MemTable is not get freed while creating index

[rockeet]

We run myrocks with sysbench:

sysbench --db-driver=mysql --threads=20 --mysql-host=192.168.100.10  \
         --mysql-port=3306 --mysql-user=test  --mysql-password=1234 \
         --mysql-db=test --tables=100 --table_size=100000000 oltp_write_only \
         --mysql_storage_engine='rocksdb default charset latin1' prepare

When creating index, data insertion on other tables is still running, we noticed that MemTables were not get freed during this period, the memory usage is very high, once index creation is done, the memory usage is decreased. On small memory machine, the mysqld process were kill for OOM.