Add: [Video] move GameLoop into its own thread

This allows drawing to happen while the GameLoop is doing an
iteration too.

Sadly, not much drawing currently can be done while the GameLoop
is running, as for example PollEvent() or UpdateWindows() can
influence the game-state. As such, they first need to acquire a
lock on the game-state before they can be called.

Currently, the main advantage is the time spend in Paint(), which
for non-OpenGL drivers can be a few milliseconds. For OpenGL this
is more like 0.05 milliseconds; in these instances this change
doesn't add any benefits for now.

This is an alternative to the former "draw-thread", which moved
the drawing in a thread for some OSes. It has similar performance
gain as this does, although this implementation allows for more
finer control over what suffers when the GameLoop takes too
long: drawing or the next GameLoop. For now they both suffer
equally.

1 files changed, 84 insertions, 22 deletions

diff --git a/src/video/video_driver.cpp b/src/video/video_driver.cpp
index 49337c03a..e313329d5 100644
--- a/src/video/video_driver.cpp
+++ b/src/video/video_driver.cpp
@@ -8,9 +8,9 @@
 /** @file video_driver.cpp Common code between video driver implementations. */
 
 #include "../stdafx.h"
-#include "../debug.h"
 #include "../core/random_func.hpp"
 #include "../network/network.h"
+#include "../debug.h"
 #include "../gfx_func.h"
 #include "../progress.h"
 #include "../thread.h"
@@ -19,40 +19,84 @@
 
 bool _video_hw_accel; ///< Whether to consider hardware accelerated video drivers.
 
-void VideoDriver::Tick()
+void VideoDriver::GameLoop()
 {
-	auto cur_ticks = std::chrono::steady_clock::now();
+	this->next_game_tick += this->GetGameInterval();
 
-	if (cur_ticks >= this->next_game_tick) {
-		this->next_game_tick += this->GetGameInterval();
-		/* Avoid next_game_tick getting behind more and more if it cannot keep up. */
-		if (this->next_game_tick < cur_ticks - ALLOWED_DRIFT * this->GetGameInterval()) this->next_game_tick = cur_ticks;
+	/* Avoid next_game_tick getting behind more and more if it cannot keep up. */
+	auto now = std::chrono::steady_clock::now();
+	if (this->next_game_tick < now - ALLOWED_DRIFT * this->GetGameInterval()) this->next_game_tick = now;
+
+	{
+		std::lock_guard<std::mutex> lock(this->game_state_mutex);
 
-		/* The game loop is the part that can run asynchronously.
-		 * The rest except sleeping can't. */
-		this->UnlockVideoBuffer();
 		::GameLoop();
-		this->LockVideoBuffer();
+	}
+}
+
+void VideoDriver::GameThread()
+{
+	while (!_exit_game) {
+		this->GameLoop();
+
+		auto now = std::chrono::steady_clock::now();
+		if (this->next_game_tick > now) {
+			std::this_thread::sleep_for(this->next_game_tick - now);
+		} else {
+			/* Ensure we yield this thread if drawings wants to take a lock on
+			 * the game state. This is mainly because most OSes have an
+			 * optimization that if you unlock/lock a mutex in the same thread
+			 * quickly, it will never context switch even if there is another
+			 * thread waiting to take the lock on the same mutex. */
+			std::lock_guard<std::mutex> lock(this->game_thread_wait_mutex);
+		}
+	}
+}
+
+/* static */ void VideoDriver::GameThreadThunk(VideoDriver *drv)
+{
+	drv->GameThread();
+}
+
+void VideoDriver::StartGameThread()
+{
+	if (this->is_game_threaded) {
+		this->is_game_threaded = StartNewThread(&this->game_thread, "ottd:game", &VideoDriver::GameThreadThunk, this);
+	}
+
+	DEBUG(driver, 1, "using %sthread for game-loop", this->is_game_threaded ? "" : "no ");
+}
+
+void VideoDriver::StopGameThread()
+{
+	if (!this->is_game_threaded) return;
+
+	this->game_thread.join();
+}
+
+void VideoDriver::Tick()
+{
+	if (!this->is_game_threaded && std::chrono::steady_clock::now() >= this->next_game_tick) {
+		this->GameLoop();
 
 		/* For things like dedicated server, don't run a separate draw-tick. */
 		if (!this->HasGUI()) {
 			::InputLoop();
-			UpdateWindows();
+			::UpdateWindows();
 			this->next_draw_tick = this->next_game_tick;
 		}
 	}
 
-	/* Prevent drawing when switching mode, as windows can be removed when they should still appear. */
-	if (this->HasGUI() && cur_ticks >= this->next_draw_tick && (_switch_mode == SM_NONE || _game_mode == GM_BOOTSTRAP || HasModalProgress())) {
+	auto now = std::chrono::steady_clock::now();
+	if (this->HasGUI() && now >= this->next_draw_tick) {
 		this->next_draw_tick += this->GetDrawInterval();
 		/* Avoid next_draw_tick getting behind more and more if it cannot keep up. */
-		if (this->next_draw_tick < cur_ticks - ALLOWED_DRIFT * this->GetDrawInterval()) this->next_draw_tick = cur_ticks;
+		if (this->next_draw_tick < now - ALLOWED_DRIFT * this->GetDrawInterval()) this->next_draw_tick = now;
 
 		/* Keep the interactive randomizer a bit more random by requesting
 		 * new values when-ever we can. */
 		InteractiveRandom();
 
-		while (this->PollEvent()) {}
 		this->InputLoop();
 
 		/* Check if the fast-forward button is still pressed. */
@@ -64,23 +108,41 @@ void VideoDriver::Tick()
 			this->fast_forward_via_key = false;
 		}
 
-		::InputLoop();
-		UpdateWindows();
+		{
+			/* Tell the game-thread to stop so we can have a go. */
+			std::lock_guard<std::mutex> lock_wait(this->game_thread_wait_mutex);
+			std::lock_guard<std::mutex> lock_state(this->game_state_mutex);
+
+			this->LockVideoBuffer();
+
+			while (this->PollEvent()) {}
+			::InputLoop();
+
+			/* Prevent drawing when switching mode, as windows can be removed when they should still appear. */
+			if (_game_mode == GM_BOOTSTRAP || _switch_mode == SM_NONE || HasModalProgress()) {
+				::UpdateWindows();
+			}
+
+			this->PopulateSystemSprites();
+		}
 
 		this->CheckPaletteAnim();
 		this->Paint();
+
+		this->UnlockVideoBuffer();
 	}
 }
 
 void VideoDriver::SleepTillNextTick()
 {
-	/* See how much time there is till we have to process the next event, and try to hit that as close as possible. */
-	auto next_tick = std::min(this->next_draw_tick, this->next_game_tick);
+	auto next_tick = this->next_draw_tick;
 	auto now = std::chrono::steady_clock::now();
 
+	if (!this->is_game_threaded) {
+		next_tick = min(next_tick, this->next_game_tick);
+	}
+
 	if (next_tick > now) {
-		this->UnlockVideoBuffer();
 		std::this_thread::sleep_for(next_tick - now);
-		this->LockVideoBuffer();
 	}
 }