Buffer memory usage on Linux server

16g of the total 32g memory was allocated as heap memory.
I know that lucene uses buffer memory, and I know that buffer memory is managed automatically without any control.
However, since the resource monitoring system calculates the total memory usage as actual usage + buffer memory, I am wondering if I can limit the buffer memory usage.
I tried applying the option below but it doesn't seem to work.
-Djdk.nio.maxCachedBufferSize = 262144 and -XX: MaxDirectMemorySize = 2048m