ha_rocksdb::external_lock use many CPU time for memory only readonly workload

[rockeet]

It seems myrocks and rocksdb doing many useless works: Clear, Reinitialize:

In the flame graph, CSPP_WBWI is our WriteBatchWithIndex implementation, used to replace rocksdb default WriteBatchWithIndex.

Although CPU usage of ha_rocksdb::external_lock is 9% of total, it is more than 50% of storage engine layer(except pack/unpack).

We expect ha_external_lock should be light weight for memory only readonly workload, could myrocks add some state variable for checking and skip expensive transaction init works ?

branch: fb-mysql-8.0.28 9cc489f
test is sysbench oltp_point_select

[Yanqin Jin]

Second graph shows some CPU wasted on DBImpl::ReleaseSnapshot() due to the db mutex, I hope facebook/rocksdb#7516 can help mitigate.