Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual download pdf (Page 250)

Resource Management

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Virtual Machine Memory

Each virtual machine consumes memory based on its configured size, plus additional

overhead memory for virtualization.

The dynamic memory allocation for a virtual machine is bounded by its minimum and

maximum size parameters. The maximum size is the amount of memory configured

for use by the guest operating system running in the virtual machine. By default,

virtual machines operate at their maximum allocation, unless memory is

overcommitted.

The minimum size is a guaranteed lower bound on the amount of memory that is

allocated to the virtual machine, even when memory is overcommitted. The

minimum size should be set to a level that ensures the virtual machine has sufficient

memory to run efficiently, without excessive paging.

The maximum size can be set to a higher level to allow the virtual machine to take

advantage of excess memory, when available.

Overhead memory includes space reserved for the virtual machine frame buffer and

various virtualization data structures. A virtual machine configured with less than 1GB

of memory requires 32MB of overhead memory. Larger virtual machines require an

additional 4MB of overhead memory per additional gigabyte of configured main

memory. For example, a virtual machine with a configured maximum memory size of

2GB requires 36MB of overhead memory.

Memory Sharing

Many workloads present opportunities for sharing memory across virtual machines.

For example, several virtual machines may be running instances of the same guest

operating system, have the same applications or components loaded or contain

common data. ESX Server uses a proprietary transparent page sharing technique to

securely eliminate redundant copies of memory pages.

With memory sharing, a workload consisting of multiple virtual machines often

consumes less memory than it would when running on physical machines. As a result,

the system can support higher levels of overcommitment efficiently.

The amount of memory saved by memory sharing is highly dependent on workload

characteristics. A workload consisting of many nearly-identical virtual machines may

free up more than 30 percent of memory, while a more diverse workload may result in

savings of less than 5 percent of memory.

To determine the effectiveness of memory sharing for a given workload, try running

the workload, and observe the actual savings via the VMware Management Interface.

The amount of memory shared and the associated savings are reported on the

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Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual Page 250