The server ran out of memory

[ping86]

I am doing load testing with the program.

When I start the test server has 2585 MB of memory in use:

1) 2585MB 0 users

Connect 500.

2) 2641MB 500 user

Connect 524 user.

3) 2692MB 1024 user

Disconnect all user

4) 2724MB 0 user

The server not free memory when users disconnect.

5) I DO: Kill -9

Memory 2585MB

How is this possible?

Thanks.