VMware vSphere vCenter Server 4.0 - Manual (Page 28)

Figure 3-1. ESX/ESXi Memory Mapping
virtual machine
1
guest virtual memory
guest physical memory
machine memory
a b
a
a b b c
b
c b
b c
virtual machine
2
n The boxes represent pages, and the arrows show the different memory mappings.
n The arrows from guest virtual memory to guest physical memory show the mapping maintained by the
page tables in the guest operating system. (The mapping from virtual memory to linear memory for x86-
architecture processors is not shown.)
n The arrows from guest physical memory to machine memory show the mapping maintained by the VMM.
n The dashed arrows show the mapping from guest virtual memory to machine memory in the shadow
page tables also maintained by the VMM. The underlying processor running the virtual machine uses the
shadow page table mappings.
Because of the extra level of memory mapping introduced by virtualization, ESX/ESXi can effectively manage
memory across all virtual machines. Some of the physical memory of a virtual machine might be mapped to
shared pages or to pages that are unmapped, or swapped out.
An ESX/ESXi host performs virtual memory management without the knowledge of the guest operating system
and without interfering with the guest operating system’s own memory management subsystem.
Performance Considerations
When you use hardware assistance, you eliminate the overhead for software memory virtualization. In
particular, hardware assistance eliminates the overhead required to keep shadow page tables in
synchronization with guest page tables. However, the TLB miss latency when using hardware assistance is
significantly higher. As a result, whether or not a workload benefits by using hardware assistance primarily
depends on the overhead the memory virtualization causes when using software memory virtualization. If a
workload involves a small amount of page table activity (such as process creation, mapping the memory, or
context switches), software virtualization does not cause significant overhead. Conversely, workloads with a
large amount of page table activity are likely to benefit from hardware assistance.
Administering Memory Resources
Using the vSphere Client you can view information about and make changes to memory allocation settings.
To administer your memory resources effectively, you must also be familiar with memory overhead, idle
memory tax, and how ESX/ESXi hosts reclaim memory.
When administering memory resources, you can specify memory allocation. If you do not customize memory
allocation, the ESX/ESXi host uses defaults that work well in most situations.
You can specify memory allocation in several ways.
n Use the attributes and special features available through the vSphere Client. The vSphere Client GUI
allows you to connect to an ESX/ESXi host or a vCenter Server system.
n Use advanced settings.
n Use the vSphere SDK for scripted memory allocation.
vSphere Resource Management Guide
28 VMware, Inc.