VMs perform memory allocation in the same way an operating system handles memory allocation and deallocation. The guest operating system frees a piece of physical memory by adding memory page numbers to the guest free list.

The guest operating system's free list is not accessible to the hypervisor; thus, it is difficult for the hypervisor to know when to free the host physical memory and when the guest physical memory needs to be freed. The hypervisor is completely unaware of which pages are free or allocated to the guest operating system, and because of this, it cannot reclaim the host physical memory when the guest operating system frees guest physical memory.

So the VMware hypervisor relies on memory reclamation techniques to reclaim the host physical memory that is freed by the guest operating system. These are the memory reclamation techniques:

• Transparent Page Sharing (TPS)
• Memory ballooning
• Host-level (or hypervisor) swapping