STL container memory issue

I'm implementing my own graph library in linux (Fedora 10 and CentOS
5) with gcc 4.3.2 and using STL containers then I found some problems
with memory. When I build my graph, I use a lot of memory enough to be
view in _top_ or another memory usage tool. I'm sure that I'm
deallocating that memory (I reviewed the code again and again and I
used valgrind to check for memory leak), but the memory remains in use
(I can view this in _top_ or _cat /proc/meminfo_) and when I create
the graph once again, it does not increase memory usage, apparently
reusing the allocated memory.

After several days of debugging, I created a very simple code that has
the same problem.

#include <iostream>
#include <list>

// Object that occupies 128KB
class MyObject
{
public:
    int * a;
    int * b;
    int * c;
    int * d;

    MyObject( )
    {
        a = new int[ 8192 ];
        b = new int[ 8192 ];
        c = new int[ 8192 ];
        d = new int[ 8192 ];
    }

    MyObject( const MyObject & m )
    {
        a = new int[ 8192 ];
        b = new int[ 8192 ];
        c = new int[ 8192 ];
        d = new int[ 8192 ];
    }

    ~MyObject( )
    {
        delete [] a;
        delete [] b;
        delete [] c;
        delete [] d;
    }
};

typedef std::list< MyObject > list_t;

#define MB_TO_ALLOC 1000 // Size in MB that the program must alloc.

#define SLEEP_TIME 5 // Time in seconds that the program must wait
until go to another step.
                        // It's used to give sufficient time for tools update the memory
usage

int main( )
{
    std::cout << "Alloc..." << std::endl;

    list_t * list = new list_t( );

    // Number of objects for alloc MB_TO_ALLOC amount of memory
    int nObjects = MB_TO_ALLOC * 1024 / 128;

    for( int i = 0; i < nObjects; ++i )
        list->push_back( MyObject( ) );

    std::cout << SLEEP_TIME << "s to Dealloc..." << std::endl;

    // Wait some time for a tool (like top) to update the memory usage
    sleep( SLEEP_TIME );

    std::cout << "Dealloc..." << std::endl;

    delete list;

    std::cout << SLEEP_TIME << "s to Alloc..." << std::endl;

    // Wait some time for a tool (like top) to update the memory usage
    sleep( SLEEP_TIME );

    //Repeats the procedure for evaluating the reuse of memory
    std::cout << "Alloc..." << std::endl;

    list = new list_t( );

    for( int i = 0; i < nObjects; ++i )
        list->push_back( MyObject( ) );

    std::cout << SLEEP_TIME << "s to Dealloc..." << std::endl;

    sleep( SLEEP_TIME );

    delete list;
}

I tried to use simple array or my own list class, but in these cases,
the memory is deallocated normally.

Does anyone know what's going on? How to prevent this memory to be
"reserved"?

Thanks!

--
Bruno Caponi

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]