The Shuffle Tuning Knobs

When you cannot eliminate a shuffle, you tune it, and there is a small set of knobs beyond the partition count that shape how a shuffle performs. None of them is as powerful as removing the shuffle or sizing the partitions, but together they trim the edges, and knowing they exist is part of a complete picture. Shuffle compression is on by default and usually worth keeping, because the CPU cost of compressing is small next to the disk and network it saves; the data being shuffled is often compressible row data. The buffer sizes govern how much memory the sort and fetch can use before spilling, so raising them can relieve spill on large partitions, though sizing the partitions smaller is usually the cleaner fix. Fetch concurrency balances how aggressively a reduce task pulls against how much

About This Interactive Section

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