To make a game like an RTS networked, I've seen a number of answers here suggest to make the game completely deterministic; then you only have to transfer the users' actions to each other, and lag what's displayed a little bit in order to "lock in" everyone's input before the next frame is rendered. Then things like unit's positions, health, etc. don't need to be constantly updated over the network, because every player's simulation will be exactly the same. I've also heard the same thing suggested for making replays.
However, since floating-point calculations are non-deterministic between machines, or even between different compilations of the same program on the same machine, is this really possible to do? How do we prevent that fact from causing small differences between players (or replays) that ripple throughout the game?
I've heard some people suggest avoiding floating-point numbers altogether and using
int to represent the quotient of a fraction, but that doesn't sound practical to me - what if I need to, for example, take the cosine of an angle? Do I seriously need to rewrite an entire math library?
Note that I am mainly interested in C#, which as far as I can tell, has exactly the same problems as C++ in this regard.
The answer to this question is from the link you posted. Specifically you should read the quote from Gas Powered Games:
I work at Gas Powered Games and i can tell you first hand that floating point math is deterministic. You just need the same instruction set and compiler and of course the user’s processor adheres to the IEEE754 standard, which includes all of our PC and 360 customers. The engine that runs DemiGod, Supreme Commander 1 and 2 rely upon the IEEE754 standard. Not to mention probably all other RTS peer to peer games in the market.
And then below that one is this:
If you store replays as controller inputs, they cannot be played back on machines with different CPU architectures, compilers, or optimization settings. In MotoGP, this meant we could not share saved replays between Xbox and PC.
A deterministic game will only be deterministic when using the identically compiled files and run on systems that adhere to the IEEE standards. Cross platform synchronized network simulations or replays will not possible.
Edit: A link to a fixed point class (Buyer beware! - I haven't used it...)
You will pay a performance penalty, but this may or may not be a problem, as .net will not be especially performant here as it won't use simd instructions. Benchmark!
N.B. Apparently someone at intel appears to have a solution to allow you to use the intel performance primitives library from c#. This may help vectorising fixed point code to compensate for the slower performance.
Note that I am mainly interested in C#, which as far as I can tell, has exactly the same problems as C++ in this regard.
Yes, C# has the same problems as C++. But it also has a lot more.
For example, take this statement from Shawn Hawgraves:
If you store replays as controller inputs, they cannot be played back on machines with different CPU architectures, compilers, or optimization settings.
It's "easy enough" to ensure that this happens in C++. In C#, however, that's going to be a lot harder to deal with. This is thanks to JIT.
What do you suppose would happen if the interpreter ran your code interpreted once, but then JIT'd it the second time? Or maybe it interprets it twice on someone else's machine, but JIT's it after that?
JIT is not deterministic, because you have very little control over it. This is one of the things you give up to use the CLR.
And God help you if one person is using .NET 4.0 to run your game, while someone else is using the Mono CLR (using the .NET libraries of course). Even .NET 4.0 vs. .NET 5.0 could be different. You simply need more control over the low-level details of a platform to guarantee this kind of thing.
You ought to be able to get away with fixed-point math. But that's about it.
Use fixed point arithmetics. Or choose an authoritative server and have it sync game state once in a while - that's what MMORTS do. (At least, Elements of War works like this. It's written in C# too.) This way, errors do not have a chance to accumulate.
I realized after writing this answer that it doesn't actually answer the question, which was specifically about floating-point nondeterminism. But maybe this is helpful to him anyway if he's going to make a networked game in this manner.
Along with the input sharing that you're broadcasting to all players, it can be very useful to create and broadcast a checksum of important game state, such as player positions, health, etc. When processing input, assert that the game state checksums for all remote players are in sync. You are guaranteed to have out of sync (OOS) bugs to fix and this will make it easier - you will have an earlier notice that something's gone wrong (which will help you figure out reproduction steps), and you should be able to add more game state logging in suspect code to allow you to bracket whatever is causing the OOS.
I think the idea from the blog linked to still needs periodic synchronisation to be viable - I've seen enough bugs in networked RTS games which don't take that approach.
Networks are lossy, slow, have latency and might even pollute your data. "Floating point determinism", (which sounds buzzwordy enough to make me skeptical) is the least of your worries in reality... esp if you use a fixed time step. with variable time steps you will need to interpolate between fixed time steps to avoid determinism problems too. I think this is usually what is meant by non-deterministic "floating point" behaviour - just that variable time steps cause integrations to diverge - not anything to do with math libraries or low level functions.
Synchronisation is key though.
I did a lot of reading on this issue a few years back when I wanted to write an RTS using the same lockstep architecture you do.
My conclusions about hardware floating-points were:
I concluded that it's impossible to use the built in floating point types in .net deterministically.
Thus I needed workarounds. I considered:
FixedPoint32in C#. While this is not too hard(I have a half finished implementation) the very small range of values makes it annoying to use. You have to be careful at all times so you neither overflow, nor lose too much precision. In the end I found this not easier than using integers directly.
FixedPoint64in C#. I found this rather hard to do. For some operations intermediate integers of 128bit would be useful. But .net doesn't offer such a type.
Decimal. But it's slow, takes a lot of memory and easily throws exceptions (division by 0, overflows). It's very nice for financial use, but no good fit for games.
Inspired by your post on StackOverflow, I've just started implementing a 32 bit floating-point type in software and the results are promising.
floatfor addition/multiplication(Single thread on a 2.66GHz i3). If anybody has good floating point benchmarks for .net please send them to me, since my current test is very rudimentary.
If anybody want to contribute tests or improve the code, just contact me, or issue a pull request on github. https://github.com/CodesInChaos/SoftFloat
There are also other sources of indeterminism in .net.
HashSet<T>returns the elements in an undefined order.
object.GetHashCode()differs from run to run.
Randomclass is unspecified, use your own.
WeakReferences lose their target is indeterministic because the GC may run at any time.
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