Increase spill read-ahead

Author: Vedin

datafusion/physical-plan/src/joins/grace_hash_join/stream.rs

@@ -57,7 +57,9 @@ use parking_lot::Mutex;
 /// prevent explosive fan-out.
 const MAX_REPARTITION_PARTITIONS: usize = 256;
 /// Upper bound for concurrent spill chunk read-ahead when loading partitions.
-const SPILL_READAHEAD_BYTES: usize = 64 * 1024 * 1024;
+/// Use a generous cap to keep IO saturated while staying well below the
+/// per-partition memory budget.
+const SPILL_READAHEAD_BYTES: usize = 512 * 1024 * 1024;
 /// Below this size we avoid further repartitioning to keep file counts under control.
 const MIN_REPARTITION_BYTES: usize = 16 * 1024 * 1024;