Before You Hunt a Desync, Prove Your Own Simulation Is Deterministic
The test nobody runs first When the system catches a desync between two players, the match dies, someone files a bug, and the hunt begins. Which client was wrong? Which subsystem? Which tick? Two machines, two builds, two network stacks, two sets of hardware, and somewhere in there one value went a different way. The hardest part of a desync is the hunt itself. What makes it worse is that a large…
Before running a desync investigation, it's crucial to first verify that your simulation is deterministic. Determinism means that given the same starting state and inputs, the simulation should always produce the same result, no matter which machine it's run on. This is a fundamental requirement for lockstep netcode systems.
To test for determinism, you can use a simple procedure called the self-check. It requires just one computer, one recording of gameplay, and one command. The self-check consists of running the simulation twice with the exact same inputs, and comparing the two recordings. If the recordings are identical, the simulation is deterministic. If not, there's a bug somewhere in the code.
The self-check can reveal several common sources of non-determinism. These include iteration order in data structures like HashMaps, using time measurements instead of fixed tick values, uninitialized or stale state, and global mutable state. The self-check can catch these issues even when the simulation runs on a single machine, without needing a second client or network connection.
The self-check is particularly useful because it can be run offline and in a matter of seconds, before any players are involved. It's a lightweight test that can catch problems before they become larger issues that affect players. However, it's important to remember that the self-check only proves determinism on one machine with one build and one process. It does not guarantee determinism across different machines, compilers, or hardware configurations.
To get a more comprehensive test, you can use Tickwise's --chaos flag, which injects known sources of non-determinism at specific ticks. By recording a session with chaos turned on, you can see exactly where the self-check fails and what types of issues are causing the divergence. This can help guide debugging efforts and ensure that your simulation remains deterministic across all platforms.
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