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[CUDA] Fallback Event impl when there is no hardware cpu/gpu coherency - #3070
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| // 2. hostNativeAtomicSupported == true | ||
| // => use cuda::atom_ref on pinned host memory | ||
| // 2. no hardware cpu/gpu coherency | ||
| // => use cuda::atom_ref on device memory |
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Which category (2 or 3) do typical consumer gpus fall into (4090, 5090 etc)?
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4090 is cat 3, I don't know about 5090. Cat 2 is probably only theoretically possible, but it is very simple to support.
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For 4090 there is no hardware cpu/gpu coherency, so we can not synchronize between CPU and GPU by sharing atomics. This PR provides a fallback implementation of
AtomicEventfor this case: we store the atomic in device-only memory, and do CPU waiting with a busy-wait loop that reads the value withcudaMemcpy.This is a very inefficient fallback but it mostly serves to make tests pass,
AtomicEventhas always been a slow fallback fromCudaEventfor infrequent use cases.Also includes some small changes:
atomic_refinstead ofatomicto make initialization easier for device-only memory.