Technically, persistent and semi-persistent memory within VoltDB is managed using code written in C++. Temporary memory is managed using code written in Java. What language the source code is written in is not usually relevant, except in the case of memory, because different languages manage memory differently. C++ uses the traditional explicit allocation and deallocation of memory, where the application code controls exactly how and when memory is assigned and deassigned. In Java, memory is not explicitly allocated and deallocated. Instead, Java uses what is called "garbage collection" to free up memory that is not in use.
To complicate matters, the language libraries themselves do some performance optimizations to avoid allocating and deallocating memory from the operating system too frequently. So even if VoltDB explicitly frees memory in persistent or semi-persistent storage, that memory may not be immediately returned to the operating system or alter the process's perceived RSS value.
For temporary storage (which is managed in Java), VoltDB cannot explicitly control memory allocation and deallocation and relies on the Java virtual machine (JVM) to manage memory appropriately. The JVM decides when and how to collect free space from unused objects. This means that the VoltDB server cannot directly control if and when the memory associated with temporary storage is returned to the operating system.