Redis - How to completely flush/clear memory after a flushdb?

After doing several flushdb on a Redis database, I wonder how to completely flush/clear the memory use ?

Indeed, I made some tests :

• watching memory with a htop -> 800 Mo used
• fill in a Redis database -> goes to 1500 Mo used
• flushing the database => memory use stay at about 1500 Mo...

Any ideas ? Thanks