Some questions regarding game loop, tick and real time programming

Question

First I want to apologize for my approximate English, as I'm French. I'm currently making a real-time game in java, using LWJGL. I have some questions regarding game loops:

• I'm running the rendering routine in a thread. Is it a good idea? Usually, the rendering routine is fairly slow and should not slow down the world update (tick) routine, which is way more important. So I guess using a thread here seems like a good idea (minus the complications from using a thread).
• In the world update routine, I'm updating a list of entities with the current time. Each entity can then compute their own deltaTime, corresponding to the last time they were updated. This differs from the usual update loop, which updates every entity in the list with the same deltaTime. This seemed appropriate because of the threaded rendering. Is it a good idea? Should I use the second method instead? If so, is the threaded rendering still needed? If so, do I have to add a maximum deltaTime?
• In general, is it a good idea to have a maximum deltaTime?

Thanks for your time!

Answer 1

1. Is it a good idea? Separate threads are fairly advanced stuff, I see no reason to do multithreading to begin with. All the mobile games I have worked on so far have not needed multiple threads, even though they are 'real-time'. Hardcore PC and console games are where multithreading really starts to come into play. Here is a link to a recent talk on the subject if interested : http://archive.assembly.org/2011/seminars/adventures-in-multithreaded-gameplay-coding.

2. Sounds like this could cause some strange things if the physics are not handled in one go. Not sure about this. Colliding an object that has already been updated to another position with an object that comes another time, for example, correcting this sort of situation may become problematic? Fast moving collisions may need to be subdivided, which may be why you have the separate update thread, but why not have them all calculated as happening at the same time?

3. 'Variable timestep' and 'Fixed timestep' are the options available for rendering. Most games at the moment seem to choose a 30 fps fixed timestep. The rendering has to be kept under the limits so no catching up should be needed. One problem with variable timestep is you are forced to pass deltaTime to all time-dependent areas. Fixed timestep is handy as you can assume you are running at say 30 fps, and use that value everywhere. It is a preferred method at the moment as far as I know.

Answer 2

Though this question is a few years old…

AFAIK,

Rendering is usually done in separated processor — GPU, so they're already a separated thread. But, drawing command must be processed by graphics driver (which is running in CPU) before dispatched to GPU, and this processing may be saved by being multi-threaded. Anyway in this case, you're responsible to manage synchronization between logics and rendering thread.

Generally speaking, games are all about interactions between objects, and it's very hard to divide state-graph into fully separated divisions. As a result, whole game state usually becomes single graph, and this graph cannot be updated while being rendered. In this case, you have no benefit by being multi-threaded.

If you can keep a separated immutable data for rendering, than you may gain some benefit from rendering in separated thread. But otherwise, I don't recommend it.

In addition, you should consider GC if you truly want a realtime game. GC related performance issues usually the biggest obstacles to make realtime stuffs.