Command line used for benchmarking:

./sysbench --batch --batch-delay=10 --test=oltp --mysql-host=172.31.23.147 \
--mysql-port=3308 --mysql-user=root --mysql-password=pwd --mysql-db=test  \
--oltp-table-size=20000000 --max-time=180 --max-requests=0 \
--mysql-table-engine=rocksdb --db-ps-mode=disable --mysql-engine-trx=yes  \
--oltp-table-name=sbtest1 --oltp-read-only \
   --oltp-skip-trx --oltp-test-mode=simple --oltp-point-select-all-cols  \
--oltp-secondary --oltp-auto-inc=off --oltp-dist-type=uniform  \
--oltp-connect-delay=0 --percentile=99 --seed-rng=1460080700 --num-threads=8 run

- InnoDB, created with the above sysbench call, innodb_buffer_pool_size=8G
  (the datadir was ~5.1G)
- InnoDB, created with the above sysbench call, innodb_buffer_pool_size=8G,
  and oltp-secondary=false (the datadir was ~4.7G)

- MyRocks, created with the above sysbench call, default settings
  (the datadir was ~4.7G)
- MyRocks, created with the above sysbench call, default settings
  and oltp-secondary=false (the datadir was ~4.1G)

Initially I was getting about 2500 QPS with MyRocks.
the time was spent in reading from disk, so I've increased
rocksdb_block_cache_size=10G, and then it got faster.