VMware VCM 5.3 User Manual download pdf (Page 30)

Sizing Memory for Virtual Machines

Deploying Microsoft Windows 7 Virtual Desktops with VMware View Applied Best Practices

Introduction

The amount of RAM allocated to a virtual machine has a direct effect on a number of factors that

impact both capacity and performance requirements for a virtual machine. This appendix briefly

explains how to allocate RAM to Windows 7 virtual machines. For the sake of simplicity,

memory overhead of the ESX host will not be included in any calculations.

This appendix also assumes that users have a good understanding of ESX memory management

technologies such as memory over-commitment, transparent page sharing (TPS) and vswap

usage. More information can be found in the vSphere Resource Management Guide found at

www.vmware.com.

As an advanced configuration consideration, if you do not over-commit memory, then it is

possible to disable TPS which will free up CPU cycles which the virtual machines can use. TPS

can be disabled by setting

Mem.ShareScanGhz to zero under the advanced settings for an ESX

host. This can provide a 5 percent performance boost to the virtual machines on the host.

Virtual Machine Active Memory (Working Set)

The amount of RAM that is actively used by a virtual machine is referred to as the active

working set. This can be seen with the %ACTV counter in esxtop or the “Active” memory

counter in vCenter under the

Resource Allocation tab. On a virtual machine with 2 GB of

RAM, if the max working set size is 1GB (%ACTV == 50%) then the virtual machine is using 50

percent of its allocated RAM. Assuming other virtual machines exhibit similar behavior, a 2:1

over-commit ratio could be used without causing excessive swapping by the ESX host.

On the ESX server, if the active working set for all virtual machines is less than the total

available host memory, then all the virtual machines run at full speed because each virtual

machine is able to address all the RAM it requires without the hypervisor ballooning or swapping

virtual machine memory pages to disk.

Conversely, when the sum of all active working sets on the host exceeds the amount of available

RAM on the ESX host, the hypervisor will be forced to swap pages from the virtual machine

memory to vswap. The hypervisor has no knowledge of which pages are in the active working set

of the virtual machine and will swap the pages to meet the memory demands placed on the ESX

host. This will lead to very poor performance of the virtual machine and should be avoided at all

times.

Refer to Table 1 for an example in which a host with 32 GB of RAM is hosting virtual machines

each co

igured for 2 GB of RAM. Each virtual machine has on average 50 percent active

memory.

Table 1 Memory over-commit

VM Count Active Memory in host Comments

16 * 2 GB * 50% = 16 GB

Without memory over-commit, only 50% of the host’s

memory is actively in use.

32 * 2 GB * 50% = 32GB

Memory is over-committed by 200% but active memory

is equal to host memory. Virtual machines will run at

full speed until usage exceeds 100% of host memory.

48 * 2 GB * 50% = 48 GB,

limited to

32 GB

by host

These virtual machines want 48 GB of RAM but are

limited to the 32 GB that is installed on the host. ESX

must swap to allow these machines to run

and performance will be degraded.

When used appropriately, the memory over-commit feature allows virtual infrastructure

administrators to drive the ESX hosts to high memory utilization without degrading performance

which is not possible with most physical systems.

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