(dtime);
+		server->step();
+	}
+
+	if (!m_is_paused)
+		client->step(dtime);
 }
 
 static void pauseNodeAnimation(PausedNodesList &paused, scene::ISceneNode *node) {

src/network/connection.cpp

@@ -1418,15 +1418,15 @@ void Connection::Disconnect()
 	putCommand(ConnectionCommand::disconnect());
 }
 
-bool Connection::Receive(NetworkPacket *pkt, u32 timeout)
+bool Connection::ReceiveTimeoutMs(NetworkPacket *pkt, u32 timeout_ms)
 {
 	/*
 		Note that this function can potentially wait infinitely if non-data
 		events keep happening before the timeout expires.
 		This is not considered to be a problem (is it?)
 	*/
 	for(;;) {
-		ConnectionEventPtr e_ptr = waitEvent(timeout);
+		ConnectionEventPtr e_ptr = waitEvent(timeout_ms);
 		const ConnectionEvent &e = *e_ptr;
 
 		if (e.type != CONNEVENT_NONE) {
@@ -1467,14 +1467,14 @@ bool Connection::Receive(NetworkPacket *pkt, u32 timeout)
 
 void Connection::Receive(NetworkPacket *pkt)
 {
-	bool any = Receive(pkt, m_bc_receive_timeout);
+	bool any = ReceiveTimeoutMs(pkt, m_bc_receive_timeout);
 	if (!any)
 		throw NoIncomingDataException("No incoming data");
 }
 
 bool Connection::TryReceive(NetworkPacket *pkt)
 {
-	return Receive(pkt, 0);
+	return ReceiveTimeoutMs(pkt, 0);
 }
 
 void Connection::Send(session_t peer_id, u8 channelnum,

src/network/connection.h

@@ -714,7 +714,8 @@ class Connection
 	void Connect(Address address);
 	bool Connected();
 	void Disconnect();
-	void Receive(NetworkPacket* pkt);
+	bool ReceiveTimeoutMs(NetworkPacket *pkt, u32 timeout_ms);
+	void Receive(NetworkPacket *pkt);
 	bool TryReceive(NetworkPacket *pkt);
 	void Send(session_t peer_id, u8 channelnum, NetworkPacket *pkt, bool reliable);
 	session_t GetPeerID() const { return m_peer_id; }
@@ -747,8 +748,6 @@ class Connection
 	// Command queue: user -> SendThread
 	MutexedQueue<ConnectionCommandPtr> m_command_queue;
 
-	bool Receive(NetworkPacket *pkt, u32 timeout);
-
 	void putEvent(ConnectionEventPtr e);
 
 	void TriggerSend();

src/server.cpp

@@ -106,26 +106,33 @@ void *ServerThread::run()
 	/*
 	 * The real business of the server happens on the ServerThread.
 	 * How this works:
-	 * AsyncRunStep() runs an actual server step as soon as enough time has
-	 * passed (dedicated_server_loop keeps track of that).
-	 * Receive() blocks at least(!) 30ms waiting for a packet (so this loop
-	 * doesn't busy wait) and will process any remaining packets.
+	 * AsyncRunStep() (which runs the actual server step) is called at the
+	 * server-step frequency. Receive() is used for waiting between the steps.
 	 */
 
 	try {
-		m_server->AsyncRunStep(true);
+		m_server->AsyncRunStep(0.0f, true);
 	} catch (con::ConnectionBindFailed &e) {
 		m_server->setAsyncFatalError(e.what());
 	} catch (LuaError &e) {
 		m_server->setAsyncFatalError(e);
 	}
 
+	float dtime = 0.0f;
+
 	while (!stopRequested()) {
 		ScopeProfiler spm(g_profiler, "Server::RunStep() (max)", SPT_MAX);
+
+		u64 t0 = porting::getTimeUs();
+
+		const Server::StepSettings step_settings = m_server->getStepSettings();
+
 		try {
-			m_server->AsyncRunStep();
+			m_server->AsyncRunStep(step_settings.pause ? 0.0f : dtime);
 
-			m_server->Receive();
+			const float remaining_time = step_settings.steplen
+					- 1e-6f * (porting::getTimeUs() - t0);
+			m_server->Receive(remaining_time);
 
 		} catch (con::PeerNotFoundException &e) {
 			infostream<<"Server: PeerNotFoundException"<<std::endl;
@@ -135,6 +142,8 @@ void *ServerThread::run()
 		} catch (LuaError &e) {
 			m_server->setAsyncFatalError(e);
 		}
+
+		dtime = 1e-6f * (porting::getTimeUs() - t0);
 	}
 
 	END_DEBUG_EXCEPTION_HANDLER
@@ -574,15 +583,8 @@ void Server::stop()
 	infostream<<"Server: Threads stopped"<<std::endl;
 }
 
-void Server::step(float dtime)
+void Server::step()
 {
-	// Limit a bit
-	if (dtime > DTIME_LIMIT)
-		dtime = DTIME_LIMIT;
-	{
-		MutexAutoLock lock(m_step_dtime_mutex);
-		m_step_dtime += dtime;
-	}
 	// Throw if fatal error occurred in thread
 	std::string async_err = m_async_fatal_error.get();
 	if (!async_err.empty()) {
@@ -595,30 +597,18 @@ void Server::step(float dtime)
 	}
 }
 
-void Server::AsyncRunStep(bool initial_step)
+void Server::AsyncRunStep(float dtime, bool initial_step)
 {
-
-	float dtime;
-	{
-		MutexAutoLock lock1(m_step_dtime_mutex);
-		dtime = m_step_dtime;
-	}
-
 	{
 		// Send blocks to clients
 		SendBlocks(dtime);
 	}
 
-	if((dtime < 0.001) && !initial_step)
+	if ((dtime < 0.001f) && !initial_step)
 		return;
 
 	ScopeProfiler sp(g_profiler, "Server::AsyncRunStep()", SPT_AVG);
 
-	{
-		MutexAutoLock lock1(m_step_dtime_mutex);
-		m_step_dtime -= dtime;
-	}
-
 	/*
 		Update uptime
 	*/
@@ -1048,25 +1038,27 @@ void Server::AsyncRunStep(bool initial_step)
 	m_shutdown_state.tick(dtime, this);
 }
 
-void Server::Receive()
+void Server::Receive(float timeout)
 {
+	const u64 t0 = porting::getTimeUs();
+	const float timeout_us = timeout * 1e6f;
+	auto remaining_time_us = [&]() -> float {
+		return std::max(0.0f, timeout_us - (porting::getTimeUs() - t0));
+	};
+
 	NetworkPacket pkt;
 	session_t peer_id;
-	bool first = true;
 	for (;;) {
 		pkt.clear();
 		peer_id = 0;
 		try {
-			/*
-				In the first iteration *wait* for a packet, afterwards process
-				all packets that are immediately available (no waiting).
-			*/
-			if (first) {
-				m_con->Receive(&pkt);
-				first = false;
-			} else {
-				if (!m_con->TryReceive(&pkt))
-					return;
+			if (!m_con->ReceiveTimeoutMs(&pkt,
+					(u32)remaining_time_us() / 1000)) {
+				// No incoming data.
+				// Already break if there's 1ms left, as ReceiveTimeoutMs is too coarse
+				// and a faster server-step is better than busy waiting.
+				if (remaining_time_us() < 1000.0f)
+					break;
 			}
 
 			peer_id = pkt.getPeerId();
@@ -1085,8 +1077,6 @@ void Server::Receive()
 			DenyAccess(peer_id, SERVER_ACCESSDENIED_UNEXPECTED_DATA);
 		} catch (const con::PeerNotFoundException &e) {
 			// Do nothing
-		} catch (const con::NoIncomingDataException &e) {
-			return;
 		}
 	}
 }
@@ -3953,21 +3943,24 @@ void dedicated_server_loop(Server &server, bool &kill)
 
 	IntervalLimiter m_profiler_interval;
 
-	static thread_local const float steplen =
-			g_settings->getFloat("dedicated_server_step");
-	static thread_local const float profiler_print_interval =
-			g_settings->getFloat("profiler_print_interval");
+	constexpr float steplen = 0.05f; // always 50 ms
+	const float profiler_print_interval = g_settings->getFloat("profiler_print_interval");
+
+	server.setStepSettings(Server::StepSettings{
+			g_settings->getFloat("dedicated_server_step"),
+			false
+		});
 
 	/*
-	 * The dedicated server loop only does time-keeping (in Server::step) and
-	 * provides a way to main.cpp to kill the server externally (bool &kill).
+	 * The dedicated server loop only provides a way to main.cpp to kill the
+	 * server externally (bool &kill).
 	 */
 
 	for(;;) {
 		// This is kind of a hack but can be done like this
 		// because server.step() is very light
-		sleep_ms((int)(steplen*1000.0));
-		server.step(steplen);
+		sleep_ms((int)(steplen*1000.0f));
+		server.step();
 
 		if (server.isShutdownRequested() || kill)
 			break;

src/server.h

@@ -38,6 +38,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "chatmessage.h"
 #include "sound.h"
 #include "translation.h"
+#include <atomic>
 #include <string>
 #include <list>
 #include <map>
@@ -157,12 +158,12 @@ class Server : public con::PeerHandler, public MapEventReceiver,
 
 	void start();
 	void stop();
-	// This is mainly a way to pass the time to the server.
 	// Actual processing is done in another thread.
-	void step(float dtime);
+	// This just checks if there was an error in that thread.
+	void step();
 	// This is run by ServerThread and does the actual processing
-	void AsyncRunStep(bool initial_step=false);
-	void Receive();
+	void AsyncRunStep(float dtime, bool initial_step = false);
+	void Receive(float timeout);
 	PlayerSAO* StageTwoClientInit(session_t peer_id);
 
 	/*
@@ -293,6 +294,14 @@ class Server : public con::PeerHandler, public MapEventReceiver,
 	inline bool isSingleplayer() const
 			{ return m_simple_singleplayer_mode; }
 
+	struct StepSettings {
+		float steplen;
+		bool pause;
+	};
+
+	void setStepSettings(StepSettings spdata) { m_step_settings.store(spdata); }
+	StepSettings getStepSettings() { return m_step_settings.load(); }
+
 	inline void setAsyncFatalError(const std::string &error)
 			{ m_async_fatal_error.set(error); }
 	inline void setAsyncFatalError(const LuaError &e)
@@ -624,10 +633,8 @@ class Server : public con::PeerHandler, public MapEventReceiver,
 	/*
 		Threads
 	*/
-	// A buffer for time steps
-	// step() increments and AsyncRunStep() run by m_thread reads it.
-	float m_step_dtime = 0.0f;
-	std::mutex m_step_dtime_mutex;
+	// Set by Game
+	std::atomic<StepSettings> m_step_settings{{0.1f, false}};
 
 	// The server mainly operates in this thread
 	ServerThread *m_thread = nullptr;