Robj/prefetching#681
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Signed-off-by: Rob Johnson <rob@robjohnson.io>
Signed-off-by: Rob Johnson <rob@robjohnson.io>
Compaction (trunk_branch_merger): thread prefetch_budget from splinterdb_config through trunk_config into build_merge_itor, which divides the budget across branches (max(2, budget/extent_size/N)) and calls btree_iterator_set_prefetch_lookahead on each. Measured ~1.4x faster full-leaf optimize vs the notification-fixes baseline. Backward scan prefetch: add mini_meta_cursor_prev() which walks the doubly-linked meta-page list in reverse, keeping the current page alive during WOULD_BLOCK so prev_meta_addr stays accessible for retry (unlike the forward cursor which releases on miss). Add fill_backward, pump_backward, on_boundary_backward, and start_backward symmetric to the forward equivalents. btree_iterator_prefetch_on_advance now takes a going_forward parameter; on a direction change it restarts the cursor (resetting the ramp) so both directions get the same slow-start treatment. btree_iterator_prev_leaf/async now call prefetch_on_advance rather than killing the cursor. Measured ~40x improvement for backward scans (baseline had zero prefetch -- prev_leaf was calling btree_prefetch_cursor_deinit on every leaf step). Also add --optimize-only mode to scan_benchmark for measuring cold compaction throughput. Validated: unit_test btree 5/5, splinterdb_quick 33/33 (release + ASAN); backward scan returns correct tuple count; no ASAN errors. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Rob Johnson <rob@robjohnson.io>
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Signed-off-by: Rob Johnson <rob@robjohnson.io>
Signed-off-by: Rob Johnson <rob@robjohnson.io>
Signed-off-by: Rob Johnson <rob@robjohnson.io>
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Implement deeper prefetching by using the mini allocator as the source of addresses.