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floatingwoods

OpenGL
Game thread synchronization (display & game logic)

4 posts in this topic

I am wondering how others handle this situation:

 

You have an application (e.g. a game) that calculates a state (e.g. the game logic) and also displays this one. From what I read it seems that the game logic is most of the time running in a different thread than the display. Which brings me to the question:

 

How is the display synchronized with the game logic? If there is no synchronization, we can have following situations:

 

- game is stepped forward, displayed, stepped forward twice, displayed, etc. --> the display will appear shacking!

- while the game is stepped forward a frame is displayed --> the display can appear "strange" (e.g. the bullet can appear as hitting the game character, but since its state was not yet updated, it will actually not hit it)

- A game character can be removed from the scene during the game logic calculation. If the display happens at that time, there might be a crash.

 

We can synchronize to some extent the 2 threads by locking resources (the last example above can be handled by deferring object destruction). But in order to avoid all above mentionned problems, one should run the 2 threads in alternance (or similar, e.g.  step the game twice, render, step the game twice, render, etc.). Doing so makes the use of 2 threads not interesting anymore, since a single thread would be running at the same speed (more or less) and all the resource locking synchronization would not have to be taken care of: a single thread would be easier and have the same result. No?

 

I guess that the game state must be demultiplied in some way (e.g. all positions would be stored as "current" and "forDisplay", and at the end of a game step, "current" would be copied to "forDisplay", so that the rendering thread would be able to run concurrently).

 

And what happens if the game logic needs to use some OpenGL commands? e.g. to render to FBO and do some simple image processing on that? Then there will be again the need to synchronize the 2 threads in order to be able to correctly switch the OpenGL contexts!

 

Just curious how things are done usually ;)

 

 

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Hodgman and Matias,

 

Thanks a lot for the very clear and exhaustive explanations. The links you mentionned were also helpful.

To give a little bit more background: I used to get inspired a lot on the gamedev forums, I however am more in the field of simulation. There, it is of importance if something gets rendered wrongly, or if a frame gets skipped without specific reason. Interpolation between two states could work, but might lead to some unrealistic renderings too and confusion, specially when stepping generated videos later on (usually there is one frame per simulation step, which helps debugging certain set-ups). Finally, the simulation (or game logic) uses the openGL functionality, in order to generate virtual images, operate on them (e.g. image processing) and create an output. The time at which the "internal" or FBO rendering occures depends on the simulation loop and how it is programmed. So there I get another heavy restriction regarding multithreading: the rendering thread and "game logic" thread can both generate OpenGL commands, and need to switch contexts every time. In that case, locking (or rather blocking) the other thread is the only option.

Given the many constraints and limitations, I concluded that having an additional thread in charge of rendering would not give me much speed increase, but would complicate drastically the architecture.

My application uses basically one single thread (of course it also uses worker threads for specific tasks) that handles the "game logic" and the visualization. But I wanted to evaluate the benefits of spliting the task into 2 different threads, and maybe even offering the 2 alternatives.

 

Again thanks for the insightful replies!

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I see you intent issueing OpenGL calls from multiple threads. As you said, this is a very bad idea, and I personally avoid them due to lots of issues in the past; unless you keep 100% independent GL context for each thread.

Otherwise switching contexts is so error-prone (and driver bugs may appear, to be honest) that any performance gain you intend to get from going multithreading is going to be nullified, or worse; just leave it single threaded.

 

If your logic needs to issue rendering calls, you're not abstracting enough rendering from logic. If you're on a tight schedule, well ok; but if you've got the time, take your time to rethink how the systems relate; and whenever logic needs something from OpenGL, it requests to the render thread, and periodically check for results arrived.

 

For what you describe, your project appears to involve a lot of image processing from what has been rendered already (am I right?) in that case, since your game logic is rendering, and could not be decoupled, you should go single-threaded and rely on a method more like what Hodgman described (map buffer from GPU to CPU, then issue N threads to work on the received image, wait for all of them to finish) for multithreaded approaches.

Edited by Matias Goldberg
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