6.5.1

ESXi memory virtualization adds lile time overhead to memory accesses. Because the processor's paging
hardware uses page tables (shadow page tables for software-based approach or two level page tables for
hardware-assisted approach) directly, most memory accesses in the virtual machine can execute without
address translation overhead.
The memory space overhead has two components.
n
A xed, system-wide overhead for the VMkernel.
n
Additional overhead for each virtual machine.
Overhead memory includes space reserved for the virtual machine frame buer and various virtualization
data structures, such as shadow page tables. Overhead memory depends on the number of virtual CPUs
and the congured memory for the guest operating system.
Overhead Memory on Virtual Machines
Virtual machines require a certain amount of available overhead memory to power on. You should be aware
of the amount of this overhead.
The following table lists the amount of overhead memory a virtual machine requires to power on. After a
virtual machine is running, the amount of overhead memory it uses might dier from the amount listed in
the table. The sample values were collected with VMX swap enabled and hardware MMU enabled for the
virtual machine. (VMX swap is enabled by default.)
N The table provides a sample of overhead memory values and does not aempt to provide
information about all possible congurations. You can congure a virtual machine to have up to 64 virtual
CPUs, depending on the number of licensed CPUs on the host and the number of CPUs that the guest
operating system supports.
Table 6‑1. Sample Overhead Memory on Virtual Machines
Memory (MB) 1 VCPU 2 VCPUs 4 VCPUs 8 VCPUs
256 20.29 24.28 32.23 48.16
1024 25.90 29.91 37.86 53.82
4096 48.64 52.72 60.67 76.78
16384 139.62 143.98 151.93 168.60
How ESXi Hosts Allocate Memory
A host allocates the memory specied by the Limit parameter to each virtual machine, unless memory is
overcommied. ESXi never allocates more memory to a virtual machine than its specied physical memory
size.
For example, a 1GB virtual machine might have the default limit (unlimited) or a user-specied limit (for
example 2GB). In both cases, the ESXi host never allocates more than 1GB, the physical memory size that
was specied for it.
When memory is overcommied, each virtual machine is allocated an amount of memory somewhere
between what is specied by Reservation and what is specied by Limit. The amount of memory granted to
a virtual machine above its reservation usually varies with the current memory load.
A host determines allocations for each virtual machine based on the number of shares allocated to it and an
estimate of its recent working set size.
n
Shares — ESXi hosts use a modied proportional-share memory allocation policy. Memory shares
entitle a virtual machine to a fraction of available physical memory.
vSphere Resource Management
34 VMware, Inc.