Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14926
This fixes a gap in the MultiScan prefetch waste accounting introduced by https://github.com/facebook/rocksdb/pull/14876. That change added reverse MultiScan support and introduced `unreleased_prefetch_blocks_` to track prefetched blocks still owned by the `ReadSet`, because reverse scans can prefetch a suffix of the collected block list and release blocks in non-prefix order. However, those unreleased prefetched blocks were only released during `MultiScanIndexIterator` destruction; they were not included in `MULTISCAN_PREFETCH_BLOCKS_WASTED`. As a result, scans that stopped before consuming all prefetched blocks underreported wasted prefetch blocks.
This change counts unreleased prefetched blocks as wasted before destructor cleanup, while avoiding counting the currently positioned block as wasted when it has been reached.
Reviewed By: xingbowang
Differential Revision: D111531409
fbshipit-source-id: 5e605ce686037256bf6847ab7ca363c3f7701401
eca1a114e0