[LoweringContext] Support an optimized parameter mapping for SPMD #8460
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| Re-opened from #8453, cleaned up the merge commit. |
tengyifei approved these changes Dec 5, 2024
rpsilva-aws force-pushed the rpsilva_lc_mapping_v3 branch from 8fd7ac7 to 9858577 Compare tengyifei added a commit that referenced this pull request Jan 2, 2025
Previously scan uses `parameter_id_tensor_mapping` to fetch tensors hoisted to HLO parameters e.g. the fn being scanned may create additional tensors while its running. `parameter_id_tensor_mapping` will fetch those tensors back to host as XLA literals and create new tensors wrapphing those, resulting in additional host RAM usage. PR #8460 added `device_parameter_id_tensor_mapping` that returns the actual device backed tensors instead of another copy. So we'll use that and test that this avoids host transfers.
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Currently, the existing parameter mapping for the lowering context is not well suited for SPMD. In case of large models, it will cause a large synchronous bottleneck when transferring all device data to the host. This is caused by each ReplicateShardedData computation that gathers and reassembles each sharded data across multiple devices. This is by design, since it is expected to collect all parameters regardless of their allocation.
In this PR, we introduce a new mapping that does not invoke the sharded replication, but instead uses references to the device data. This is generally sufficient and preferred in most cases, where the user only wants to access the validate parameters (those that are not returned as
-1fromtensor_parameter_id, as 'fake' parameters).