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  • Gerd Knor of Suse, Rusty Russell, Jon Mason and Ryan Harper of IBM
  • POPF doesn’t trap if executed with insufficient privilege
  • Commercial / production use Released under the GPL; Guest OSes can be any licensce
  • X86 hard to virtualize so benefits from more modifications than is necessary for IA64/PPC popf Avoiding fault trapping : cli/sti can be handled at guest level
  • Important for fork
  • RaW hazards
  • Use update_va_mapping in preference
  • typically 2 pages writeable : one in other page table
  • RaW hazards
  • RaW hazards
  • Most benchmark results identical. In addition to these, some slight slowdown
  • Xen has always supported SMP hosts Important to make virtualization ubiquitous, enterprise
  • Xen has always supported SMP hosts Important to make virtualization ubiquitous, enterprise
  • Pacifica
  • At least double the number of faults Memory requirements Hiding which machine pages are in use: may interfere with page colouring, large TLB entries etc.
  • Framebuffer console NUMA
  • Net-restart
  • newring
  • We have run 128 guests Tickerless patch may help for large numbers of guests
  • disk is easy new IO isn’t as fast as not all ring 0 ; win big time
  • 8(diff) : using QoS controls to split resources in 1:2:3:4:5:6:7:8 ratios. OSDB-OLTP suffers due to current poor disk scheduling – should be fixed in future Xen release

Transcript

  • 1. Xen 3.0 and the Art of Virtualization Ian Pratt Keir Fraser, Steven Hand, Christian Limpach, Andrew Warfield, Dan Magenheimer (HP), Jun Nakajima (Intel), Asit Mallick (Intel) Computer Laboratory
  • 2. Outline
    • Virtualization Overview
    • Xen Architecture
    • New Features in Xen 3.0
    • VM Relocation
    • Xen Roadmap
  • 3. Virtualization Overview
    • Single OS image: Virtuozo, Vservers, Zones
      • Group user processes into resource containers
      • Hard to get strong isolation
    • Full virtualization: VMware, VirtualPC, QEMU
      • Run multiple unmodified guest OSes
      • Hard to efficiently virtualize x86
    • Para-virtualization: UML, Xen
      • Run multiple guest OSes ported to special arch
      • Arch Xen/x86 is very close to normal x86
  • 4. Virtualization in the Enterprise X
    • Consolidate under-utilized servers to reduce CapEx and OpEx
    • Avoid downtime with VM Relocation
    • Dynamically re-balance workload to guarantee application SLAs
    • Enforce security policy
    X X
  • 5. Xen Today : Xen 2.0.6
    • Secure isolation between VMs
    • Resource control and QoS
    • Only guest kernel needs to be ported
      • User-level apps and libraries run unmodified
      • Linux 2.4/2.6, NetBSD, FreeBSD, Plan9, Solaris
    • Execution performance close to native
    • Broad x86 hardware support
    • Live Relocation of VMs between Xen nodes
  • 6. Para-Virtualization in Xen
    • Xen extensions to x86 arch
      • Like x86, but Xen invoked for privileged ops
      • Avoids binary rewriting
      • Minimize number of privilege transitions into Xen
      • Modifications relatively simple and self-contained
    • Modify kernel to understand virtualised env.
      • Wall-clock time vs. virtual processor time
        • Desire both types of alarm timer
      • Expose real resource availability
        • Enables OS to optimise its own behaviour
  • 7. Xen 2.0 Architecture Event Channel Virtual MMU Virtual CPU Control IF Hardware (SMP, MMU, physical memory, Ethernet, SCSI/IDE) Native Device Driver GuestOS (XenLinux) Device Manager & Control s/w VM0 Native Device Driver GuestOS (XenLinux) Unmodified User Software VM1 Front-End Device Drivers GuestOS (XenLinux) Unmodified User Software VM2 Front-End Device Drivers GuestOS (XenBSD) Unmodified User Software VM3 Safe HW IF Xen Virtual Machine Monitor Back-End Back-End
  • 8. Xen 3.0 Architecture Event Channel Virtual MMU Virtual CPU Control IF Hardware (SMP, MMU, physical memory, Ethernet, SCSI/IDE) Native Device Driver GuestOS (XenLinux) Device Manager & Control s/w VM0 Native Device Driver GuestOS (XenLinux) Unmodified User Software VM1 Front-End Device Drivers GuestOS (XenLinux) Unmodified User Software VM2 Front-End Device Drivers Unmodified GuestOS (WinXP)) Unmodified User Software VM3 Safe HW IF Xen Virtual Machine Monitor Back-End Back-End VT-x x86_32 x86_64 IA64 AGP ACPI PCI SMP
  • 9. x86_32
    • Xen reserves top of VA space
    • Segmentation protects Xen from kernel
    • System call speed unchanged
    • Xen 3 now supports PAE for >4GB mem
    ring 3 Kernel User 4GB 3GB 0GB Xen S S U ring 1 ring 0
  • 10. x86_64
    • Large VA space makes life a lot easier, but:
    • No segment limit support
    • Need to use page-level protection to protect hypervisor
    Kernel User 2 64 0 Xen U S U Reserved 2 47 2 64 -2 47
  • 11. x86_64
    • Run user-space and kernel in ring 3 using different pagetables
      • Two PGD’s (PML4’s): one with user entries; one with user plus kernel entries
    • System calls require an additional syscall/ret via Xen
    • Per-CPU trampoline to avoid needing GS in Xen
    Kernel User Xen U S U syscall/sysret r3 r0 r3
  • 12. Para-Virtualizing the MMU
    • Guest OSes allocate and manage own PTs
      • Hypercall to change PT base
    • Xen must validate PT updates before use
      • Allows incremental updates, avoids revalidation
    • Validation rules applied to each PTE:
      • 1. Guest may only map pages it owns*
      • 2. Pagetable pages may only be mapped RO
    • Xen traps PTE updates and emulates, or ‘unhooks’ PTE page for bulk updates
  • 13. Writeable Page Tables : 1 – Write fault MMU Guest OS Xen VMM Hardware page fault first guest write guest reads Virtual -> Machine
  • 14. Writeable Page Tables : 2 – Emulate? MMU Guest OS Xen VMM Hardware first guest write guest reads Virtual -> Machine emulate? yes
  • 15. Writeable Page Tables : 3 - Unhook MMU Guest OS Xen VMM Hardware guest writes guest reads Virtual -> Machine X
  • 16. Writeable Page Tables : 4 - First Use MMU Guest OS Xen VMM Hardware page fault guest writes guest reads Virtual -> Machine X
  • 17. Writeable Page Tables : 5 – Re-hook MMU Guest OS Xen VMM Hardware validate guest writes guest reads Virtual -> Machine
  • 18. MMU Micro-Benchmarks L X V U Page fault ( µ s) L X V U Process fork ( µ s) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 lmbench results on Linux (L), Xen (X), VMWare Workstation (V), and UML (U)
  • 19. SMP Guest Kernels
    • Xen extended to support multiple VCPUs
      • Virtual IPI’s sent via Xen event channels
      • Currently up to 32 VCPUs supported
    • Simple hotplug/unplug of VCPUs
      • From within VM or via control tools
      • Optimize one active VCPU case by binary patching spinlocks
  • 20. SMP Guest Kernels
    • Takes great care to get good SMP performance while remaining secure
      • Requires extra TLB syncronization IPIs
    • Paravirtualized approach enables several important benefits
      • Avoids many virtual IPIs
      • Allows ‘bad preemption’ avoidance
      • Auto hot plug/unplug of CPUs
    • SMP scheduling is a tricky problem
      • Strict gang scheduling leads to wasted cycles
  • 21. I/O Architecture
    • Xen IO-Spaces delegate guest OSes protected access to specified h/w devices
      • Virtual PCI configuration space
      • Virtual interrupts
      • (Need IOMMU for full DMA protection)
    • Devices are virtualised and exported to other VMs via Device Channels
      • Safe asynchronous shared memory transport
      • ‘ Backend’ drivers export to ‘frontend’ drivers
      • Net: use normal bridging, routing, iptables
      • Block: export any blk dev e.g. sda4,loop0,vg3
    • (Infiniband / Smart NICs for direct guest IO)
  • 22. VT-x / (Pacifica)
    • Enable Guest OSes to be run without para-virtualization modifications
      • E.g. legacy Linux, Windows XP/2003
    • CPU provides traps for certain privileged instrs
    • Shadow page tables used to provide MMU virtualization
    • Xen provides simple platform emulation
      • BIOS, Ethernet (ne2k), IDE emulation
    • (Install paravirtualized drivers after booting for high-performance IO)
  • 23. Native Device Drivers Control Panel (xm/xend) Front end Virtual Drivers Linux xen64 Xen Hypervisor Device Models Guest BIOS Unmodified OS Domain N Linux xen64 Callback / Hypercall VMExit Virtual Platform 0D Guest VM (VMX) (32-bit) Backend Virtual driver Native Device Drivers Domain 0 Event channel 0P 1/3P 3P FE Virtual Drivers Guest BIOS Unmodified OS VMExit Virtual Platform Guest VM (VMX) (64-bit) FE Virtual Drivers 3D I/O: PIT, APIC, PIC, IOAPIC Processor Memory Control Interface Hypercalls Event Channel Scheduler
  • 24. MMU Virtualizion : Shadow-Mode MMU Accessed & dirty bits Guest OS VMM Hardware guest writes guest reads Virtual -> Pseudo-physical Virtual -> Machine Updates
  • 25. VM Relocation : Motivation
    • VM relocation enables:
      • High-availability
        • Machine maintenance
      • Load balancing
        • Statistical multiplexing gain
    Xen Xen
  • 26. Assumptions
    • Networked storage
      • NAS: NFS, CIFS
      • SAN: Fibre Channel
      • iSCSI, network block dev
      • drdb network RAID
    • Good connectivity
      • common L2 network
      • L3 re-routeing
    Xen Xen Storage
  • 27. Challenges
    • VMs have lots of state in memory
    • Some VMs have soft real-time requirements
      • E.g. web servers, databases, game servers
      • May be members of a cluster quorum
      • Minimize down-time
    • Performing relocation requires resources
      • Bound and control resources used
  • 28. Relocation Strategy Stage 0: pre-migration Stage 1: reservation Stage 2: iterative pre-copy Stage 3: stop-and-copy Stage 4: commitment VM active on host A Destination host selected (Block devices mirrored) Initialize container on target host Copy dirty pages in successive rounds Suspend VM on host A Redirect network traffic Synch remaining state Activate on host B VM state on host A released
  • 29. Pre-Copy Migration: Round 1
  • 30. Pre-Copy Migration: Round 1
  • 31. Pre-Copy Migration: Round 1
  • 32. Pre-Copy Migration: Round 1
  • 33. Pre-Copy Migration: Round 1
  • 34. Pre-Copy Migration: Round 2
  • 35. Pre-Copy Migration: Round 2
  • 36. Pre-Copy Migration: Round 2
  • 37. Pre-Copy Migration: Round 2
  • 38. Pre-Copy Migration: Round 2
  • 39. Pre-Copy Migration: Final
  • 40. Writable Working Set
    • Pages that are dirtied must be re-sent
      • Super hot pages
        • e.g. process stacks; top of page free list
      • Buffer cache
      • Network receive / disk buffers
    • Dirtying rate determines VM down-time
      • Shorter iterations -> less dirtying -> …
  • 41. Writable Working Set
    • Set of pages written to by OS/application
    • Pages that are dirtied must be re-sent
      • Hot pages
        • E.g. process stacks
        • Top of free page list (works like a stack)
      • Buffer cache
      • Network receive / disk buffers
  • 42. Page Dirtying Rate
    • Dirtying rate determines VM down-time
      • Shorter iters -> less dirtying -> shorter iters
      • Stop and copy final pages
    • Application ‘phase changes’ create spikes
    time into iteration #dirty
  • 43. Rate Limited Relocation
    • Dynamically adjust resources committed to performing page transfer
      • Dirty logging costs VM ~2-3%
      • CPU and network usage closely linked
    • E.g. first copy iteration at 100Mb/s, then increase based on observed dirtying rate
      • Minimize impact of relocation on server while minimizing down-time
  • 44. Web Server Relocation
  • 45. Iterative Progress: SPECWeb 52s
  • 46. Iterative Progress: Quake3
  • 47. Quake 3 Server relocation
  • 48. Extensions
    • Cluster load balancing
      • Pre-migration analysis phase
      • Optimization over coarse timescales
    • Evacuating nodes for maintenance
      • Move easy to migrate VMs first
    • Storage-system support for VM clusters
      • Decentralized, data replication, copy-on-write
    • Wide-area relocation
      • IPSec tunnels and CoW network mirroring
  • 49. Current 3.0 Status           Driver Domains     64-on-64     VT ?   4TB 16GB   >4GB memory         ~tools Save/Restore/Migrate     new!     SMP Guests           Domain U           Domain 0 Power IA64 x86_64 x86_32p x86_32  
  • 50. 3.1 Roadmap
    • Improved full-virtualization support
      • Pacifica / VT-x abstraction
    • Enhanced control tools project
    • Performance tuning and optimization
      • Less reliance on manual configuration
    • Infiniband / Smart NIC support
    • (NUMA, Virtual framebuffer, etc)
  • 51. IO Virtualization
    • IO virtualization in s/w incurs overhead
      • Latency vs. overhead tradeoff
        • More of an issue for network than storage
      • Can burn 10-30% more CPU
    • Solution is well understood
      • Direct h/w access from VMs
        • Multiplexing and protection implemented in h/w
      • Smart NICs / HCAs
        • Infiniband, Level-5, Aaorhi etc
        • Will become commodity before too long
  • 52. Research Roadmap
    • Whole-system debugging
      • Lightweight checkpointing and replay
      • Cluster/dsitributed system debugging
    • Software implemented h/w fault tolerance
      • Exploit deterministic replay
    • VM forking
      • Lightweight service replication, isolation
    • Secure virtualization
      • Multi-level secure Xen
  • 53. Conclusions
    • Xen is a complete and robust GPL VMM
    • Outstanding performance and scalability
    • Excellent resource control and protection
    • Vibrant development community
    • Strong vendor support
    • http://xen.sf.net
  • 54. Thanks!
    • The Xen project is hiring, both in Cambridge UK, Palo Alto and New York
    • [email_address]
    Computer Laboratory
  • 55. Backup slides
  • 56. Isolated Driver VMs
    • Run device drivers in separate domains
    • Detect failure e.g.
      • Illegal access
      • Timeout
    • Kill domain, restart
    • E.g. 275ms outage from failed Ethernet driver
    0 50 100 150 200 250 300 350 0 5 10 15 20 25 30 35 40 time (s)
  • 57. Device Channel Interface
  • 58. Scalability
    • Scalability principally limited by Application resource requirements
      • several 10’s of VMs on server-class machines
    • Balloon driver used to control domain memory usage by returning pages to Xen
      • Normal OS paging mechanisms can deflate quiescent domains to <4MB
      • Xen per-guest memory usage <32KB
    • Additional multiplexing overhead negligible
  • 59. System Performance L X V U SPEC INT2000 (score) L X V U Linux build time (s) L X V U OSDB-OLTP (tup/s) L X V U SPEC WEB99 (score) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Benchmark suite running on Linux (L), Xen (X), VMware Workstation (V), and UML (U)
  • 60. TCP results L X V U Tx, MTU 1500 (Mbps) L X V U Rx, MTU 1500 (Mbps) L X V U Tx, MTU 500 (Mbps) L X V U Rx, MTU 500 (Mbps) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 TCP bandwidth on Linux (L), Xen (X), VMWare Workstation (V), and UML (U)
  • 61. Scalability L X 2 L X 4 L X 8 L X 16 0 200 400 600 800 1000 Simultaneous SPEC WEB99 Instances on Linux (L) and Xen(X)
  • 62. Resource Differentation 2 4 8 8(diff) OSDB-IR 2 4 8 8(diff) OSDB-OLTP 0.0 0.5 1.0 1.5 2.0 Simultaneous OSDB-IR and OSDB-OLTP Instances on Xen Aggregate throughput relative to one instance
  • 63.  
  • 64.  
  • 65.