Explain How XGBoost Parallelizes Training
Scope
Describe how XGBoost achieves parallelism:
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Within a single machine
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Histogram-based split finding and why it enables feature- or data-parallel computation
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Handling of sparse features and missing values
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Cache-friendly column/block data layout
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Thread-level work partitioning and reductions
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Across multiple machines
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Data-parallel training with all-reduce/ring-reduce
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Synchronization points per tree/level/node
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Determinism and reproducibility considerations
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How these choices affect scalability, overfitting, and reproducibility