Deterministic float game state - strictfp?

Networking & Multiplayer
#1

Hi,

Developing the networking for TT, I’d like to keep things simple and simply assure that all game state is altered in the exact same way on all clients. That is, only game state altering events will be sent over the network, and no correction of different states on different machines will be needed.

The downside to this approach, is that I need to compute game states updates exactly the same, even on different platforms, and then I wondered what to do about the floating point game state we need? The tedious solution is fixed point integer math, but is there any easy way out through the strictfp keyword? As I see it, the keyword assures that all IEEE floating point operations follow the same rules, but I’d like the be certain. Game state differences through floating point differences would probably be hard to detect, and I can’t figure out a good way to test/prove it. And I wouldn’t wan’t to build game state on floating points and then months later discover I need to change it all to ints.

  • elias
#2

If float accuracy is your main concern, you seem to have extremely stable algortihms concerning framerate, latency and such. Are you sure this is really true?

#3

It is indeed accuracy that is the problem - but not in the way you might think: I don’t care how accurate the floating point computations are, but I must be certain that the accuracy is the same on all platforms the game is running on. According to the JVM specs, VM’s are allowed to use the extended precision floating point operations available on the FPU units. That’s normally a nice feature, because if you execute a complex floating point operation (for example a square root) on the FPU, you decrease rounding errors per step.

However, different FPUs can very possible have different internal precision in its calculations, and the same computation on different FPUs can give different results. And that’s where I believe the strictfp keyword comes in - it disables any extra precision representations on FPUs making all computations the same on every CPU (because floats and doubles have the same number of bits on all platforms). I gain deterministic behaviour in the game simulation at the expense of accuracy.

  • elias
#4

Yes, I believe the point of strictfp is to ensure that the same math input produces the samee xact output on all Java VMs.