Elastic search using more RAM and not release

I have a problem with elastic search uses a lot of os memory (non buff / cache).Here are some details:
1.I am using a 64 GB Ram machine .
2.I have allocated 26gb as ES heap size.
3.only elasticsearch service running on machine.

After running for a while, I see that elastic search is using up most of the RAM,

Meanwhile ,Of the 26 GB heap space allocated, only 16 GB has been used up.
I noticed that in the process mapping file / proc / PID / smaps, elastic search has applied for about 500 anonymous memory spaces with the size of 50MB.

Is this the behavior of elastic search or Lucene? Is it normal?

This is the OS doing what is should do and is caching constantly used files in memory.