Understanding Memory Overhead
Virtualization of memory resources has some associated overhead.
ESX/ESXi memory virtualization adds little time overhead to memory accesses. Because the processor's paging hardware uses page tables (shadow page tables for software-based approach or nested page tables for hardware-assisted approach) directly, most memory accesses in the virtual machine can execute without address translation overhead.
For ESX, the service console typically uses 272MB and the VMkernel uses a smaller amount of memory. The amount depends on the number and size of the device drivers that are being used.
Overhead memory includes space reserved for the virtual machine frame buffer and various virtualization data structures, such as shadow page tables. Overhead memory depends on the number of virtual CPUs and the configured memory for the guest operating system.
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