Summary:
Previously we enabled tracking expected state changes during
`FinishInitDb()`, as soon as the DB was opened. This meant tracing was
enabled during `VerifyDb()`. This cost extra CPU by requiring
`DBImpl::trace_mutex_` to be acquired on each read operation. It was
unnecessary since we know there are no expected state changes during the
`VerifyDb()` phase. So, this PR delays tracking expected state changes
until after the `VerifyDb()` phase has completed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9470
Test Plan:
Measured this PR reduced `VerifyDb()` 76% (387 -> 92 seconds) with
`-disable_wal=1` (i.e., expected state tracking enabled).
- benchmark command: `./db_stress -max_key=100000000 -ops_per_thread=1 -destroy_db_initially=1 -expected_values_dir=/dev/shm/dbstress_expected/ -db=/dev/shm/dbstress/ --clear_column_family_one_in=0 --disable_wal=1 --reopen=0`
- without this PR, `VerifyDb()` takes 387 seconds:
```
2022/01/30-21:43:04 Initializing worker threads
Crash-recovery verification passed :)
2022/01/30-21:49:31 Starting database operations
```
- with this PR, `VerifyDb()` takes 92 seconds
```
2022/01/30-21:59:06 Initializing worker threads
Crash-recovery verification passed :)
2022/01/30-22:00:38 Starting database operations
```
Reviewed By: riversand963
Differential Revision: D33884596
Pulled By: ajkr
fbshipit-source-id: 5f259de8087de5b0531f088e11297f37ed2f7685
c7ce03dce1