In the Druid Basic Cluster Tuning guide: https://druid.apache.org/docs/latest/operations/basic-cluster-tuning/#total-memory-usage under “Total Memory Usage” section, there is a statement:

The Historical will use any available free system memory (i.e., memory not used by the Historical JVM and heap/direct memory buffers or other processes on the system) for memory-mapping of segments on disk.

What does “Free System Memory” corelate to, in terms of the output of

free -m

command. Is “Free System Memory” => “FREE” or “Cache/Buffer Memory”?

I believe

free

is available memory that Druid can then use to memory-mapping of segments on disk. While the

Cache/Buffer Memory

can include memory that is used for memory-mapped files, among other things.