Flink managed memory is much lower than configure

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Conceptually, Flink splits the heap into three regions: Network buffers: A number of 32 KiByte buffers used by the network stack to buffer records for network transfer. Allocated on TaskManager startup. By default 2048 buffers are used, but can be adjusted via "taskmanager.network.numberOfBuffers".

Memory Manager pool: A large collection of buffers (32 KiBytes) that are used by all runtime algorithms whenever they need to buffer records. Records are stored in serialized form in those blocks. The memory manager allocates these buffers at startup.

Remaining (Free) Heap: This part of the heap is left to the user code and the TaskManager's data structures. Since those data structures are rather small, that memory is mostly available to the user code.

https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=53741525