boosting virtualization performance with flexible memory...

Virtualization

Reprinted from Dell Power Solutions, 2011 Issue 2. Copyright © 2011 Dell Inc. All rights reserved. Reprinted from Dell Power Solutions, 2011 Issue 2. Copyright © 2011 Dell Inc. All rights reserved.

To accelerate gains in IT efficiency and

cost savings, organizations are highly

focused on virtualizing systems for

optimal hardware utilization of shared

processor, memory, and storage resources. System

memory dramatically affects performance, so it is

usually sized to help ensure acceptable response

even at peak workloads, however infrequent they

may be. When virtual machine (VM) workloads

vary at different times or rates, the capability to

dynamically add and remove memory from the

VMs enables organizations to meet on-demand

requirements and maximize the use of hardware

resources without compromising performance.

To address this growing need, Microsoft introduced

the Dynamic Memory (DM) feature in the Hyper-V

virtualization hypervisor as part of Windows Server 2008

R2, Service Pack 1 (SP1). The DM feature allows a VM

to draw upon and release memory based on priority

settings and specific VM OS or application requirements.

At the same time, it provides the ability to control the

minimum and maximum memory allotted to a VM and

also the priority of a VM to get memory from the host’s

memory pool during peak periods.

48 2011 Issue 02 | dell.com/powersolutions

Delivered as part of SP1 for the Microsoft® Windows Server® 2008 R2

OS, the Dynamic Memory feature heightens memory efficiency for the

Microsoft Hyper-V™ hypervisor—enabling increased VM density and

hardware utilization when VM workloads fluctuate.

By Barun Chaudhary and David Waggoner

Boosting virtualization performance with flexible memory configurations

dell.com/powersolutions | 2011 Issue 02 49

Virtualization


Setting up Dynamic Memory

The DM feature requires no additional hardware

beyond typical Hyper-V requirements. This

complimentary enhancement is available to

deployed servers after a Windows Server 2008

R2 SP1 upgrade has been completed on the host

machine. Figure 1 lists guest operating systems

for which DM may be enabled.

Making sure each legacy guest OS is

updated to support DM is important. Both

32- and 64-bit versions of guest operating

systems are supported. Organizations with

Microsoft System Center Virtual Machine

Manager (SCVMM)–managed environments

should also confirm SCVMM is at the right

service pack level to comprehensively support

DM, based on the following guidelines:

• Before SP1 release, VM memory was allocated

during creation and would not change unless

the VM was powered off and the static settings

were changed.

• After SP1 release, static memory remains

the default, but it can be replaced by the

DM setting. To enable DM, the VM must

be powered off, and the DM parameters

configured from VM settings. Right-click

the VM, then select Settings and navigate to

memory options (see Figure 2).

Startup RAM is the basic starting memory

for each VM. This value should be the minimum

memory recommended by Microsoft for

individual VMs, depending on the guest OS in

use. For the guest operating systems listed in

Figure 1, Microsoft-recommended minimum

startup RAM settings are as follows:

• 128 MB for Windows Server 2003–based systems

• 512 MB for Windows Vista® and subsequent

operating systems

If DM is inadvertently enabled on a VM

running a non-DM-capable guest OS, the startup

value is treated as a static setting; however,

problems may occur, so enabling DM in this case

is not recommended.

Maximum RAM is the highest value to which

the VM memory can expand. By default, this

setting is 64 GB—the maximum supported VM

memory value. However, it is not useful to set this

value higher than the guest OS limit because that

limit cannot be exceeded. For example, Windows

Server 2008 R2 Standard is limited to 32 GB, so

setting a higher maximum RAM value has no

effect on the guest OS. When the VM is running,

the following settings can be adjusted:

• Memory buffer: This setting enables the host

to keep a cushion of RAM assigned to address

quick spikes, because the DM adjustment is

not instantaneous.

• Memory weight: This setting communicates to

the host the VM’s priority for memory resources

when there is contention. A best practice

is to carefully consider the memory weight

setting on mission-critical VMs—for example,

a runaway application may cause significant

Guest operating systems for VMs Version

Windows Server 2008 R2 with SP1 Standard, Enterprise, Web, and Datacenter

Windows Server 2008 R2 RTM* Enterprise and Datacenter

Windows Server 2008 SP2* Standard, Enterprise, Web, and Datacenter

Windows Server 2003 R2 SP2* Standard, Enterprise, Web, and Datacenter

Windows Server 2003 SP2* Standard, Enterprise, Web, and Datacenter

Windows 7 with SP1 Enterprise and Ultimate

Windows 7 and Vista SP2* Enterprise and Ultimate

* Requires updating Hyper-V integration services and installing Microsoft Quick Fix Engineering (QFE) update KB2230887 for the Standard and Web editions of Windows Server 2008 R2 SP2. For details, visit support.microsoft.com/kb/2230887.

Figure 1. Operating systems in which DM can be enabled

Dynamic Memory and other SP1 benefits

Windows Server 2008 R2 SP1 offers robust features and enhancements. Listen to Oliver Rist of the Microsoft Windows Server team discuss DM and other key features of the SP1 release in this TechNet podcast.

bit.ly/mCyKpD

http://support.microsoft.com/kb/2230887

http://bit.ly/mCyKpD


Virtualization


memory utilization in a particular VM,

resulting in the host trying to reallocate

memory away from certain critical VMs

to support those machines with a higher

priority. If all VMs are left at their defaults,

the host may select an undesired VM to

protect if memory becomes constrained.

Defining Dynamic Memory

Similar to the Hot-Add Memory1 support

already available on certain Windows

Server versions, updated integration

services allow the hypervisor to

communicate directly with the enlightened

guest OS memory manager through a

synthetic driver—but with substantially

enhanced flexibility. The Microsoft Hyper-V

host monitors the VM’s memory usage

and demands, and provides additional

memory to the VM based on priority

settings and overall host workload. When

the host needs to reclaim memory, it

triggers a balloon driver that starts to

occupy memory in the guest—using the

same mechanism a regular driver uses

to allocate memory. Once occupied, the

driver locks that memory in the guest, so

that nothing else can access it, and reports

to the hypervisor that it can reallocate that

space. The memory is then reassigned

based on priority and/or demand.

Testing Dynamic Memory

As a Microsoft Premier Partner, Dell has

been involved with early builds of SP1 to

validate this feature on a wide range of

Dell™ PowerEdge™ servers designed for

real-time applications. Dell servers that

support Microsoft Hyper-V are capable of

supporting the DM feature without added

cost or hardware. Advanced systems

such as the Dell PowerEdge R810 rack

server, PowerEdge M915 blade server, and

PowerEdge R815 rack server—all supporting

up to 512 GB RAM—are designed to

provide an easy, optimized way to set up a

high-density data center. The DM feature

was tested on a variety of servers, memory

configurations, and storage to evaluate the

feature and ensure it behaves consistently

across these servers. One test of DM by Dell

engineers at Dell Labs in March 2011 on a

server with medium memory configuration

produced the example results that follow.

On a PowerEdge T610 system with

the Intel® Xeon® E5530 processor at

2.40 GHz, 8 GB of physical memory, and

running Windows Server 2008 R2 SP1 as

the host OS, two VMs were created with the

Windows Server 2008 R2 SP1 OS.

In the first test, each VM was

configured with a static memory of

1,024 MB, as well as a dynamically

increasing load set to consume system

memory. Regardless of the workload in

the VMs, the memory assigned to each

VM was limited to 1,024 MB—represented

by the blue line in Figure 3—to a collective

total of 2,048 MB, resulting in performance

impact on the load.

In the second test, DM was enabled

on each of the VMs with the following

parameter configuration:

• Startup RAM – 1,024 MB

• Maximum RAM – 6,000 MB

• Memory buffer – 20 percent

• Memory weight – 50 percent

When the same load as in the first

test was applied to both VMs—memory

assignment for each is represented by

the green and red lines in Figure 3—the

collective memory assignment for both—

represented by the orange line in Figure 3—

grew to approximately 7,000 MB, enhancing

Figure 2. Virtual machine memory settings for enabling DM

1 For details on Hot-Add Memory, see msdn.microsoft.com/en-us/windows/hardware/gg487553.

dell.com/powersolutions | 2011 Issue 02 51

Virtualization


utilization of the host’s physical memory

(see Figure 3). These example test scenarios

indicate that with the increase in load, VM

memory utilization increases, leading to the

assignment of additional memory to the

VMs. When the VMs reach their maximum

memory assignment, their shares both

remain steady until one VM’s load decreases

and the host starts reclaiming memory—

while reallocating memory to the other VM

to handle the increasing load.

Figure 4 shows the significance and

behavior of memory weight, which

defines the priority for VM memory

assignments. Both VMs were configured

with 1,024 MB startup RAM, 6,000 MB

maximum RAM, and 20 percent memory

buffer, and both were set with the same

increasing load. In addition, VM-1 was

configured with 75 percent memory

weight, and VM-2 was configured with

25 percent memory weight.

With a growing load, both VMs

showed similar memory utilization until

they reached the maximum physical

limit in the system, approximately

7.5 GB total. Because the load in both

VMs continued to demand more memory,

once the maximum value was reached the

hypervisor began to reassign memory—

providing increased memory to the VM

with the higher memory weight, VM-1,

and drawing it from the VM with the lower

memory weight, VM-2—while maintaining

the collective usage at the same level,

approximately 7.5 GB.

Scaling memory on demand

The primary advantage of DM is the

ability of the hypervisor host to scale

the VM memory to support fluctuating

demand. Increasing memory on demand

enhances performance within the VM

and resident applications. Figure 5

shows a comparison of the memory

utilization and hard faults that can occur

within a VM with static versus dynamic

memory settings.

In a hard fault scenario, the system

tries to locate a memory address that has

been paged out to the disk and is not

present in the physical memory. Whenever

a hard fault occurs, the system has to

load the memory content from the disk to

physical memory, which can significantly

impact performance. As shown in the

last graph of each snapshot in Figure 5,

hard faults become prominent during

heavy system memory usage and when

maximum memory utilization suddenly

drops. With static memory set, hard faults

were prominent, contributing to sluggish

behavior of the application running within

the VM. In contrast, with dynamic memory

set, hard faults decreased significantly,

enhancing the VM’s performance.

Figure 3. System memory utilization with static and dynamic memory

Figure 4. Memory assignment for VMs having different memory weight with DM

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

Mem

ory

uti

liza

tio

n (

meg

abyt

es)

Load in VM

0 5 10 15

Static memory (per VM)

VM-1 (dynamic memory)

VM-2 (dynamic memory)

Total system memory utilization (dynamic memory)

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

Mem

ory

uti

liza

tio

n (

meg

abyt

es)

Load in VM

0 5 10 15 20

VM-1 (memory weight—75 percent)

VM-2 (memory weight—25 percent)

Total system memory utilization


Virtualization


Accelerating virtualization cost-efficiencies

with Dynamic Memory

The Microsoft Hyper-V DM feature provides

efficient, on-demand memory scaling to

dynamic and changing IT requirements. DM

offers a beneficial, easy-to-use, cost-free

addition to the Hyper-V hypervisor feature set,

and is suitable for a wide range of organizations

from small businesses to large enterprise data

centers. By ensuring that particular applications

are supported for DM,2 and configuring

memory weight settings to match VMs with

their correct priorities, organizations can use

DM to realize increased hardware utilization,

which can be a critical component of a

successful virtualization strategy.

Figure 5. Comparing static memory and dynamic memory utilization in the VM

2 For an overview of system requirements, see the Hardware Virtualization section of “Exchange 2010 System Requirements” at technet.microsoft.com/en-us/library/aa996719.aspx.

Static memory

utilization

Dynamic memory

utilization

Authors

Barun Chaudhary is a software engineer on the

Dell Enterprise Server Operating System team,

specializing in Windows Server virtualization and

networking technology.

David Waggoner is a technologist on the Dell

Enterprise Server Operating System team, specializing

in Windows Server and other Microsoft technologies.

Learn more

Hyper-V Dynamic Memory Configuration Guide:

bit.ly/mcNX2b

Hyper-V and Dynamic Memory in Depth:

bit.ly/kUUsbu

http://technet.microsoft.com/en-us/library/aa996719.aspx

http://technet.microsoft.com/en-us/library/aa996719.aspx

http://bit.ly/mcNX2b

http://bit.ly/kUUsbu

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