Virtual Machine Monitors: Technology and Trends

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Virtual Machine MonitorsTechnology and Trends

• Jonathan Kaldor
• CS614 / F07

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Virtual machine Monitors (VMMs)

• Allow users to run multiple commodity OSes on a
  single piece of hardware
• Applications unchanged
• Resources fairly distributed and multiplexed
• Can get, but doesnt need help from hardware/OS
• Main difference between two papers

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Why a VMM?
Server A
Server B
OS A
OS B
Application A
Application B
Client (Windows)
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation

OS A
Application A
Application B
Client (Windows)
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation

OS A
Application A
Application B
Client (Windows)
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation

OS A
???
Application A
Application B
Windows
Linux
Client (Windows)
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation

OS A
Application A
Application B
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation

OS A
Application A
Application B
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation
• While preserving boundaries

OS A
Application A
Application B
Client (Windows)
Client (Linux)
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Why a VMM?
Server A

• Hardware consolidation
• While preserving boundaries

VMM
Guest OS A
Guest OS B
Application A
Application B
Client (Windows)
Client (Windows)
Client (Linux)
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Applications

• Server consolidation
• Application hosting
• Application mobility
• Security
• Reducing need for dual-booting

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VMM Organizational Types
App
App
App
App
App
App
App
App
GuestOS
GuestOS
GuestOS
GuestOS
GuestOS
VM
VM
VM
VM
VM
VMM
App
VMM
HostOS
Hardware
Hardware

• Exokernel-like layer (Type I)
• Multiplexes and manages hardware through virtual
  layer

• Layered on Host OS (Type II)
• Use HostOS to interface with hardware

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To Host or Not to Host

• Hosted eases development
• Can use HostOS drivers to interface with hardware
• But performs poorly
• Hybrid systems (modify HostOS for performance)

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Performance

• Run code directly on CPU for speed
• Conflicting requirements
• VMM needs to maintain control
• OS assumes it is privileged
• Solution run Guest OS code directly in
  less-privileged level
• How to deal with the consequences?

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Hardware Issues(or why no one has ever called
x86 elegant, part 15,023)

• Allows multiple privilege levels (ring 0-3)
• ISA can be ill-defined in virtualized environment
• Silent failures, multiple outcomes, etc
• Hardware page tables
• Nontrivial for VMM to exert control

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(Para-) Virtualization

• OS no longer has complete control over hardware
• Paravirtualization
• Provide alternatives to privileged instructions
• Requires modifying source code of GuestOS
• Binary Translation
• Translate privileged instructions to virtualized
  alternatives while running

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(Para-) Virtualization, cont.

• Arguments for paravirtualization
• Can improve performance
• Important virtual/nonvirtual concepts
• Time!
• Architecture doesnt necessarily work well with
  full virtualization

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(Para-) Virtualization, cont.

• Arguments for binary translation
• Does not require access to source
• Unrealistic at times to modify the OS
• coughWindowscough
• Legacy apps compatible with older OSes
• Can be reasonably fast?

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Design Decisions
Para-Virtualized Binary Translation
Exokernel Xen VMWare ESX
Hosted VMWare WS (sort of)
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An example Page Tables
Application
OS
VMWare
Shadow Table
Page Table

• VMWare Keeps a shadow copy of page table

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An example Page Tables
Application
OS
VMWare
Shadow Table
Page Table

• VMWare Keeps a shadow copy of page table
• Detects when change is made, makes corresponding
  change to shadow table
• Translation from OS address to machine address

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An example Page Tables
Application
OS
Xen
Page Table

• Xen OS tells Xen about the page table,
  relinquishes write control

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An example Page Tables
Application
OS
Xen
Add page 10
Page Table

• Xen OS tells Xen about the page table,
  relinquishes write control
• OS tells Xen what updates it wants to make
• Xen ensures updates are legal, can batch

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Xen Improving Performance

• Minimize TLB flushes
• Xen lives at top 64MB of every address space
• Allow batch updates/requests to Xen
• I/O, page tables, etc
• OS-specified handlers
• Need to guarantee safety

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Xen I/O

• Use ring structure to queue requests / responses
• Enables batching, reordering
• Virtual Network Interface
• Rules used to correctly route packets
• Avoids copying via page trading

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Oversubscribing Memory

• Hundreds of OSes, each with 128MB of maximum
  memory
• Need to efficiently allocate memory among OSes,
  effectively page to disk
• Disk paging at VMM level can result in poor
  behavior

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Disk Paging Policy Decisions at the VMM Level

• VMM decides to take a page from the OS

VMM
Guest OS
Main Memory
Page A
Page B
Page C
Page D
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Disk Paging Policy Decisions at the VMM Level

• VMM decides to take a page from the OS

VMM
Guest OS
Main Memory
Page A
Page B
Page C
Page D
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Disk Paging Policy Decisions at the VMM Level

• VMM decides to take a page from the OS
• OS decides to page to disk as well, picks same
  page

VMM
Guest OS
Main Memory
Page A
Page B
Page C
Page D
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Disk Paging Policy Decisions at the VMM Level

• VMM decides to take a page from the OS
• OS decides to page to disk as well, picks same
  page
• VMM now needs to reload page from disk

VMM
Guest OS
Main Memory
Page A
Page B
Page C
Page D
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Disk Paging Policy Decisions at the VMM Level

• VMM decides to take a page from the OS
• OS decides to page to disk as well, picks same
  page
• VMM now needs to reload page from disk
• solely so the Guest OS can write it back out to
  disk!

VMM
Guest OS
Main Memory
Page A
Page B
Page C
Page D
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Using the OS paging algorithm

• Lesson The VMM is necessarily a poor estimator
  of which page to claim
• Use OS paging algorithm instead
• Balloon process

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Using a Balloon Process
request memory
Guest OS
VMM
Balloon Process
Main Memory
Page A
Page B
Page C
Page D
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Using a Balloon Process
request memory
Guest OS
VMM
Balloon Process
Main Memory
Page A
Page B
Page C
Page D
process needs pages badly!
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Using a Balloon Process
request memory
Guest OS
VMM
Balloon Process
Main Memory
Page A
Page B
Page C
Page D
process needs pages badly!
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Using a Balloon Process
he gave me page c
Guest OS
VMM
Balloon Process
Main Memory
Page A
Page B
Page C
Page D
take page c
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Additional Memory Tricks

• Still need a paging algorithm in case ballooning
  fails
• Potentially many copies of the same page
• Detect these, remap them with copy-on-write
• VMWare 7-30 memory savings in real world

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Performance
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Performance
L Native Linux, X Xen, V VMWare, U User-Mode
Linux
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Multi-OS Performance
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Xen Versus VMWare ESX (with a bucket of salt)
From A Performance Comparison of Commercial
Hypervisors, XenSource http//www.xensource.com/D
ocuments/hypervisor_performance_comparison_1_0_5_w
ith_esx-data.pdf
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Conclusions

• Either approach works well in practice
• Small but noticeable performance penalty
• Becoming a nonissue
• OS support for virtualization
• Microsoft Windows (?!)
• Hardware support

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The Future

• Virtualization is probably going to become more
  commonplace
• Hardware support will hopefully eliminate some
  issues
• In a way, back to where we started
• Resurrection of an old research idea to solve new
  problems