VMware vSphere vCenter Server 4.0 - Manual (Page 30)

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.
The memory space overhead has two components.
n A fixed, system-wide overhead for the VMkernel and (for ESX only) the service console.
n Additional overhead for each virtual machine.
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.
ESX/ESXi also provides optimizations such as memory sharing to reduce the amount of physical memory used
on the underlying server. These optimizations can save more memory than is taken up by the overhead.
Overhead Memory on Virtual Machines
Virtual machines incur overhead memory. You should be aware of the amount of this overhead.
Table 3-2 lists the overhead memory (in MB) for each number of VCPUs.
Table 3-2. Overhead Memory on Virtual Machines
Memory
(MB)
1 VCPU 2 VCPUs 3 VCPUs 4 VCPUs 5 VCPUs 6 VCPUs 7 VCPUs 8 VCPUs
256 113.17 159.43 200.53 241.62 293.15 334.27 375.38 416.50
512 116.68 164.96 206.07 247.17 302.75 343.88 385.02 426.15
1024 123.73 176.05 217.18 258.30 322.00 363.17 404.34 445.52
2048 137.81 198.20 239.37 280.53 360.46 401.70 442.94 484.18
4096 165.98 242.51 283.75 324.99 437.37 478.75 520.14 561.52
8192 222.30 331.12 372.52 413.91 591.20 632.86 674.53 716.19
16384 334.96 508.34 550.05 591.76 900.44 942.98 985.52 1028.07
32768 560.27 863.41 906.06 948.71 1515.75 1559.42 1603.09 1646.76
65536 1011.21 1572.29 1616.19 1660.09 2746.38 2792.30 2838.22 2884.14
131072 1912.48 2990.05 3036.46 3082.88 5220.24 5273.18 5326.11 5379.05
262144 3714.99 5830.60 5884.53 5938.46 10142.83 10204.79 10266.74 10328.69
How ESX/ESXi Hosts Allocate Memory
An ESX/ESXi host allocates the memory specified by the Limit parameter to each virtual machine, unless
memory is overcommitted. An ESX/ESXi host never allocates more memory to a virtual machine than its
specified physical memory size.
For example, a 1GB virtual machine might have the default limit (unlimited) or a user-specified limit (for
example 2GB). In both cases, the ESX/ESXi host never allocates more than 1GB, the physical memory size that
was specified for it.
When memory is overcommitted, each virtual machine is allocated an amount of memory somewhere between
what is specified by Reservation and what is specified by Limit. The amount of memory granted to a virtual
machine above its reservation usually varies with the current memory load.
vSphere Resource Management Guide
30 VMware, Inc.