Support multihost SPMD execution #4573
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yeounoh
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Feb 8, 2023
| @@ -88,7 +89,7 @@ def mark_sharding(t: Union[torch.Tensor, XLAShardedTensor], mesh: Mesh, | |||
| Examples | |||
| —------------------------------ | |||
| mesh_shape = (4, 2) | |||
| num_devices = len(xm.get_xla_supported_devices()) | |||
| num_devices = pjrt.global_device_count() | |||
yeounoh
reviewed
Feb 8, 2023
| const std::vector<std::string>& devices) { | ||
| std::unordered_map<int, int> device_index; | ||
| for (int i = 0; i < devices.size(); ++i) { | ||
| int global_ordinal = ParseDeviceString(devices[i]).ordinal(); |
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The first global device gets the local index 0, so the order of the input devices list is important. Is this a correct understanding? Can we add some comments on this?
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The first device in the list gets local index 0, but the order of the global ordinals within devices doesn't matter. I'll add some more documentation around this.
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yeounoh
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| @@ -931,30 +931,24 @@ std::vector<torch::lazy::BackendDataPtr> CreateTensorsData( | |||
| std::vector<xla::ComputationClient::DataPtr> new_handles; // out | |||
| if (shardings[i] != nullptr) { | |||
| xla::OpSharding sharding = shardings[i]->sharding; | |||
| // TODO(yeounoh) PJRT runs a process per host for SPMD and without cross | |||
| // host communications. This means that we may need to manually shard | |||
| // across global devices for multi-host training. | |||
| std::vector<std::string> local_devices = | |||
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Does GetLocalDevices() return local devices with global ordinals? If so, let's leave a comment.
yeounoh
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Feb 10, 2023
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The only main change to support multihost execution is to restrict the generated shards in ShardTensor to those which belong to addressable devices.