RocksDB is developed in C/C++ programming language and has a JAVA binding which provides a set of JAVA classes to access RocksDB. In a Java application, the memory is managed by JVM. Object allocations are done on JVM heap and GC manages the heap. But RocksDB allocates C/C++ object using the OS malloc library. When RocksDB is used in a Java application we need to extra careful about the allocations made by RocksDB.
For a Java process these are native memory allocations which are not on the heap. If Java process doesn't delete those objects or close the handles properly after use then they will remain and the JAVA process resident memory size (RSS) starts growing. Even application is well written and properly handling the allocations, some time depending on workload, it may need to access several SSTables (RocksDB sst files) to serve client requests. This case cause uncontrollable growth if RocksDB not tuned properly and can cause Out Of Memory and eventually lead to process crash by Kernel OOM killer. There is a lengthy discussion on the thread specified in Reference section which provide lot of details and how to monitor it and resolve it if you are experiencing this problem.