An admission control policy ensures that sufficient unreserved memory and server swap space is available before a VM is allowed to power on. Machine memory must be reserved for the guaranteed min size, as well as additional overhead memory required for virtualization, for a total of min + overhead. Overhead memory includes space for the VM graphics frame buffer and various virtualization data structures such as the pmap and shadow page tables (see Section 2). Typical VMs reserve 32 MB for overhead, of which 4 to 8 MB is devoted to the frame buffer, and the remainder contains implementation-specific data structures. Additional memory is required for VMs larger than 1 GB.
Disk swap space must be reserved for the remaining VM memory; i.e. max min. This reservation ensures the system is able to preserve VM memory under any circumstances; in practice, only a small fraction of this disk space is typically used. Similarly, while memory reservations are used for admission control, actual memory allocations vary dynamically, and unused reservations are not wasted.