Add microbenchmarks for DB::GetMergeOperands()

[ajkr]

The new microbenchmarks, DBGetMergeOperandsInMemtable and DBGetMergeOperandsInSstFile, correspond to the two different LSMs tested: all data in one memtable and all data in one SST file, respectively. Both cases are parameterized by thread count (1 or 8) and merge operands per key (1, 32, or 1024). The SST file case is additionally parameterized by whether data is in block cache or mmap'd memory.

Test Plan:

$ TEST_TMPDIR=/dev/shm/db_basic_bench/ ./db_basic_bench --benchmark_filter=DBGetMergeOperands                        
The number of inputs is very large. DBGet will be repeated at least 192 times.
The number of inputs is very large. DBGet will be repeated at least 192 times.
2022-05-09T13:15:40-07:00
Running ./db_basic_bench
Run on (36 X 2570.91 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x18)
  L1 Instruction 32 KiB (x18)
  L2 Unified 1024 KiB (x18)
  L3 Unified 25344 KiB (x1)
Load Average: 4.50, 4.33, 4.37
----------------------------------------------------------------------------------------------------------------------------
Benchmark                                                                  Time             CPU   Iterations UserCounters...
----------------------------------------------------------------------------------------------------------------------------
DBGetMergeOperandsInMemtable/entries_per_key:1/threads:1                 846 ns          846 ns       849893 db_size=0
DBGetMergeOperandsInMemtable/entries_per_key:32/threads:1               2436 ns         2436 ns       305779 db_size=0
DBGetMergeOperandsInMemtable/entries_per_key:1024/threads:1            77226 ns        77224 ns         8152 db_size=0
DBGetMergeOperandsInMemtable/entries_per_key:1/threads:8                 116 ns          929 ns       779368 db_size=0
DBGetMergeOperandsInMemtable/entries_per_key:32/threads:8                330 ns         2644 ns       280824 db_size=0
DBGetMergeOperandsInMemtable/entries_per_key:1024/threads:8            12466 ns        99718 ns         7200 db_size=0
DBGetMergeOperandsInSstFile/entries_per_key:1/mmap:0/threads:1          1640 ns         1640 ns       461262 db_size=21.7826M
DBGetMergeOperandsInSstFile/entries_per_key:1/mmap:1/threads:1          1693 ns         1693 ns       439936 db_size=21.7826M
DBGetMergeOperandsInSstFile/entries_per_key:32/mmap:0/threads:1         3999 ns         3999 ns       172881 db_size=19.6981M
DBGetMergeOperandsInSstFile/entries_per_key:32/mmap:1/threads:1         5544 ns         5543 ns       135657 db_size=19.6981M
DBGetMergeOperandsInSstFile/entries_per_key:1024/mmap:0/threads:1      78767 ns        78761 ns         8395 db_size=19.6389M
DBGetMergeOperandsInSstFile/entries_per_key:1024/mmap:1/threads:1     157242 ns       157238 ns         4495 db_size=19.6389M
DBGetMergeOperandsInSstFile/entries_per_key:1/mmap:0/threads:8           231 ns         1848 ns       347768 db_size=21.7826M
DBGetMergeOperandsInSstFile/entries_per_key:1/mmap:1/threads:8           214 ns         1715 ns       393312 db_size=21.7826M
DBGetMergeOperandsInSstFile/entries_per_key:32/mmap:0/threads:8          596 ns         4767 ns       142088 db_size=19.6981M
DBGetMergeOperandsInSstFile/entries_per_key:32/mmap:1/threads:8          720 ns         5757 ns       118200 db_size=19.6981M
DBGetMergeOperandsInSstFile/entries_per_key:1024/mmap:0/threads:8      11613 ns        92460 ns         7344 db_size=19.6389M
DBGetMergeOperandsInSstFile/entries_per_key:1024/mmap:1/threads:8      19989 ns       159908 ns         4440 db_size=19.6389M

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[jay-zhuang]

LGTM.

Please fix the build failure.

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@ajkr has updated the pull request. You must reimport the pull request before landing.

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@ajkr has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[ajkr]

LGTM.

Please fix the build failure.

Done. Thanks for the review!