using jemalloc 5.2.1 and memory seems not being released to the operation system

mem_not_released.cpp

// COMPILE:          g++ a.cpp -g -Wall -std=c++11 -I./jemalloc/include/ -ljemalloc -L./jemalloc/lib/
// jemalloc version: 5.2.1
// TEST:
// $ export MALLOC_CONF="prof:true,prof_active:true,prof_prefix:jeprof.out,stats_print:true,muzzy_decay_ms:3000,dirty_decay_ms:3000"
// $ cat > test.sh << EOF
// > a.out &
// > top -p $! | grep $USER
// EOF
// $ sh test.sh
/**
 * this programe can allocate 62 gigabytes virtual memory at most, and after it reached its peak, it never falls, even after I had
 * released all the allocated memory, the amount of virtual memory keeps not changed.
 */

#include <iostream>
#include <memory>
#include <map>
#include <unistd.h>

using namespace std;
constexpr int SIZE = 1024 * 50;
int i = 0;
int main()
{
    {
        std::map<int, std::unique_ptr<char[]>> map1;
        int i = 0;
        cout << "before allocate" << endl;
        while (i < SIZE) {
            map1.emplace_hint(map1.end(), i, new char[rand() % (2 * 1024 * 1024) + 1]());
            ++i;
        }
        cout << "All allocated" << endl;
        while (i >= 0) {
            map1.erase(i);
            --i;
        }
        cout << "All freed" << endl;
        while (i < SIZE) {
            map1.emplace_hint(map1.end(), i, new char[rand() % (2 * 1024 * 1024) + 1]());
            ++i;
        }
        cout << "All reallocated" << endl;
    }
    cout << "map deallocated" << endl;
    sleep(30); 
    return 0;
}