Ballooning is a completely different memory reclamation technique compared to page sharing. Before describing the technique, it is helpful to review why the hypervisor needs to reclaim memory from virtual machines. Due to the virtual machine’s isolation, the guest operating system is not aware that it is running inside a virtual machine and is not aware of the states of other virtual machines on the same host. When the hypervisor runs multiple virtual machines and the total amount of the free host memory becomes low, none of the virtual machines will free guest physical memory because the guest operating system cannot detect the host’s memory shortage. Ballooning makes the guest operating system aware of the low memory status of the host.