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Embedded System Lab.
Xen and the Art of
Virtualization
Yoon Jun Kee
Embedded System Lab.
Basic idea
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Embedded System Lab.
☠ Xen
What is Xen?
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A high performance resourcemanaged virtual machine
monitor (VMM).
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Zen enables applications.
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Xen 4.5
server consolidation
co-located hosting facilities
distributed web services
secure computing platforms
application mobility.
Embedded System Lab.
INTRODUCTION
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VMM : High performance resource-managed virtual machine monitor.
Successful partitioning
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VMs must be isolated
To support a variety of different OS
The performance overhead introduced by virtualization should be small
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Xen enables to dynamically instantiate an operating system.
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Xen enables to dynamically instantiate an operating system.
performance isolation
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The scheduling,
memory demand,
network traffic, disk
accesses
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Impact the
performance
of others
Embedded System Lab.
XEN: APPROACH & OVERVIEW
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Paravirtualization vs Full virtualization
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Embedded System Lab.
XEN: APPROACH & OVERVIEW
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design principles
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Support for unmodified application binaries is essential, or users will not
transition to Xen..
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Supporting full multi-application operating systems is important, as this
allows complex server configurations to be virtualized within a single
guest OS instance.
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Paravirtualization is necessary to obtain high performance and strong
resource isolation on uncooperative machine architectures such as x86.
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Even on cooperative machine architectures, completely hiding the effects
of resource virtualization from guest OSes risks both correctness and
performance.
Embedded System Lab.
XEN: APPROACH & OVERVIEW
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Xen design
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Xen is intended to scale to approximately 100 virtual machines running
industry standard applications and services.
1)
Denali does not target existing ABIs, and so can elide certain
architectural features from their VM interface.
2)
The Denali implementation does not address the problem of supporting
application multiplexing, nor multiple address spaces, within a single
guest OS.
3)
In the Denali architecture the VMM performs all paging to and from disk.
4)
Denali virtualizes the ‘namespaces’ of all machine resources, taking the
view that no VM can access the resource allocations of another VM if it
cannot name them.
Embedded System Lab.
The Virtual Machine Interface
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Such as memory management, are specific to the x86, many aspects
(such as our virtual CPU and I/O devices) can be readily applied to
other machine architectures.
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Typical System call
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Embedded System Lab.
Xen CPU
Xen: CPU
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X86 supports 4 levels of
privileges
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0 for OS, and 3 for applications
Xen downgrades the privilege
of OSes
System-call and page-fault
handlers registered to Xen
“fast handlers” for most
exceptions, Xen isn’t involved
Embedded System Lab.
Control transfer
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Virtualizing Device I/O
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Xen exposes device abstractions.
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I/O data is transferred to and from each domain via Xen, using shared
memory, asynchronous buffer descriptor rings.
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Embedded System Lab.
Data Transfer: I/O Rings
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Two main factors have shaped the design of our I/O-transfer
mechanism.
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resource management
event notification
I/O buffers are protected during data transfer by pinning the
underlying page frames within Xen.
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Embedded System Lab.
Data Transfer: I/O Rings
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Decouple the production of requests or responses from the notification
of the other party.
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Requests case : a domain may enqueue multiple entries before invoking
a hypercall to alert Xen.
Responses case : a domain can defer delivery of a notification event by
specifying a threshold number of responses.
This allows each domain to trade-off latency and throughput
requirements, similarly to the flow-aware interrupt dispatch in the
ArseNIC Gigabit Ethernet interface.
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Embedded System Lab.
Memory management
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Hardware provides a softwaremanaged TLB. Unfortunately, x86 does
not have a software-managed TLB.
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TLB is not tagged, address space switches typically require a
complete TLB flush.
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Given these limitations to made two decisions.
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Guest OSes are responsible for allocating and managing the hardware
page tables.
Xen exists in a 64MB section at the top of every address space, thus
avoiding a TLB flush when entering and leaving the hypervisor.
The OS must relinquish direct write privileges to the page-table
memory: all subsequent updates must be validated by Xen.
Embedded System Lab.
Xen Control and Management
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Embedded System Lab.
Subsystem Virtualization
CPU scheduling
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Xen currently schedules domains according to the Borrowed Virtual
Time (BVT) scheduling algorithm
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Fast dispatch is particularly important to minimize the effect of
virtualization on OS subsystems that are designed to run in a timely
fashion
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Embedded System Lab.
Subsystem Virtualization
Time and timers
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Each guest OS can program a pair of alarm timers, one for real time
and the other for virtual time
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Real time : Real time is expressed in nanoseconds passed since machine
boot and is maintained to the accuracy of the processor’s cycle counter
and can be frequency-locked to an external time source
Virtual time : virtual time only advances while it is executing
Guest OSes are expected to maintain internal timer queues and use
the Xen-provided alarm timers to trigger the earliest timeout
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Embedded System Lab.
Subsystem Virtualization
Virtual address translation
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No shadow pages (VMWare)
Xen provides constrained but direct MMU updates
All guest OSes have read-only accesses to page tables
Updates are batched into a single hypercall
Updates must be validated by Xen
Guest OSes are responsible for allocation and managing pages within
their own domain
Xen exists in a generally unused section at the top of every address
space to prevent paging out
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Embedded System Lab.
Subsystem Virtualization
Physical memory
 Reserved at domain creation times
 Memory statically partitioned among domains
 Does not guarantee contiguous regions of memory
 Supports hardware~physical mapping by providing shared translation
array readable by all domains
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Embedded System Lab.
Subsystem Virtualization
Network
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Virtual firewall-router attached to all domains
Round-robin packet scheduler
To send a packet, enqueue a buffer descriptor into the transmit rang
Use scatter-gather DMA (no packet copying)
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A domain needs to exchange page frame to avoid Copying
Page-aligned buffering
Embedded System Lab.
Subsystem Virtualization
Disk
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Only Domain0 has direct access to disks
Other domains need to use virtual block devices
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Use the I/O ring
Reorder requests prior to enqueuing them on the ring
If permitted, Xen will also reorder requests to improve
Performance
Use DMA (zero copy)
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Embedded System Lab.
The Cost of Porting an OS to Xen
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More changes were required in Windows XP, mainly due to the
presence of legacy 16-bit emulation code and the need for a
somewhat different boot-loading mechanism.
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Embedded System Lab.
Evaluation
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Based on Linux 2.4.21(neither XP nor NetBSD fully functional)
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Thoroughly compared to 2 other systems
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–VMware Workstation (binary translation)
–UML (run Linux as a Linux process)
Performs better than solutions with restrictive licenses (ESX Server)
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Embedded System Lab.
Relative Performance
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Embedded System Lab.
Operating System Benchmarks
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As expected fork, exec and shrequire large number of page updates
which slow things down
On the up side these can be batched (up to 8MB of address space
constructed per hypercall)
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Embedded System Lab.
Operating System Benchmarks
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Overhead due to a hypercall when switching context in a guest OS (in
order to change base of page table)
The larger the working set the smaller the relative overhead
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Embedded System Lab.
Operating System Benchmarks
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2 transitions into XEN
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One for the page fault handler
One to actually get the page
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Operating System Benchmarks
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Page flipping really pays off –no unnecessary data copying
More overhead for smaller packets –we still need to deal with every
header
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Embedded System Lab.
Concurrent Virtual Machines
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Unexpectedly low SMP
performance for 1
instance of Apache
As expected adding
another domain leads to
a sharp jump in
performance under XEN
More domains –more
overhead
Embedded System Lab.
Concurrent Virtual Machines
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Performance
differentiation works as
expected with IR
But fails with OLTP
Probably due to
inefficiencies with the
disk scheduling
algorithm
Embedded System Lab.
Isolation
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Run uncooperative
user applications, see
if they bring down the
system
2 “bad” domains vs 2
“good” ones
XEN delivers good
performance even in
this case
Embedded System Lab.
Scalability
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Very low footprint per domain (4 -6MB memory, 20KB state)
Benchmark is compute-bound and Linux assigns long time slices,
XEN needs some tweaking
Even without it does pretty well (but no absolute values)
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Embedded System Lab.
Thank you!
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Any questions?
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Embedded System Lab.