Hyper-V Dynamic Memory in Depth

Bryon Surace Senior Program ManagerWindows Server, Microsoft

Agenda

•Memory & Virtualization •Memory Optimization Techniques• Hyper-V Dynamic Memory• Architecture & Concepts• Demo• System Impact

•Q&A

Memory & Virtualization

Virtualization & Memory•How much memory does a server actually need?• IIS Server?• Print server?• File server?• Branch Cache?• Direct Access?

•How much will performance be affected if you halved the amount of memory in a VM?

No one wants to size VMs Memory

• “New virtual machines get 1GB of RAM [no matter what the VM is running]. I only give people more memory if they complain about performance”

• “All VMs get 4GB of RAM [I have no idea what is happening with that memory] and no one complains”

• “I take the minimum system requirements and add (insert one: 50%, 100%, 150%)”

• “A vendor tells me their app needs 4GB of RAM. I do not have the time to test this to find out if it is true or not”

Virtualization & Memory• Memory• Key Factor to the number of running VMs• The most utilized asset in the system, but also a more expensive asset

• Statistics on resource utilization of workloads • CPU 10%• Memory 40%• Network I/O <5%• Disk I/O <5%

• Customer Requirements• Maximum density, without sacrificing performance• Maintain consistent performance• Don’t provide a feature that’s unsuitable for production use

Dynamic Memory Goals•Higher VM consolidation ratios with minimal performance

impact• Dependent on:• How much variation in memory utilization the workloads have• How good a job you did of sizing the systems in the first place

•Work well for both server and desktop workloads•Add minimal overhead to the system• Pass the “that looks right” test

Understanding philosophical differences

•Host and Guest Collaborate• Understands what guest information to use• Trying to get the “best bang for buck” in virtual memory

management

•Host works in Isolation• Does not trust guest information• Building a “black box” solution

Dynamic Memory, not Overcommit

•Overloaded Term• Over-subscription vs. Over-committment

•No one wants to overcommit their resource• You don’t overcommit other resources (really – you do not)• VMware does not want you to overcommit memory (really)

•Dynamic Memory treat memory like we treat CPU resources• Dynamically schedulable resource

Memory Management & Optimization Techniques

Ballooning• How it works:• Increasing the size of the balloon forces the guest to react to memory pressure by

releasing unused pages• Decreasing the size makes more memory available to the guest

VM

Memory

Host Computer

Balloon

Disk

VM

Host Computer

Memory

Balloon

DiskInflate

Swap Out

Ballloon

Deflate

VM

Host Computer

Memory

Disk

Swap In

Balloon

Deflate

Second Level Paging

•How it works: • Paging at the virtualization layer by swapping VMs

memory out to disk•Many problems:• Swapping Guest Kernel Resources•Double Paging•Disks are slow

•But it always works…

External Page Sharing• How it works: • Eliminate redundant copies of memory pages common to more than one

virtual machine1. Hash all memory and store it in a table…2. Identify the common hashes and then…3. Perform a bit by bit comparison

• Problems• Page Sharing not dynamic• Can take hours to share pages• The largest benefit are zero pages• Doesn’t work with large pages

Other Techniques•Guest directed page sharing•Memory compression•And on…

Hyper-V R2 SP1Dynamic Memory Architecture & Concepts

System Requirements

•Parent Requirements:•Windows Server 2008 R2 SP1, Windows Server 2012•Hyper-V Server 2008 R2 SP1, Hyper-V Server 2012

•Guest Requirements:•Windows Server 2003, 2008 & 2008 R2, 2012• 32-bit & 64-bit versions

•Windows Vista, Windows 7, Windows 8• Enterprise and Ultimate Editions only• 32-bit & 64-bit versions

Adding/Removing Memory

•Adding Memory• Enlightened fashion• Synthetic Memory Driver (VSP/VSC Pair)

•No hardware emulation• Light weight

•Removing Memory•Wanted to remove memory•Ballooning is more efficient•Messes up task manager in the guest OS

Adding/Removing Memory

•Active Memory addition•Memory is added immediately when VM needs it

• Passive Memory reclamation•Memory is not removed when there is no immediate need• Unutilized memory is collected every 5 minutes

Startup & Max

Windows Server 2008 R2 SP1• Startup: amount of memory to boot VM• BIOS does not know about DM• Guest OS may not know about DM• Default: 512MB

•Max: don’t let the VM above this amount• Default: 64GB

Windows Server 2012• Startup: amount of memory to boot VM•Min: amount of memory the VM can balloon down to•Max: don’t let the VM above this amount

Controlling Memory Availability

•Availability is a concept•How much memory does the VM have?•How much memory does the VM want?• The difference is the availability

•Mechanisms in place to control Memory Availability through Buffer and Priority settings

Memory Buffer & Priority

• Buffer: How much “free” memory should we try and keep in the VM?• Allows for responsiveness to bursty workloads• Can be used for file cache

“I like to configure my virtual machines so that they have ~20% free memory”

• Priority: which VM gets the memory first• 1-10,000: default is 5,000• The higher the priority, the higher the availability

Demo:Dynamic Memory in Action

System Impact

Changes to Root Reserve

•Hyper-V has always had the concept of a reserve of memory that is kept for the parent partition

Host Reserve - Auto-calculated based on:If the machine SLAT capabilityTotal size of host RAMNUMA architecture

Host Reserve VM Overhead Memory utilized by VMs

VM Reserve

Changes to Root Reserve

•DM allows VMs to push up against the reserve consistently•New behavior to better protect the parent partition

from rampaging virtual machines•New registry key in place• Allows you to reserve static memory for the parent partition

•May result in less memory being available for VMs

Changes to NUMA management…•Wait – what is NUMA?•Why do I care?• How does this work today?

What is “NUMA”?• A traditional computer:

Computer

CPU CPU CPU CPU

BUS

Memory

VM VM VM VM

VM VM VM VM

What is “NUMA”?• A NUMA computer:

Computer

CPU CPU CPU CPU

BUS

Memory

VM VM VM VM

VM VM VM VM

Back Channel

Memory

BUS

Node 1 Node 2

Why do I care?• VM memory should come from the “local NUMA node”

Computer

CPU CPU CPU CPU

BUS

Memory

VM VM VM VM

VM VM VM VM

Back Channel

Memory

BUS

Node 1 Node 2

Good!

Why do I care?• Ideally VM memory should come from the “local NUMA node”

Computer

CPU CPU CPU CPU

BUS

Memory

VM VM VM VM

VM VM VM VM

Back Channel

Memory

BUS

Node 1 Node 2

Bad

How does this work today?

• Hyper-V tries to get all memory for a virtual machine from a single NUMA node•When it cannot – the virtual machine “spans” NUMA nodes• Users can set preferred NUMA nodes for virtual machines in order

to get the best distribution

Changes to NUMA management•Dynamic memory can result in more virtual machines

spanning NUMA nodes• A virtual machine might start all on one node – but added

memory might come from another node•New option to disable NUMA node spanning

Disabling NUMA Spanning• Makes the system behave like multiple small computers

Computer

CPU CPU CPU CPU

BUS

Memory

VM VM VM VM

VM VM VM VM

Back Channel

Memory

BUS

Node 1 Node 2

Questions and Answers