save key comparisons in BlockIter::BinarySeek #7068
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Summary: This is a followup to facebook#6646. In that PR, for simplicity I just appended a comparison against the 0th restart key in case `BinarySeek()`'s binary search landed at index 0. As a result there were `2/(N+1) + log_2(N)` key comparisons. This PR does it differently. Now we expand the binary search range by one so it also covers the case where target is at or before the restart key at index 0. As a result, it involves `log_2(N+1)` key comparisons. Pull Request resolved: facebook#7068 Test Plan: ran readrandom with mostly default settings and counted key comparisons using `PerfContext`. before: `user_key_comparison_count = 28881965` after: `user_key_comparison_count = 27823245` setup command: ``` $ TEST_TMPDIR=/dev/shm/dbbench ./db_bench -benchmarks=fillrandom,compact -write_buffer_size=1048576 -target_file_size_base=1048576 -max_bytes_for_level_base=4194304 -max_background_jobs=12 -level_compaction_dynamic_level_bytes=true -num=10000000 ``` benchmark command: ``` $ TEST_TMPDIR=/dev/shm/dbbench/ ./db_bench -use_existing_db=true -benchmarks=readrandom -disable_auto_compactions=true -num=10000000 -compression_type=none -reads=1000000 -perf_level=3 ``` Reviewed By: anand1976 Differential Revision: D22357032 Pulled By: ajkr fbshipit-source-id: 8b01e9c1c2a4e9d02fc9dfe16c1cc0327f8bdf24
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This is a followup to #6646. In that PR, for simplicity I just appended a comparison against the 0th restart key in case
BinarySeek()'s binary search landed at index 0. As a result there were2/(N+1) + log_2(N)key comparisons. This PR does it differently. Now we expand the binary search range by one so it also covers the case where target is at or before the restart key at index 0. As a result, it involveslog_2(N+1)key comparisons.Test Plan:
ran readrandom with mostly default settings and counted key comparisons
using
PerfContext.before:
user_key_comparison_count = 28881965after:
user_key_comparison_count = 27823245setup command:
benchmark command: