This is up and running as a C++ library. Currently transactional variable reads and writes are ~50x slower than ordinary variables. 40% the overhead is transaction bookkeeping, 40% is concurrent garbage collection bookkeeping, 20% is platform atomics. Many optimizations to go.https://twitter.com/TimSweeneyEpic/status/1210260682605764611 …
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For comparison, UE3 UnrealScript and pre-il2cpp Unity Mono bytecode interpretation were ~30x slower than native.
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One great side-effect of transactions is that you can automatically undo variable writes upon failure, for example so you can write something like "if(move a bunch of actors) ... else ...", and if any operations fail, then all of their effects are undone.
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I've had to re-learn my code optimization intuition from the late 90's. C++ compiler optimization is magic now, and Skylake can reliably issue 4-6 instructions per clock. But control flow misprediction has become wildly expensive.
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Anyway, there are two competing theories on how we'll unlock higher performance through parallelism. One is the data oriented design approach, asking programmers to rewrite gameplay code as highly parallel fragments of algorithms that pipe inputs and outputs among stages.
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The other is transactions, hoping we can just write gameplay code using "var<int> Health;" instead of "int health;", write code to minimize unnecessary contention for shared state, and have the engine and API magically sort out concurrency for us.
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End of conversation
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FWIW: The implementors of the STM monad in Haskell found that committing many things together ended up merging away a LOT of the overhead (which matches the intuition from databases.) https://www.microsoft.com/en-us/research/wp-content/uploads/2005/01/2005-ppopp-composable.pdf … http://hackage.haskell.org/package/stm-2.5.0.0/docs/Control-Monad-STM.html … This is a reason for hope!
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