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Discussion Determinism across platforms

Discussion in 'DOTS Dev Blitz Day 2022 - Q&A' started by Goularou, Dec 7, 2022.

  1. Goularou

    Goularou

    Joined:
    Oct 19, 2018
    Posts:
    54
    I understood in Laurent Gibert's (bonjour, Monsieur DOTS, et merci!) announcements that determinism across platforms is planned to be integrated in the DOTS Physics: could be be more specific about:
    1) the road map: when, and what platforms
    2) the "compatibility" if netcode for entities as the multiplayer use remains the main expectation in my view
    3) the extent: are we talking about "full" determinism = exact same float outputs at the same times
    Thanks in advance.
     
  2. philsa-unity

    philsa-unity

    Unity Technologies

    Joined:
    Aug 23, 2022
    Posts:
    118
    We've done some initial exploration and have a technical plan for the work necessary, but it's a significant investment we haven't scheduled yet. We cannot share details at this point.

    If this is an important feature for you, we would encourage you to add your feedback here: https://unity.com/roadmap/unity-platform/dots . You may write feedback on topics that aren't presently on the roadmap under the "Didn't find what you were looking for?" card at the bottom of the page, after selecting a level of importance
     
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  3. TheOtherMonarch

    TheOtherMonarch

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    Jul 28, 2012
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    888
    My understand is that Burst is already deterministic when one target architecture is selected in AOT settings.

    I am still not sure what cross-platform determinism means. There are at least three possible interpretations. I assume it is in the broader sense of applying to the x86-64 and ARM architectures. Rather than in the narrower sense of instruction set architecture. Or does it refer to microarchitecture? For example, Intel vs. AMD, along with different generations of hardware from the same manufacturer? Does platform correspond to the Unity Build settings?
     
  4. Kmsxkuse

    Kmsxkuse

    Joined:
    Feb 15, 2019
    Posts:
    306
    IIRC it's visually confirmed to be determinsitic across same platform archetecture but the Burst team hasnt implemented the necessary tests to fully confirm determinism as it'll require a significant amount of work to iron out all the edge cases.
     
  5. TheOtherMonarch

    TheOtherMonarch

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    Jul 28, 2012
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    Which is why they should focus on one target and validate that. Then they can limit testing to a reasonable number of configurations around 25? Not all of SIMD. They have done some work but I am not sure exactly what their focus was.

    From my own research SIMD appears to be the best way to implement deterministic floating points. This strategy has been executed successfully since SSE2. With some games being released well over a decade ago. Now that x86-64 is ubiquitous, this makes even more sense. However, I am also aware that some CPU microarchitectures exhibit errata, although some of these bugs have been patched. I am also not positive that all instructions are designed to be deterministic; as far as I know, they should be. It is hard to know exactly what behavior to expect without widespread validation.


    I know that they are a company called Unity but the huge performance penalty to target the Apple Mac is not going to be worth it to most developers. It would be valuable to have a deterministic floating-point option that uses SIMD. Doing so will significantly enhance performance. It does not matter which target is selected. However, restricting determinism to AVX2 would be perfectly acceptable, assuming all AVX2 enabled CPU microarchitectures exhibit deterministic behavior. AVX2, in the Steam survey, has 90.13% market share. The PlayStation 5 and Xbox Series X/S also support AVX2. The vast preponderance of modern client systems going forward will be configured with AVX2 support.


    Testing and proving determinism is critical to being able to rely on determinism. Burst AOT Settings currently allow for CPU target architectures to be selected. When FloatMode is set to Default/Strict and a single target CPU architecture is selected, we don't know what the state of floating point determinism will be. But we know that a huge performance reduction compared to SIMD can be expect from platform agnostic floating points. IEEE 754 only guarantees determinism for a few math operations.
     
    Last edited: Dec 8, 2022
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