Code:
----
import std.stdio;
import core.stdc.stdlib : calloc, free;

void main() {
	readln(); /// ~ 1332 K

	void* ptr = .calloc(100_000, int.sizeof);

	readln(); /// ~ 1720 K

	free(ptr);
	ptr = null;

	readln(); /// Still ~ 1720 K
}
----

If I compile this code with dmd test.d and execute it, I see a memory usage of ~ 1332K. If I press enter the programm allocate 100_000 int's and the memory usage increased to  ~ 1720 K. If I now press enter I assumed that the memory usage decreased to ~ 1332 K again, but it is still ~ 1720 K. Why?

Namespace:

> If I compile this code with dmd test.d and execute it, I see a memory usage of ~ 1332K. If I press enter the programm allocate 100_000 int's and the memory usage increased to  ~ 1720 K. If I now press enter I assumed that the memory usage decreased to ~ 1332 K again, but it is still ~ 1720 K. Why?

This is system-dependent. Sometimes the memory allocated by a program is released to the OS only when the a program ends. But that memory is available to your C heap, so if you call calloc again after the second readln, the total memory used should change very little.

Bye,
bearophile